This document summarizes Hubble Space Telescope ultraviolet spectroscopy of four white dwarfs known to accrete planetary debris. The spectra reveal absorption lines from a variety of metals including carbon, oxygen, aluminum, silicon, phosphorus, sulfur, chromium, iron, and nickel. Combined with optical data, between five and eleven different metals are detected in each white dwarf, indicating substantial chemical diversity in the accreted planetary debris. The abundances observed provide insights into the composition and origin of the debris, with some debris resembling bulk Earth composition and others showing evidence of melting and differentiation.
This document summarizes a study that uses cosmological simulations to model the formation of stellar halos around galaxies via the tidal disruption of accreted dwarf galaxies. The simulations follow the dynamical evolution and disruption of satellites from high redshift in a fully cosmological setting. The simulations produce stellar halos with masses and density profiles consistent with observations of the Milky Way and M31. The stellar halos show complex structures composed of well-mixed components, tidal streams, shells, and other substructures rather than smooth distributions.
The panoramic view_on_the_recent_star_formation_in_ic2574Sérgio Sacani
This document summarizes research on star formation activity in the dwarf galaxy IC 2574 using deep imaging from the Large Binocular Telescope. The imaging is used to identify star-forming regions and construct maps of stellar age and mass surface density on sub-kiloparsec scales. The analysis finds an older burst of star formation occurred about 100 Myr ago within 4 kpc of the galaxy center, while a younger burst in the last 10 Myr occurred between 4-8 kpc. No correlation is seen between star formation and atomic hydrogen gas on local scales, suggesting star formation does not locally expel or ionize a significant fraction of hydrogen. The stellar populations in known hydrogen holes in IC 2574 are also analyzed, finding
Unique Meteorite from Early Amazonian Mars: Water-Rich Basaltic Breccia North...Carlos Bella
1) NWA 7034 is a 319.8g martian meteorite found in Morocco in 2011. It is a porous basaltic breccia composed of feldspar and pyroxene phenocrysts in a fine-grained groundmass.
2) Chemical and isotopic analyses indicate it formed on Mars 2.089 billion years ago from a magma enriched in the martian crust. It contains over 6000ppm water, more than most martian meteorites.
3) NWA 7034 matches basalts analyzed at Gusev Crater by the Mars Exploration Rover and the average martian crust measured by orbiters, unlike other martian meteorites. Its composition suggests
1) NWA 7034 is a 319.8g martian meteorite found in Morocco in 2011. It is a porous basaltic breccia composed of feldspar and pyroxene phenocrysts in a fine-grained groundmass containing magnetite.
2) Chemical and isotopic analyses indicate NWA 7034 formed on Mars 2.089 billion years ago from a geochemically enriched crustal source. It contains over 6000ppm water, more than most martian meteorites.
3) NWA 7034's composition is unlike SNC meteorites but matches basalts analyzed at Gusev Crater and the average martian crust measured by orbiters, providing the
1) The document describes an experiment where an ice mixture simulating interstellar medium composition was ultraviolet irradiated at 12K and analyzed after warming.
2) 16 amino acids were detected in the residue, including some that are also found in meteorites. Chiral amino acids showed enantiomeric separation.
3) The results demonstrate that amino acids can be spontaneously generated in interstellar ice analogs upon ultraviolet irradiation, supporting the hypothesis that amino acids could have been delivered to Earth by comets or meteorites.
This document reports on spectroscopic observations of z-band dropout galaxy candidates in the NTT Deep Field. Two galaxies are confirmed to be at redshift ~6.7 based on detected Lyman-alpha emission lines. For NTTDF-474, a faint line is detected at 9270 Angstroms, identified as Lyman-alpha at z=6.623. For NTTDF-1917, an emission line is seen at 8415 Angstroms, which may be Lyman-alpha or H-beta. No significant features were found for the other candidates. These results provide further evidence of galaxy populations in the early universe approaching the epoch of reionization.
The harps serach_for_southern_exoplanets_02Sérgio Sacani
The document presents an analysis of additional radial velocity measurements of the M dwarf stars Gl 433 and Gl 667C, obtained with the HARPS spectrograph. It finds:
- For Gl 433, one planet (Gl 433b) with a minimum mass of about 5.5 Earth masses and an orbital period of about 7 days. Longer period radial velocity variations are also detected, likely due to magnetic activity cycles.
- For Gl 667C, two planets are found. Gl 667Cb has a minimum mass of about 5.5 Earth masses and an orbital period of about 7 days. Gl 667Cc has a minimum mass of about 4.25 Earth masses and orbits in the habitable zone of its star with a
1) Astronomers observed comet Hale-Bopp at 30.7 AU from the Sun using the ESO 2.2m telescope in Chile on December 4, 2010.
2) They detected the comet with a total brightness of R=23.3 mag, corresponding to an absolute brightness of R(1,1,0)=8.3 mag.
3) The profile of the comet was star-like without any evidence of an extended coma or tail, indicating a cessation of matter production from the comet. However, the measured brightness corresponds to a reflecting surface area nine times smaller than three years prior, suggesting some low-level activity may still be occurring.
This document summarizes a study that uses cosmological simulations to model the formation of stellar halos around galaxies via the tidal disruption of accreted dwarf galaxies. The simulations follow the dynamical evolution and disruption of satellites from high redshift in a fully cosmological setting. The simulations produce stellar halos with masses and density profiles consistent with observations of the Milky Way and M31. The stellar halos show complex structures composed of well-mixed components, tidal streams, shells, and other substructures rather than smooth distributions.
The panoramic view_on_the_recent_star_formation_in_ic2574Sérgio Sacani
This document summarizes research on star formation activity in the dwarf galaxy IC 2574 using deep imaging from the Large Binocular Telescope. The imaging is used to identify star-forming regions and construct maps of stellar age and mass surface density on sub-kiloparsec scales. The analysis finds an older burst of star formation occurred about 100 Myr ago within 4 kpc of the galaxy center, while a younger burst in the last 10 Myr occurred between 4-8 kpc. No correlation is seen between star formation and atomic hydrogen gas on local scales, suggesting star formation does not locally expel or ionize a significant fraction of hydrogen. The stellar populations in known hydrogen holes in IC 2574 are also analyzed, finding
Unique Meteorite from Early Amazonian Mars: Water-Rich Basaltic Breccia North...Carlos Bella
1) NWA 7034 is a 319.8g martian meteorite found in Morocco in 2011. It is a porous basaltic breccia composed of feldspar and pyroxene phenocrysts in a fine-grained groundmass.
2) Chemical and isotopic analyses indicate it formed on Mars 2.089 billion years ago from a magma enriched in the martian crust. It contains over 6000ppm water, more than most martian meteorites.
3) NWA 7034 matches basalts analyzed at Gusev Crater by the Mars Exploration Rover and the average martian crust measured by orbiters, unlike other martian meteorites. Its composition suggests
1) NWA 7034 is a 319.8g martian meteorite found in Morocco in 2011. It is a porous basaltic breccia composed of feldspar and pyroxene phenocrysts in a fine-grained groundmass containing magnetite.
2) Chemical and isotopic analyses indicate NWA 7034 formed on Mars 2.089 billion years ago from a geochemically enriched crustal source. It contains over 6000ppm water, more than most martian meteorites.
3) NWA 7034's composition is unlike SNC meteorites but matches basalts analyzed at Gusev Crater and the average martian crust measured by orbiters, providing the
1) The document describes an experiment where an ice mixture simulating interstellar medium composition was ultraviolet irradiated at 12K and analyzed after warming.
2) 16 amino acids were detected in the residue, including some that are also found in meteorites. Chiral amino acids showed enantiomeric separation.
3) The results demonstrate that amino acids can be spontaneously generated in interstellar ice analogs upon ultraviolet irradiation, supporting the hypothesis that amino acids could have been delivered to Earth by comets or meteorites.
This document reports on spectroscopic observations of z-band dropout galaxy candidates in the NTT Deep Field. Two galaxies are confirmed to be at redshift ~6.7 based on detected Lyman-alpha emission lines. For NTTDF-474, a faint line is detected at 9270 Angstroms, identified as Lyman-alpha at z=6.623. For NTTDF-1917, an emission line is seen at 8415 Angstroms, which may be Lyman-alpha or H-beta. No significant features were found for the other candidates. These results provide further evidence of galaxy populations in the early universe approaching the epoch of reionization.
The harps serach_for_southern_exoplanets_02Sérgio Sacani
The document presents an analysis of additional radial velocity measurements of the M dwarf stars Gl 433 and Gl 667C, obtained with the HARPS spectrograph. It finds:
- For Gl 433, one planet (Gl 433b) with a minimum mass of about 5.5 Earth masses and an orbital period of about 7 days. Longer period radial velocity variations are also detected, likely due to magnetic activity cycles.
- For Gl 667C, two planets are found. Gl 667Cb has a minimum mass of about 5.5 Earth masses and an orbital period of about 7 days. Gl 667Cc has a minimum mass of about 4.25 Earth masses and orbits in the habitable zone of its star with a
1) Astronomers observed comet Hale-Bopp at 30.7 AU from the Sun using the ESO 2.2m telescope in Chile on December 4, 2010.
2) They detected the comet with a total brightness of R=23.3 mag, corresponding to an absolute brightness of R(1,1,0)=8.3 mag.
3) The profile of the comet was star-like without any evidence of an extended coma or tail, indicating a cessation of matter production from the comet. However, the measured brightness corresponds to a reflecting surface area nine times smaller than three years prior, suggesting some low-level activity may still be occurring.
This document summarizes a study of the galaxy NGC 3801 using panchromatic data from radio, ultraviolet, optical and infrared wavelengths. The study finds that NGC 3801 is a post-merger early-type galaxy that shows evidence of recent star formation in spiral wisps as well as feedback from active galactic nucleus jets. Specifically, the galaxy has a young radio jet that has not yet impacted the outer gaseous regions, but will likely quench star formation within the next 10 million years and further transform the galaxy into a red elliptical. The multi-wavelength data reveal complex gas and dust structures that may be due to a kinematically decoupled core or warped disk.
This document summarizes the results of a study using Herschel Space Observatory data to analyze the physical properties of dust in the RCW 120 H ii region. The study finds:
1) Dust temperatures in RCW 120 range from around 30 K in the interior decreasing to 20 K in the photo-dissociation region and 10 K in nearby infrared dark clouds.
2) There is tentative evidence of an anti-correlation between dust temperature and the dust emissivity index β, with cooler dust having higher β values, consistent with some previous studies.
3) RCW 120 appears to be in the process of destroying its photo-dissociation region boundary and leaked radiation from the interior may be influencing the
This document describes the discovery and characterization of CFBDSIR2149, a late T dwarf found to have an unusually red J-K color and enhanced K-band flux. Spectroscopy with X-Shooter revealed it has a T7 spectral type with low surface gravity features, indicating it is a young substellar object. Comparison to atmospheric models suggests it has a temperature of 650-750K, surface gravity of logg=3.75-4.0, and a mass in the planetary regime of 4-7 Jupiter masses. An analysis of its proper motion finds an 87% probability it is a member of the young AB Doradus moving group, which would make it a free-floating planet with an
Ngc1912 and ngc 1907_close_encounter_between_open_clustersSérgio Sacani
1) The open clusters NGC 1912 and NGC 1907 have a projected separation of 18.8 pc and similar distances and ages based on previous studies, suggesting they may be a physical pair.
2) New simulations of the clusters' orbits over the past 200 Myr using their spatial velocities show they were born in different regions of the Galaxy and are currently experiencing a close fly-by encounter.
3) Detailed N-body simulations of the clusters' approach support the conclusion that while they have a close projected proximity now, NGC 1912 and NGC 1907 did not originate from the same star-forming complex and are not a true physical pair.
This study examines the evolution of the yellow hypergiant star HR 8752 over nearly a century based on spectroscopic observations and photometry. The star underwent successive gas ejections, seen as downward excursions in effective temperature. During each ejection, a pseudo-photosphere formed with lower gravity and higher turbulence. After the ejected shells dispersed, a hotter, more compact photosphere emerged. Analysis of observations shows variation over time in the star's effective temperature, luminosity, radius, and mass, suggesting it is evolving away from an unstable region in the HR diagram known as the "yellow evolutionary void."
This document describes a study that determines the relative abundances of elements in the warm absorber of the active galaxy Mrk 509 using X-ray spectra from XMM-Newton and Chandra observations. The study finds relative abundances that are consistent with proto-solar values, with the exception of sulfur which is slightly underabundant. Specifically, the study measures relative abundances of carbon, nitrogen, neon, magnesium, silicon, sulfur, calcium, and iron with respect to oxygen. These abundances provide information about the enrichment processes in the host galaxy and can be compared to abundances measured in other galaxies to study abundance evolution.
This document summarizes the discovery and analysis of a Type Ia supernova (SN Ia) at a redshift of 1.71, the most distant spectroscopically confirmed SN Ia at the time. The Hubble Space Telescope was used to obtain spectroscopy and photometry of the supernova. Analysis of the spectroscopy confirms it as a SN Ia with 92% confidence. The photometry is used to determine the supernova's distance, which agrees with the predictions of the ΛCDM cosmological model. This distant supernova helps improve constraints on the nature and behavior of dark energy in the early universe.
This document summarizes the results of a 180 ks Chandra-LETGS observation of Mrk 509 as part of a larger multi-wavelength campaign. The observation detected several absorption features in the X-ray spectrum originating from an ionized absorber, including ions with three distinct ionization degrees. The lowest ionized component is slightly redshifted and not in pressure equilibrium with the others, likely belonging to the host galaxy's interstellar medium. The other two components are outflowing at velocities of around -200 and -455 km/s. Simultaneous HST-COS observations detected 13 UV kinematic components, and at least three can be associated with the X-ray components, providing evidence that the UV and X-
This document discusses ferromagnetic resonance (FMR) investigations of precipitated phases in lunar glasses. FMR was used to characterize the magnetic properties of tiny iron-bearing particles in lunar soil samples and experimentally synthesized lunar glasses. The results indicate the presence of magnetite-like phases containing iron in both the +2 and +3 oxidation states. Temperature-dependent FMR measurements match the behavior expected for magnetite and rule out metallic iron. The formation of magnetite is explained by disproportionation reactions occurring during the cooling of lunar volcanic glasses. Overall, the study provides evidence that magnetite, rather than pure iron, is the primary ferromagnetic mineral contributing to the magnetic signatures observed in lunar materials.
Observational evidence against long lived spiral arms in galaxies1Sérgio Sacani
This document presents evidence against the theory of long-lived spiral arms in galaxies. The authors analyze simulations and observations to test whether spiral patterns are quasi-stationary features with a constant pattern speed, as implied by density wave theory. They find that in simulations with imposed spiral potentials, gas and young stars are spatially ordered as expected. However, in more realistic simulations where spirals form naturally, the gas and stars are coincident with the spiral potential minors and not separated. Analyzing observations of 12 galaxies, the authors calculate angular cross-correlations between different star formation tracers but find no systematic offsets, contrary to the predictions of stationary spiral density waves. This suggests spiral density waves are not important for explaining spiral structure in
The high albedo of the hot jupiter kepler 7 bSérgio Sacani
This document summarizes the analysis of Kepler photometry data for the exoplanet Kepler-7b. Key findings include:
1) The occultation depth is measured to be 44±5 ppm, translating to a Kepler geometric albedo of 0.32±0.03, the most precise value measured for an exoplanet.
2) The planetary orbital phase lightcurve is characterized with an amplitude of 42±4 ppm.
3) Atmospheric modeling finds it unlikely that the high albedo is due to a dominant thermal component. Two possible explanations are proposed: excess reflection from clouds/hazes, or depleted atmospheric sodium and potassium allowing Rayleigh scattering to dominate.
1. The author measured proper motions of molecular hydrogen (H2) emission features in the Herbig-Haro 46/47 outflow system, finding tangential velocities ranging from tens to nearly 500 km/s.
2. The highest velocities were observed for H2 knots located along or near the jet/counterjet axes, while knots forming the wings of a large H2 bow shock moved more slowly.
3. Several H2 knots were found to have significantly changed luminosity over the 4-year study, indicating variability in H2 emission from young stellar object outflows for the first time.
This document provides an overview and context for a study of the symbiotic star SS Leporis using interferometric imaging with the PIONIER instrument on the VLTI. Key points:
- SS Leporis is a long-period interacting binary system composed of an A star accreting material from an evolved M giant companion, presenting an "Algol paradox" where the more evolved star is less massive.
- Previous studies have not fully constrained the system morphology and characteristics. New interferometric observations with PIONIER were obtained to directly probe the inner parts of the system.
- The observations were used to perform aperture synthesis imaging and model the system as a binary surrounded by a circumbinary disc. This provides the
The document summarizes a study that presents the first panoramic view of the outskirts of the Milky Way analogue galaxy NGC 891 based on ground-based imaging. The key findings are:
1) Numerous stellar overdensities and coherent substructures were detected out to distances of ~90 kpc from the center of NGC 891, including a giant stream looping around the galaxy up to ~50 kpc.
2) A previously undetected large, flat, and thick cocoon-like stellar structure was found surrounding the bulge and disk of NGC 891 out to radial distances of ~40 kpc and vertical distances of ~15 kpc.
3) The properties of the substructures suggest NGC
This study examines the clustering properties and redshift evolution of bias for 593 X-ray selected AGN from the XMM-Newton survey in the COSMOS field with spectroscopic redshifts below z=4. The authors find evidence that the bias factor increases with redshift for the total AGN population, from b=2.30 at z=0.92 to b=4.37 at z=1.94. Splitting the sample, they observe a similar increase in bias with redshift for broad-line and unobscured AGN, corresponding to a constant halo mass of logM~13.28 and logM~13, respectively. The observed bias factors cannot be reproduced by models assuming AGN are triggered
Modulations in the_radio_light_curve_of_the_typeiib_supernova_2001igSérgio Sacani
This document summarizes radio observations of the Type IIb supernova SN 2001ig made with the Australia Telescope Compact Array and Very Large Array over 700 days. The observations found periodic deviations from the standard model of radio light curves that are consistent with recurring density modulations in the circumstellar medium with a period of around 150 days. This provides evidence that the progenitor was a Wolf-Rayet star in a binary system with an eccentric 100 day orbit, causing regular build-up of circumstellar material. Such binary systems are thought to produce Type Ib/c supernovae through envelope stripping, linking these events to Type IIb supernovae.
This document presents a study characterizing active galactic nuclei (AGN) selected using mid-infrared colors from the Wide-field Infrared Survey Explorer (WISE). The authors find that a simple color criterion of W1-W2 ≥ 0.8 selects AGN with high reliability to a depth of W2 ∼ 15.0, identifying over 60 AGN candidates per square degree. This WISE selection method is effective at identifying both obscured and unobscured AGN. Using deep data in the COSMOS field, the authors explore the properties and redshift distribution of WISE-selected AGN candidates, finding the selection identifies 78% of AGN identified by previous Spitzer studies.
1) Observations of a Lyman-alpha nebula (LAB1) reveal that the Lyman-alpha emission is polarized, with a polarization fraction of around 20% at radii of 45 kpc from the center.
2) The detection of polarized radiation indicates that the Lyman-alpha photons were produced in the galaxies within the nebula and scattered by neutral hydrogen at large distances, rather than being produced in situ.
3) The polarization vectors are oriented tangentially to the overall geometry of the nebula, consistent with theoretical expectations for Lyman-alpha photons produced centrally and scattered at large radii.
Searching for signs_of_triggered_star_formation_toward_ic1848Sérgio Sacani
This document summarizes observations of three bright-rimmed clouds - SFO 11, SFO 11NE, and SFO 11E - associated with the HII region IC 1848, using submillimeter continuum data from SCUBA on the JCMT and molecular line data from CO isotopologues. The observations show evidence for protostellar cores within the clouds and recent or ongoing star formation. Based on the morphology and pressures inferred from the data, it is possible the UV illumination from the primary ionizing star HD17505 has triggered the collapse of dense molecular cores within SFO 11 and SFO 11E.
The document summarizes a study that uses strong and weak gravitational lensing to analyze the mass distribution of 28 galaxy clusters. Key findings include:
- The concentration parameter cvir is found to steeply decrease with increasing mass, approximately following cvir ∝ Mvir-0.59±0.12, consistent with theoretical predictions for high-mass clusters but observationally inferred concentrations are higher for lower-mass clusters possibly due to baryon cooling effects.
- Stacking weak lensing shear profiles also supports a steep mass-concentration relation.
- Stacking weak lensing shear maps with position angle information from strong lensing indicates significant ellipticity of the mean mass distribution, with a best-fit ellipticity
This document summarizes a study of the galaxy NGC 3801 using panchromatic data from radio, ultraviolet, optical and infrared wavelengths. The study finds that NGC 3801 is a post-merger early-type galaxy that shows evidence of recent star formation in spiral wisps as well as feedback from active galactic nucleus jets. Specifically, the galaxy has a young radio jet that has not yet impacted the outer gaseous regions, but will likely quench star formation within the next 10 million years and further transform the galaxy into a red elliptical. The multi-wavelength data reveal complex gas and dust structures that may be due to a kinematically decoupled core or warped disk.
This document summarizes the results of a study using Herschel Space Observatory data to analyze the physical properties of dust in the RCW 120 H ii region. The study finds:
1) Dust temperatures in RCW 120 range from around 30 K in the interior decreasing to 20 K in the photo-dissociation region and 10 K in nearby infrared dark clouds.
2) There is tentative evidence of an anti-correlation between dust temperature and the dust emissivity index β, with cooler dust having higher β values, consistent with some previous studies.
3) RCW 120 appears to be in the process of destroying its photo-dissociation region boundary and leaked radiation from the interior may be influencing the
This document describes the discovery and characterization of CFBDSIR2149, a late T dwarf found to have an unusually red J-K color and enhanced K-band flux. Spectroscopy with X-Shooter revealed it has a T7 spectral type with low surface gravity features, indicating it is a young substellar object. Comparison to atmospheric models suggests it has a temperature of 650-750K, surface gravity of logg=3.75-4.0, and a mass in the planetary regime of 4-7 Jupiter masses. An analysis of its proper motion finds an 87% probability it is a member of the young AB Doradus moving group, which would make it a free-floating planet with an
Ngc1912 and ngc 1907_close_encounter_between_open_clustersSérgio Sacani
1) The open clusters NGC 1912 and NGC 1907 have a projected separation of 18.8 pc and similar distances and ages based on previous studies, suggesting they may be a physical pair.
2) New simulations of the clusters' orbits over the past 200 Myr using their spatial velocities show they were born in different regions of the Galaxy and are currently experiencing a close fly-by encounter.
3) Detailed N-body simulations of the clusters' approach support the conclusion that while they have a close projected proximity now, NGC 1912 and NGC 1907 did not originate from the same star-forming complex and are not a true physical pair.
This study examines the evolution of the yellow hypergiant star HR 8752 over nearly a century based on spectroscopic observations and photometry. The star underwent successive gas ejections, seen as downward excursions in effective temperature. During each ejection, a pseudo-photosphere formed with lower gravity and higher turbulence. After the ejected shells dispersed, a hotter, more compact photosphere emerged. Analysis of observations shows variation over time in the star's effective temperature, luminosity, radius, and mass, suggesting it is evolving away from an unstable region in the HR diagram known as the "yellow evolutionary void."
This document describes a study that determines the relative abundances of elements in the warm absorber of the active galaxy Mrk 509 using X-ray spectra from XMM-Newton and Chandra observations. The study finds relative abundances that are consistent with proto-solar values, with the exception of sulfur which is slightly underabundant. Specifically, the study measures relative abundances of carbon, nitrogen, neon, magnesium, silicon, sulfur, calcium, and iron with respect to oxygen. These abundances provide information about the enrichment processes in the host galaxy and can be compared to abundances measured in other galaxies to study abundance evolution.
This document summarizes the discovery and analysis of a Type Ia supernova (SN Ia) at a redshift of 1.71, the most distant spectroscopically confirmed SN Ia at the time. The Hubble Space Telescope was used to obtain spectroscopy and photometry of the supernova. Analysis of the spectroscopy confirms it as a SN Ia with 92% confidence. The photometry is used to determine the supernova's distance, which agrees with the predictions of the ΛCDM cosmological model. This distant supernova helps improve constraints on the nature and behavior of dark energy in the early universe.
This document summarizes the results of a 180 ks Chandra-LETGS observation of Mrk 509 as part of a larger multi-wavelength campaign. The observation detected several absorption features in the X-ray spectrum originating from an ionized absorber, including ions with three distinct ionization degrees. The lowest ionized component is slightly redshifted and not in pressure equilibrium with the others, likely belonging to the host galaxy's interstellar medium. The other two components are outflowing at velocities of around -200 and -455 km/s. Simultaneous HST-COS observations detected 13 UV kinematic components, and at least three can be associated with the X-ray components, providing evidence that the UV and X-
This document discusses ferromagnetic resonance (FMR) investigations of precipitated phases in lunar glasses. FMR was used to characterize the magnetic properties of tiny iron-bearing particles in lunar soil samples and experimentally synthesized lunar glasses. The results indicate the presence of magnetite-like phases containing iron in both the +2 and +3 oxidation states. Temperature-dependent FMR measurements match the behavior expected for magnetite and rule out metallic iron. The formation of magnetite is explained by disproportionation reactions occurring during the cooling of lunar volcanic glasses. Overall, the study provides evidence that magnetite, rather than pure iron, is the primary ferromagnetic mineral contributing to the magnetic signatures observed in lunar materials.
Observational evidence against long lived spiral arms in galaxies1Sérgio Sacani
This document presents evidence against the theory of long-lived spiral arms in galaxies. The authors analyze simulations and observations to test whether spiral patterns are quasi-stationary features with a constant pattern speed, as implied by density wave theory. They find that in simulations with imposed spiral potentials, gas and young stars are spatially ordered as expected. However, in more realistic simulations where spirals form naturally, the gas and stars are coincident with the spiral potential minors and not separated. Analyzing observations of 12 galaxies, the authors calculate angular cross-correlations between different star formation tracers but find no systematic offsets, contrary to the predictions of stationary spiral density waves. This suggests spiral density waves are not important for explaining spiral structure in
The high albedo of the hot jupiter kepler 7 bSérgio Sacani
This document summarizes the analysis of Kepler photometry data for the exoplanet Kepler-7b. Key findings include:
1) The occultation depth is measured to be 44±5 ppm, translating to a Kepler geometric albedo of 0.32±0.03, the most precise value measured for an exoplanet.
2) The planetary orbital phase lightcurve is characterized with an amplitude of 42±4 ppm.
3) Atmospheric modeling finds it unlikely that the high albedo is due to a dominant thermal component. Two possible explanations are proposed: excess reflection from clouds/hazes, or depleted atmospheric sodium and potassium allowing Rayleigh scattering to dominate.
1. The author measured proper motions of molecular hydrogen (H2) emission features in the Herbig-Haro 46/47 outflow system, finding tangential velocities ranging from tens to nearly 500 km/s.
2. The highest velocities were observed for H2 knots located along or near the jet/counterjet axes, while knots forming the wings of a large H2 bow shock moved more slowly.
3. Several H2 knots were found to have significantly changed luminosity over the 4-year study, indicating variability in H2 emission from young stellar object outflows for the first time.
This document provides an overview and context for a study of the symbiotic star SS Leporis using interferometric imaging with the PIONIER instrument on the VLTI. Key points:
- SS Leporis is a long-period interacting binary system composed of an A star accreting material from an evolved M giant companion, presenting an "Algol paradox" where the more evolved star is less massive.
- Previous studies have not fully constrained the system morphology and characteristics. New interferometric observations with PIONIER were obtained to directly probe the inner parts of the system.
- The observations were used to perform aperture synthesis imaging and model the system as a binary surrounded by a circumbinary disc. This provides the
The document summarizes a study that presents the first panoramic view of the outskirts of the Milky Way analogue galaxy NGC 891 based on ground-based imaging. The key findings are:
1) Numerous stellar overdensities and coherent substructures were detected out to distances of ~90 kpc from the center of NGC 891, including a giant stream looping around the galaxy up to ~50 kpc.
2) A previously undetected large, flat, and thick cocoon-like stellar structure was found surrounding the bulge and disk of NGC 891 out to radial distances of ~40 kpc and vertical distances of ~15 kpc.
3) The properties of the substructures suggest NGC
This study examines the clustering properties and redshift evolution of bias for 593 X-ray selected AGN from the XMM-Newton survey in the COSMOS field with spectroscopic redshifts below z=4. The authors find evidence that the bias factor increases with redshift for the total AGN population, from b=2.30 at z=0.92 to b=4.37 at z=1.94. Splitting the sample, they observe a similar increase in bias with redshift for broad-line and unobscured AGN, corresponding to a constant halo mass of logM~13.28 and logM~13, respectively. The observed bias factors cannot be reproduced by models assuming AGN are triggered
Modulations in the_radio_light_curve_of_the_typeiib_supernova_2001igSérgio Sacani
This document summarizes radio observations of the Type IIb supernova SN 2001ig made with the Australia Telescope Compact Array and Very Large Array over 700 days. The observations found periodic deviations from the standard model of radio light curves that are consistent with recurring density modulations in the circumstellar medium with a period of around 150 days. This provides evidence that the progenitor was a Wolf-Rayet star in a binary system with an eccentric 100 day orbit, causing regular build-up of circumstellar material. Such binary systems are thought to produce Type Ib/c supernovae through envelope stripping, linking these events to Type IIb supernovae.
This document presents a study characterizing active galactic nuclei (AGN) selected using mid-infrared colors from the Wide-field Infrared Survey Explorer (WISE). The authors find that a simple color criterion of W1-W2 ≥ 0.8 selects AGN with high reliability to a depth of W2 ∼ 15.0, identifying over 60 AGN candidates per square degree. This WISE selection method is effective at identifying both obscured and unobscured AGN. Using deep data in the COSMOS field, the authors explore the properties and redshift distribution of WISE-selected AGN candidates, finding the selection identifies 78% of AGN identified by previous Spitzer studies.
1) Observations of a Lyman-alpha nebula (LAB1) reveal that the Lyman-alpha emission is polarized, with a polarization fraction of around 20% at radii of 45 kpc from the center.
2) The detection of polarized radiation indicates that the Lyman-alpha photons were produced in the galaxies within the nebula and scattered by neutral hydrogen at large distances, rather than being produced in situ.
3) The polarization vectors are oriented tangentially to the overall geometry of the nebula, consistent with theoretical expectations for Lyman-alpha photons produced centrally and scattered at large radii.
Searching for signs_of_triggered_star_formation_toward_ic1848Sérgio Sacani
This document summarizes observations of three bright-rimmed clouds - SFO 11, SFO 11NE, and SFO 11E - associated with the HII region IC 1848, using submillimeter continuum data from SCUBA on the JCMT and molecular line data from CO isotopologues. The observations show evidence for protostellar cores within the clouds and recent or ongoing star formation. Based on the morphology and pressures inferred from the data, it is possible the UV illumination from the primary ionizing star HD17505 has triggered the collapse of dense molecular cores within SFO 11 and SFO 11E.
The document summarizes a study that uses strong and weak gravitational lensing to analyze the mass distribution of 28 galaxy clusters. Key findings include:
- The concentration parameter cvir is found to steeply decrease with increasing mass, approximately following cvir ∝ Mvir-0.59±0.12, consistent with theoretical predictions for high-mass clusters but observationally inferred concentrations are higher for lower-mass clusters possibly due to baryon cooling effects.
- Stacking weak lensing shear profiles also supports a steep mass-concentration relation.
- Stacking weak lensing shear maps with position angle information from strong lensing indicates significant ellipticity of the mean mass distribution, with a best-fit ellipticity
This document summarizes a study of lithium and sodium abundances in stars in the globular cluster M4. The authors obtained spectra for 91 main sequence and subgiant branch stars using the FLAMES spectrograph on the VLT. They detected a weak anti-correlation between lithium and sodium abundances among unevolved main sequence stars. Notably, one star, #37934, showed an unusually high lithium abundance, comparable to estimates of the primordial lithium abundance. This high lithium abundance, coupled with the star's sodium-rich nature, suggests the lithium may have come from pollution by a previous generation of stars, though preservation of the primordial abundance cannot be ruled out. The detection provides new evidence that globular clusters
1) The document presents new observations of shell structures in the halo of galaxy NGC 7600 and a movie from a cosmological simulation showing how shell galaxies can form in a cold dark matter universe.
2) The movie illustrates how the continuous accretion of dark matter and star clumps creates diffuse circumgalactic structures, and how the disruption of a massive clump on a near-radial orbit creates a complex system of concentric shells resembling those of NGC 7600.
3) With the aid of the simulation, the authors interpret NGC 7600 in the context of the cold dark matter model of galaxy formation through the accretion and disruption of smaller companion galaxies.
This document is the lecture notes from an introductory astronomy course covering Chapter 15 on galaxies. It provides an overview of the course structure, learning objectives, quiz and exam schedule, and links to additional online resources and images of various galaxies presented in the Astronomy Picture of the Day.
The puzzling source_in_ngc6388_a_possible_planetary_tidal_disruption_eventSérgio Sacani
Artigo descreve a descoberta da destruição de um planeta ao passar próximo a uma estrela do tipo anã branca presente dentro do aglomerado globular de estrelas NGC 6388. Para isso os astrônomos utilizaram um arsenal de telescópios.
This document summarizes a study on the formation of polar ring galaxies through two proposed scenarios: the merging scenario and the accretion scenario. The merging scenario involves a head-on collision between two galaxies, where one becomes the host galaxy and the other's gas forms the polar ring. The accretion scenario involves tidal interactions that allow gas from one galaxy to be accreted by another galaxy to form a polar ring. Through simulations, the study finds that both scenarios can produce stable polar rings, but that the accretion scenario better matches observed properties of polar ring galaxies, such as the frequency and morphology of the rings. The study concludes the accretion scenario is both more likely and more supported by observations to explain how most polar ring galaxies form.
This document summarizes observations of the debris disk around the subgiant star κ CrB using Herschel and Keck. Herschel spatially resolved images of the debris disk, the first such images of a disk around a subgiant star. Keck radial velocity monitoring provided evidence for a second planetary companion around κ CrB. Keck adaptive optics imaging placed an upper limit on the mass of this companion. Modeling of the Herschel images showed the dust is broadly distributed but could not distinguish between a single wide belt or two narrow belts. The observations are consistent with dynamical depletion or collisional erosion clearing the inner regions of the disk.
This document summarizes a study analyzing far-infrared spectra of Saturn's rings obtained by the Cassini spacecraft. The authors modeled the spectra to estimate the size distribution of regolith grains on ring particles. They found that the spectra were best fit by a broad size distribution ranging from 1 micron to 1-10 cm, with a power law index of around 3. This suggests that the largest regolith grains are comparable in size to the smallest visible ring particles. The abundance of smaller grains was found to increase with decreasing solar phase angle, likely due to cooling effects in Saturn's shadow.
A 100 parsec elliptical and twisted ring of cold and dense molecular clouds r...Sérgio Sacani
The document summarizes observations from the Herschel satellite that reveal a 100-parsec elliptical and twisted ring of cold, dense molecular clouds orbiting the Galactic Center. The ring has a mass of about 3 million solar masses and semi-major axes of 100 and 60 parsecs. Its major axis is inclined 40 degrees to the plane of the sky and perpendicular to the Galactic Bar. The ring appears to trace stable x2 orbits predicted for the barred Galactic potential.
First detection of_molecular_gas_in_shells_in_cena_galaxySérgio Sacani
This document reports the first detection of molecular gas (CO) in the shells of the galaxy Centaurus A (Cen A), located 15 kpc from the galaxy's center. The ratio of CO to HI emission in the shells matches what is found in the galaxy's central regions, which is unexpected given typical metallicity gradients in galaxies. The detection of molecular gas in the shells provides evidence that molecular gas in galaxy mergers may be spread further from nuclear regions than previously thought. The dynamics of the gas can be understood if the interstellar medium is considered clumpy and less dissipative than assumed, allowing dense gas to orbit with stars and form shells.
This document presents aluminum abundances for about 100 red giant stars in each of the Galactic globular clusters 47 Tuc and M 4. The abundances were derived from intermediate-resolution FLAMES/GIRAFFE spectra, focusing on the Al I doublet at 8772-8773 Å. Previous homogeneous abundances of O, Na, Mg, and Si were also analyzed to study multiple stellar populations in these clusters. The data confirm two stellar populations are visible in M 4, while 47 Tuc shows evidence of at least three distinct stellar groups based on the abundances of O, Na, Mg, Al, and N.
X ray emission-from_strongly_asymmetric_circumstellar_material_in_the_remnant...Sérgio Sacani
This document summarizes a study of X-ray emission from circumstellar material (CSM) in the remnant of Kepler's supernova. The researchers used a statistical technique to isolate X-ray emission from CSM versus ejecta based on spectral characteristics. They found that most CSM is distributed along the bright north rim, but substantial amounts are also projected against the center, indicating a disk-like distribution of CSM from the progenitor system before the supernova. Hydrodynamic simulations support an AGB star companion as the origin of the asymmetric CSM. Quantitative analysis of magnesium emission identifies CSM and requires Kepler to have originated from a close binary system.
1. Chandra observations of the galaxy NGC 3351 revealed X-ray emission from its circumnuclear star-forming ring that is composed of numerous point-like sources embedded in diffuse hot gas.
2. The morphology of the X-ray emission is similar to but not identical with UV and H-alpha hot spots in the ring, which can be understood if star formation occurs through intermittent starbursts around the ring with different emissions tracing later evolutionary stages.
3. X-ray emission also extends beyond the ring, which is interpreted as outflowing gas from the ring into the disk and halo of NGC 3351, providing evidence for confined outflow near the plane but less restricted outflow perpendicular to
Investigating the nuclear_activity_of_ngc1672Sérgio Sacani
1) The document investigates the nuclear activity of the barred spiral galaxy NGC 1672 using X-ray data from Chandra and XMM-Newton, as well as optical data from Hubble and infrared data from Spitzer.
2) It detects 28 X-ray sources within NGC 1672, with nine being ultraluminous X-ray sources located at the ends of the galaxy's bar.
3) For the first time, it shows NGC 1672 possesses a hard nuclear X-ray source surrounded by an X-ray bright circumnuclear star-forming ring, which dominates the central X-ray emission of the galaxy. The data indicates the nuclear source is a low-luminosity active gal
Three newly discovered_globular_clusters_in_ngc6822Sérgio Sacani
This document describes the discovery of three new globular clusters in the Local Group dwarf irregular galaxy NGC 6822. The authors present optical photometry of the clusters from archival CFHT/Megacam data. Two clusters are luminous and compact, while the third is a lower luminosity diffuse cluster. The positions of the new clusters are consistent with the linear alignment of previously known clusters in NGC 6822. Studying globular cluster systems in dwarf galaxies provides insights into the formation of larger galaxies through accretion and mergers.
A multiband study_of_hercules_a_ii_multifrequency_vlaSérgio Sacani
This document summarizes a multi-frequency study of the radio galaxy Hercules A using the VLA radio telescope. The observations revealed that the bright jets and prominent rings have a flatter spectrum than the surrounding lobes and bridge, suggesting they represent a recent outburst from the active nucleus. The magnetic field closely follows the edges of the lobes, jets, and rings. There is also an asymmetry in the spectral properties between the two lobes that can be explained by relativistic beaming and light travel delays.
1) The Fermi bubbles are giant gamma-ray emitting structures extending above and below the galactic center.
2) The bubbles may have been formed by periodic capture of stars by the supermassive black hole at the galactic center, releasing energy of around 3x10^52 ergs per capture.
3) This energy injection could produce very hot plasma, accelerating electrons that produce radio and gamma-ray emission through synchrotron radiation and inverse Compton scattering.
An exceptional xray_view_of_the_young_open_cluster_ngc6231Sérgio Sacani
The XMM-Newton satellite observed the young open star cluster NGC 6231 for a total of 180 kiloseconds, providing an unprecedented X-ray view. Around 600 point-like X-ray sources were detected, including many early-type O stars. The data helped refine the relationship between X-ray and bolometric luminosities for O stars, finding less scatter than previously thought, with the main exception being X-ray emission from wind interactions in binary systems. The data also provided insight into the low-mass pre-main sequence star population and star formation history of the cluster.
This document summarizes Herschel observations of the Fornax galaxy cluster at 100, 160, 250, 350, and 500 μm. It identifies 11 bright galaxies detected at 500 μm that can be compared to previous observations of galaxies in the Virgo cluster. Stellar and far-infrared luminosities are calculated for each galaxy, and modified blackbody curves are fit to derive dust masses and temperatures. The dust properties of Fornax galaxies are found to be broadly similar to those of Virgo galaxies. Fornax shows higher far-infrared luminosity density but lower gas density than Virgo, indicating gas loss in the cluster environment.
The first hyper_luminous_infrared_galaxy_discovered_by_wiseSérgio Sacani
This document summarizes the discovery of WISE J181417.29+341224.9 (WISE 1814+3412), the first hyper-luminous infrared galaxy (LIR > 1013 L⊙) discovered by the Wide-field Infrared Survey Explorer (WISE). Follow-up images of WISE 1814+3412 revealed four nearby sources - a QSO, two Lyman Break Galaxies at z = 2.45, and an M dwarf star. The brighter LBG dominates the bolometric emission and has a star formation rate of ~300M⊙ yr−1, accounting for <10% of the bolometric luminosity. An obscured AGN combined with starburst and
The first hyper_luminous_infrared_galaxy_discovered_by_wiseSérgio Sacani
This document summarizes the discovery of WISE J181417.29+341224.9 (WISE 1814+3412), the first hyper-luminous infrared galaxy (LIR > 1013 L⊙) discovered by the Wide-field Infrared Survey Explorer (WISE). Follow-up images of WISE 1814+3412 revealed four nearby sources - a QSO, two Lyman Break Galaxies at z=2.45, and an M dwarf star. The brighter LBG dominates the bolometric emission of WISE 1814+3412 and has a star formation rate of ~300M⊙ yr−1, accounting for <10% of the total luminosity. An obscured
Creation of cosmic structure in the complex galaxy cluster merger abell 2744Sérgio Sacani
Abell 2744 is one of the most actively merging galaxy clusters known, appearing to have "dark", "ghost", "bullet", and "stripped" substructures of around 1014 solar masses each. The cluster shows a complex phenomenology that will challenge simulations to reproduce. The authors present a detailed strong lensing, weak lensing, and X-ray analysis of Abell 2744, identifying 34 strongly lensed images around the massive Southern core and producing the most detailed mass map to date. They find evidence that the Southern core and Northwestern substructure are post-merger systems similar to the Bullet Cluster viewed from an angle, and derive a new constraint on the self-interaction cross section of dark matter particles. They
Similar to Exoplanets debris around_white_dwarfs (20)
The binding of cosmological structures by massless topological defectsSérgio Sacani
Assuming spherical symmetry and weak field, it is shown that if one solves the Poisson equation or the Einstein field
equations sourced by a topological defect, i.e. a singularity of a very specific form, the result is a localized gravitational
field capable of driving flat rotation (i.e. Keplerian circular orbits at a constant speed for all radii) of test masses on a thin
spherical shell without any underlying mass. Moreover, a large-scale structure which exploits this solution by assembling
concentrically a number of such topological defects can establish a flat stellar or galactic rotation curve, and can also deflect
light in the same manner as an equipotential (isothermal) sphere. Thus, the need for dark matter or modified gravity theory is
mitigated, at least in part.
EWOCS-I: The catalog of X-ray sources in Westerlund 1 from the Extended Weste...Sérgio Sacani
Context. With a mass exceeding several 104 M⊙ and a rich and dense population of massive stars, supermassive young star clusters
represent the most massive star-forming environment that is dominated by the feedback from massive stars and gravitational interactions
among stars.
Aims. In this paper we present the Extended Westerlund 1 and 2 Open Clusters Survey (EWOCS) project, which aims to investigate
the influence of the starburst environment on the formation of stars and planets, and on the evolution of both low and high mass stars.
The primary targets of this project are Westerlund 1 and 2, the closest supermassive star clusters to the Sun.
Methods. The project is based primarily on recent observations conducted with the Chandra and JWST observatories. Specifically,
the Chandra survey of Westerlund 1 consists of 36 new ACIS-I observations, nearly co-pointed, for a total exposure time of 1 Msec.
Additionally, we included 8 archival Chandra/ACIS-S observations. This paper presents the resulting catalog of X-ray sources within
and around Westerlund 1. Sources were detected by combining various existing methods, and photon extraction and source validation
were carried out using the ACIS-Extract software.
Results. The EWOCS X-ray catalog comprises 5963 validated sources out of the 9420 initially provided to ACIS-Extract, reaching a
photon flux threshold of approximately 2 × 10−8 photons cm−2
s
−1
. The X-ray sources exhibit a highly concentrated spatial distribution,
with 1075 sources located within the central 1 arcmin. We have successfully detected X-ray emissions from 126 out of the 166 known
massive stars of the cluster, and we have collected over 71 000 photons from the magnetar CXO J164710.20-455217.
The debris of the ‘last major merger’ is dynamically youngSérgio Sacani
The Milky Way’s (MW) inner stellar halo contains an [Fe/H]-rich component with highly eccentric orbits, often referred to as the
‘last major merger.’ Hypotheses for the origin of this component include Gaia-Sausage/Enceladus (GSE), where the progenitor
collided with the MW proto-disc 8–11 Gyr ago, and the Virgo Radial Merger (VRM), where the progenitor collided with the
MW disc within the last 3 Gyr. These two scenarios make different predictions about observable structure in local phase space,
because the morphology of debris depends on how long it has had to phase mix. The recently identified phase-space folds in Gaia
DR3 have positive caustic velocities, making them fundamentally different than the phase-mixed chevrons found in simulations
at late times. Roughly 20 per cent of the stars in the prograde local stellar halo are associated with the observed caustics. Based
on a simple phase-mixing model, the observed number of caustics are consistent with a merger that occurred 1–2 Gyr ago.
We also compare the observed phase-space distribution to FIRE-2 Latte simulations of GSE-like mergers, using a quantitative
measurement of phase mixing (2D causticality). The observed local phase-space distribution best matches the simulated data
1–2 Gyr after collision, and certainly not later than 3 Gyr. This is further evidence that the progenitor of the ‘last major merger’
did not collide with the MW proto-disc at early times, as is thought for the GSE, but instead collided with the MW disc within
the last few Gyr, consistent with the body of work surrounding the VRM.
Observation of Io’s Resurfacing via Plume Deposition Using Ground-based Adapt...Sérgio Sacani
Since volcanic activity was first discovered on Io from Voyager images in 1979, changes
on Io’s surface have been monitored from both spacecraft and ground-based telescopes.
Here, we present the highest spatial resolution images of Io ever obtained from a groundbased telescope. These images, acquired by the SHARK-VIS instrument on the Large
Binocular Telescope, show evidence of a major resurfacing event on Io’s trailing hemisphere. When compared to the most recent spacecraft images, the SHARK-VIS images
show that a plume deposit from a powerful eruption at Pillan Patera has covered part
of the long-lived Pele plume deposit. Although this type of resurfacing event may be common on Io, few have been detected due to the rarity of spacecraft visits and the previously low spatial resolution available from Earth-based telescopes. The SHARK-VIS instrument ushers in a new era of high resolution imaging of Io’s surface using adaptive
optics at visible wavelengths.
Earliest Galaxies in the JADES Origins Field: Luminosity Function and Cosmic ...Sérgio Sacani
We characterize the earliest galaxy population in the JADES Origins Field (JOF), the deepest
imaging field observed with JWST. We make use of the ancillary Hubble optical images (5 filters
spanning 0.4−0.9µm) and novel JWST images with 14 filters spanning 0.8−5µm, including 7 mediumband filters, and reaching total exposure times of up to 46 hours per filter. We combine all our data
at > 2.3µm to construct an ultradeep image, reaching as deep as ≈ 31.4 AB mag in the stack and
30.3-31.0 AB mag (5σ, r = 0.1” circular aperture) in individual filters. We measure photometric
redshifts and use robust selection criteria to identify a sample of eight galaxy candidates at redshifts
z = 11.5 − 15. These objects show compact half-light radii of R1/2 ∼ 50 − 200pc, stellar masses of
M⋆ ∼ 107−108M⊙, and star-formation rates of SFR ∼ 0.1−1 M⊙ yr−1
. Our search finds no candidates
at 15 < z < 20, placing upper limits at these redshifts. We develop a forward modeling approach to
infer the properties of the evolving luminosity function without binning in redshift or luminosity that
marginalizes over the photometric redshift uncertainty of our candidate galaxies and incorporates the
impact of non-detections. We find a z = 12 luminosity function in good agreement with prior results,
and that the luminosity function normalization and UV luminosity density decline by a factor of ∼ 2.5
from z = 12 to z = 14. We discuss the possible implications of our results in the context of theoretical
models for evolution of the dark matter halo mass function.
THE IMPORTANCE OF MARTIAN ATMOSPHERE SAMPLE RETURN.Sérgio Sacani
The return of a sample of near-surface atmosphere from Mars would facilitate answers to several first-order science questions surrounding the formation and evolution of the planet. One of the important aspects of terrestrial planet formation in general is the role that primary atmospheres played in influencing the chemistry and structure of the planets and their antecedents. Studies of the martian atmosphere can be used to investigate the role of a primary atmosphere in its history. Atmosphere samples would also inform our understanding of the near-surface chemistry of the planet, and ultimately the prospects for life. High-precision isotopic analyses of constituent gases are needed to address these questions, requiring that the analyses are made on returned samples rather than in situ.
Multi-source connectivity as the driver of solar wind variability in the heli...Sérgio Sacani
The ambient solar wind that flls the heliosphere originates from multiple
sources in the solar corona and is highly structured. It is often described
as high-speed, relatively homogeneous, plasma streams from coronal
holes and slow-speed, highly variable, streams whose source regions are
under debate. A key goal of ESA/NASA’s Solar Orbiter mission is to identify
solar wind sources and understand what drives the complexity seen in the
heliosphere. By combining magnetic feld modelling and spectroscopic
techniques with high-resolution observations and measurements, we show
that the solar wind variability detected in situ by Solar Orbiter in March
2022 is driven by spatio-temporal changes in the magnetic connectivity to
multiple sources in the solar atmosphere. The magnetic feld footpoints
connected to the spacecraft moved from the boundaries of a coronal hole
to one active region (12961) and then across to another region (12957). This
is refected in the in situ measurements, which show the transition from fast
to highly Alfvénic then to slow solar wind that is disrupted by the arrival of
a coronal mass ejection. Our results describe solar wind variability at 0.5 au
but are applicable to near-Earth observatories.
Gliese 12 b: A Temperate Earth-sized Planet at 12 pc Ideal for Atmospheric Tr...Sérgio Sacani
Recent discoveries of Earth-sized planets transiting nearby M dwarfs have made it possible to characterize the
atmospheres of terrestrial planets via follow-up spectroscopic observations. However, the number of such planets
receiving low insolation is still small, limiting our ability to understand the diversity of the atmospheric
composition and climates of temperate terrestrial planets. We report the discovery of an Earth-sized planet
transiting the nearby (12 pc) inactive M3.0 dwarf Gliese 12 (TOI-6251) with an orbital period (Porb) of 12.76 days.
The planet, Gliese 12 b, was initially identified as a candidate with an ambiguous Porb from TESS data. We
confirmed the transit signal and Porb using ground-based photometry with MuSCAT2 and MuSCAT3, and
validated the planetary nature of the signal using high-resolution images from Gemini/NIRI and Keck/NIRC2 as
well as radial velocity (RV) measurements from the InfraRed Doppler instrument on the Subaru 8.2 m telescope
and from CARMENES on the CAHA 3.5 m telescope. X-ray observations with XMM-Newton showed the host
star is inactive, with an X-ray-to-bolometric luminosity ratio of log 5.7 L L X bol » - . Joint analysis of the light
curves and RV measurements revealed that Gliese 12 b has a radius of 0.96 ± 0.05 R⊕,a3σ mass upper limit of
3.9 M⊕, and an equilibrium temperature of 315 ± 6 K assuming zero albedo. The transmission spectroscopy metric
(TSM) value of Gliese 12 b is close to the TSM values of the TRAPPIST-1 planets, adding Gliese 12 b to the small
list of potentially terrestrial, temperate planets amenable to atmospheric characterization with JWST.
Gliese 12 b, a temperate Earth-sized planet at 12 parsecs discovered with TES...Sérgio Sacani
We report on the discovery of Gliese 12 b, the nearest transiting temperate, Earth-sized planet found to date. Gliese 12 is a
bright (V = 12.6 mag, K = 7.8 mag) metal-poor M4V star only 12.162 ± 0.005 pc away from the Solar system with one of the
lowest stellar activity levels known for M-dwarfs. A planet candidate was detected by TESS based on only 3 transits in sectors
42, 43, and 57, with an ambiguity in the orbital period due to observational gaps. We performed follow-up transit observations
with CHEOPS and ground-based photometry with MINERVA-Australis, SPECULOOS, and Purple Mountain Observatory,
as well as further TESS observations in sector 70. We statistically validate Gliese 12 b as a planet with an orbital period of
12.76144 ± 0.00006 d and a radius of 1.0 ± 0.1 R⊕, resulting in an equilibrium temperature of ∼315 K. Gliese 12 b has excellent
future prospects for precise mass measurement, which may inform how planetary internal structure is affected by the stellar
compositional environment. Gliese 12 b also represents one of the best targets to study whether Earth-like planets orbiting cool
stars can retain their atmospheres, a crucial step to advance our understanding of habitability on Earth and across the galaxy.
The importance of continents, oceans and plate tectonics for the evolution of...Sérgio Sacani
Within the uncertainties of involved astronomical and biological parameters, the Drake Equation
typically predicts that there should be many exoplanets in our galaxy hosting active, communicative
civilizations (ACCs). These optimistic calculations are however not supported by evidence, which is
often referred to as the Fermi Paradox. Here, we elaborate on this long-standing enigma by showing
the importance of planetary tectonic style for biological evolution. We summarize growing evidence
that a prolonged transition from Mesoproterozoic active single lid tectonics (1.6 to 1.0 Ga) to modern
plate tectonics occurred in the Neoproterozoic Era (1.0 to 0.541 Ga), which dramatically accelerated
emergence and evolution of complex species. We further suggest that both continents and oceans
are required for ACCs because early evolution of simple life must happen in water but late evolution
of advanced life capable of creating technology must happen on land. We resolve the Fermi Paradox
(1) by adding two additional terms to the Drake Equation: foc
(the fraction of habitable exoplanets
with significant continents and oceans) and fpt
(the fraction of habitable exoplanets with significant
continents and oceans that have had plate tectonics operating for at least 0.5 Ga); and (2) by
demonstrating that the product of foc
and fpt
is very small (< 0.00003–0.002). We propose that the lack
of evidence for ACCs reflects the scarcity of long-lived plate tectonics and/or continents and oceans on
exoplanets with primitive life.
A Giant Impact Origin for the First Subduction on EarthSérgio Sacani
Hadean zircons provide a potential record of Earth's earliest subduction 4.3 billion years ago. Itremains enigmatic how subduction could be initiated so soon after the presumably Moon‐forming giant impact(MGI). Earlier studies found an increase in Earth's core‐mantle boundary (CMB) temperature due to theaccumulation of the impactor's core, and our recent work shows Earth's lower mantle remains largely solid, withsome of the impactor's mantle potentially surviving as the large low‐shear velocity provinces (LLSVPs). Here,we show that a hot post‐impact CMB drives the initiation of strong mantle plumes that can induce subductioninitiation ∼200 Myr after the MGI. 2D and 3D thermomechanical computations show that a high CMBtemperature is the primary factor triggering early subduction, with enrichment of heat‐producing elements inLLSVPs as another potential factor. The models link the earliest subduction to the MGI with implications forunderstanding the diverse tectonic regimes of rocky planets.
Climate extremes likely to drive land mammal extinction during next supercont...Sérgio Sacani
Mammals have dominated Earth for approximately 55 Myr thanks to their
adaptations and resilience to warming and cooling during the Cenozoic. All
life will eventually perish in a runaway greenhouse once absorbed solar
radiation exceeds the emission of thermal radiation in several billions of
years. However, conditions rendering the Earth naturally inhospitable to
mammals may develop sooner because of long-term processes linked to
plate tectonics (short-term perturbations are not considered here). In
~250 Myr, all continents will converge to form Earth’s next supercontinent,
Pangea Ultima. A natural consequence of the creation and decay of Pangea
Ultima will be extremes in pCO2 due to changes in volcanic rifting and
outgassing. Here we show that increased pCO2, solar energy (F⨀;
approximately +2.5% W m−2 greater than today) and continentality (larger
range in temperatures away from the ocean) lead to increasing warming
hostile to mammalian life. We assess their impact on mammalian
physiological limits (dry bulb, wet bulb and Humidex heat stress indicators)
as well as a planetary habitability index. Given mammals’ continued survival,
predicted background pCO2 levels of 410–816 ppm combined with increased
F⨀ will probably lead to a climate tipping point and their mass extinction.
The results also highlight how global landmass configuration, pCO2 and F⨀
play a critical role in planetary habitability.
Constraints on Neutrino Natal Kicks from Black-Hole Binary VFTS 243Sérgio Sacani
The recently reported observation of VFTS 243 is the first example of a massive black-hole binary
system with negligible binary interaction following black-hole formation. The black-hole mass (≈10M⊙)
and near-circular orbit (e ≈ 0.02) of VFTS 243 suggest that the progenitor star experienced complete
collapse, with energy-momentum being lost predominantly through neutrinos. VFTS 243 enables us to
constrain the natal kick and neutrino-emission asymmetry during black-hole formation. At 68% confidence
level, the natal kick velocity (mass decrement) is ≲10 km=s (≲1.0M⊙), with a full probability distribution
that peaks when ≈0.3M⊙ were ejected, presumably in neutrinos, and the black hole experienced a natal
kick of 4 km=s. The neutrino-emission asymmetry is ≲4%, with best fit values of ∼0–0.2%. Such a small
neutrino natal kick accompanying black-hole formation is in agreement with theoretical predictions.
Detectability of Solar Panels as a TechnosignatureSérgio Sacani
In this work, we assess the potential detectability of solar panels made of silicon on an Earth-like
exoplanet as a potential technosignature. Silicon-based photovoltaic cells have high reflectance in the
UV-VIS and in the near-IR, within the wavelength range of a space-based flagship mission concept
like the Habitable Worlds Observatory (HWO). Assuming that only solar energy is used to provide
the 2022 human energy needs with a land cover of ∼ 2.4%, and projecting the future energy demand
assuming various growth-rate scenarios, we assess the detectability with an 8 m HWO-like telescope.
Assuming the most favorable viewing orientation, and focusing on the strong absorption edge in the
ultraviolet-to-visible (0.34 − 0.52 µm), we find that several 100s of hours of observation time is needed
to reach a SNR of 5 for an Earth-like planet around a Sun-like star at 10pc, even with a solar panel
coverage of ∼ 23% land coverage of a future Earth. We discuss the necessity of concepts like Kardeshev
Type I/II civilizations and Dyson spheres, which would aim to harness vast amounts of energy. Even
with much larger populations than today, the total energy use of human civilization would be orders of
magnitude below the threshold for causing direct thermal heating or reaching the scale of a Kardashev
Type I civilization. Any extraterrrestrial civilization that likewise achieves sustainable population
levels may also find a limit on its need to expand, which suggests that a galaxy-spanning civilization
as imagined in the Fermi paradox may not exist.
Jet reorientation in central galaxies of clusters and groups: insights from V...Sérgio Sacani
Recent observations of galaxy clusters and groups with misalignments between their central AGN jets
and X-ray cavities, or with multiple misaligned cavities, have raised concerns about the jet – bubble
connection in cooling cores, and the processes responsible for jet realignment. To investigate the
frequency and causes of such misalignments, we construct a sample of 16 cool core galaxy clusters and
groups. Using VLBA radio data we measure the parsec-scale position angle of the jets, and compare
it with the position angle of the X-ray cavities detected in Chandra data. Using the overall sample
and selected subsets, we consistently find that there is a 30% – 38% chance to find a misalignment
larger than ∆Ψ = 45◦ when observing a cluster/group with a detected jet and at least one cavity. We
determine that projection may account for an apparently large ∆Ψ only in a fraction of objects (∼35%),
and given that gas dynamical disturbances (as sloshing) are found in both aligned and misaligned
systems, we exclude environmental perturbation as the main driver of cavity – jet misalignment.
Moreover, we find that large misalignments (up to ∼ 90◦
) are favored over smaller ones (45◦ ≤ ∆Ψ ≤
70◦
), and that the change in jet direction can occur on timescales between one and a few tens of Myr.
We conclude that misalignments are more likely related to actual reorientation of the jet axis, and we
discuss several engine-based mechanisms that may cause these dramatic changes.
The solar dynamo begins near the surfaceSérgio Sacani
The magnetic dynamo cycle of the Sun features a distinct pattern: a propagating
region of sunspot emergence appears around 30° latitude and vanishes near the
equator every 11 years (ref. 1). Moreover, longitudinal flows called torsional oscillations
closely shadow sunspot migration, undoubtedly sharing a common cause2. Contrary
to theories suggesting deep origins of these phenomena, helioseismology pinpoints
low-latitude torsional oscillations to the outer 5–10% of the Sun, the near-surface
shear layer3,4. Within this zone, inwardly increasing differential rotation coupled with
a poloidal magnetic field strongly implicates the magneto-rotational instability5,6,
prominent in accretion-disk theory and observed in laboratory experiments7.
Together, these two facts prompt the general question: whether the solar dynamo is
possibly a near-surface instability. Here we report strong affirmative evidence in stark
contrast to traditional models8 focusing on the deeper tachocline. Simple analytic
estimates show that the near-surface magneto-rotational instability better explains
the spatiotemporal scales of the torsional oscillations and inferred subsurface
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1. Mon. Not. R. Astron. Soc. 000, 1–16 (2006) Printed 2 May 2012 (MN L TEX style file v2.2)
A
The chemical diversity of exo-terrestrial planetary debris
around white dwarfs
1
B.T. G¨nsicke1, D. Koester2, J. Farihi3, J. Girven1, S.G. Parsons1, E. Breedt1
a
Department of Physics, University of Warwick, Coventry CV4 7AL, UK
arXiv:1205.0167v1 [astro-ph.EP] 1 May 2012
2 Institut f¨r Theoretische Physik und Astrophysik, University of Kiel, 24098 Kiel, Germany
u
3 Department of Physics Astronomy, University of Leicester, Leicester LE1 7RH, UK
Accepted 2005. Received 2005; in original form 2005
ABSTRACT
We present Hubble Space Telescope ultraviolet spectroscopy of the white dwarfs
PG 0843+516, PG 1015+161, SDSS 1228+1040, and GALEX 1931+0117, which ac-
crete circumstellar planetary debris formed from the destruction of asteroids. Com-
bined with optical data, a minimum of five and a maximum of eleven different metals
are detected in their photospheres. With metal sinking time scales of only a few days,
these stars are in accretion/diffusion equilibrium, and the photospheric abundances
closely reflect those of the circumstellar material. We find C/Si ratios that are consis-
tent with that of the bulk Earth, corroborating the rocky nature of the debris. Their
C/O values are also very similar to those of bulk Earth, implying that the planetary
debris is dominated by Mg and Fe silicates. The abundances found for the debris
at the four white dwarfs show substantial diversity, comparable at least to that seen
across different meteorite classes in the solar system. PG 0843+516 exhibits significant
over-abundances of Fe and Ni, as well as of S and Cr, which suggests the accretion
of material that has undergone melting, and possibly differentiation. PG 1015+161
stands out by having the lowest Si abundance relative to all other detected elements.
The Al/Ca ratio determined for the planetary debris around different white dwarfs is
remarkably similar. This is analogous to the nearly constant abundance ratio of these
two refractory lithophile elements found among most bodies in the solar system.
Based on the detection of all major elements of the circumstellar debris, we cal-
culate accretion rates of ≃ 1.7 × 108 g s−1 to ≃ 1.5 × 109 g s−1 . Finally, we detect
additional circumstellar absorption in the Si iv 1394,1403 ˚ doublet in PG 0843+516
A
and SDSS 1228+1040, reminiscent to similar high-ionisation lines seen in the HST
spectra of white dwarfs in cataclysmic variables. We suspect that these lines originate
in hot gas close to the white dwarf, well within the sublimation radius.
Key words: Stars: individual: PG 0843+516, PG 1015+161,
SDSS J122859.93+104032.9, GALEX J193156.8+011745 – white dwarfs – cir-
cumstellar matter – planetary systems
1 INTRODUCTION rial detected around the white dwarf GD 362, and showed
that the composition of this material is broadly comparable
to that of the Earth-Moon system.
Most of our current insight into the interior structure of exo-
planets is derived from the bulk density of transiting planets The strong surface gravity of white dwarfs implies that
(e.g. Valencia et al. 2010), and transit spectroscopy provides metals will sink out of the photosphere on time scales that
some information on the chemical composition of their at- are orders of magnitude shorter than their cooling ages, and
mospheres (e.g. Grillmair et al. 2008). More detailed inves- therefore white dwarfs are expected to have either pure hy-
tigations of the chemistry of exo-planetary systems around drogen or helium atmospheres (Fontaine & Michaud 1979).
main-sequence host stars are beyond the reach of present Exceptions to this rule are only hot (Teff 25 000 K) white
observational instrumentation. However, Zuckerman et al. dwarfs where radiative levitation can support some heavy el-
(2007) demonstrated in a pioneering paper that the pho- ements in the photosphere (e.g. Chayer et al. 1995), and cool
tospheric abundances of polluted white dwarfs can be used (Teff 10 000 K) white dwarfs where convection may dredge
to infer the bulk abundances of the planetary debris mate- up core material (Koester et al. 1982; Fontaine et al. 1984).
c 2006 RAS
2. 2 B.T. G¨nsicke et al.
a
Yet white dwarfs with metal-contaminated atmospheres G¨nsicke et al. 2006; Vennes et al. 2010). Our sample also
a
have been known for nearly a century (van Maanen 1917), includes a small number of post-common envelope binaries
and accretion from the interstellar medium (e.g. Koester (PCEBs) in which the white dwarf accretes from the wind of
1976; Wesemael 1979; Dupuis et al. 1993) has been the the M-dwarf companion. These systems were selected from
most widely accepted scenario, despite a number of funda- Schreiber & G¨nsicke (2003) and Farihi et al. (2010b) with
a
mental problems (e.g. Aannestad et al. 1993; Friedrich et al. the same cut on white dwarf temperature and cooling age.
2004; Farihi et al. 2010a). However, the rapidly growing Under the assumption that the M-dwarfs have a solar-like
number of white dwarfs that are accreting from circum- composition, the white dwarfs in PCEBs serve as “abun-
stellar discs (e.g. Becklin et al. 2005; Kilic et al. 2005; dance standards” for our abundances analyses and diffusion
G¨nsicke et al. 2006; von Hippel et al. 2007; Farihi et al.
a calculations.
2008; Vennes et al. 2010; Dufour et al. 2012) unambigu-
ously demonstrates that debris from the tidal disrup-
2.1 HST /COS spectroscopy
tion of main-belt analogue asteroids or minor planets
(Graham et al. 1990; Jura 2003), or Kuiper-belt like ob- PG 0843+516, PG 1015+161, and
jects (Bonsor et al. 2011), likely perturbed by unseen plan- GALEX J193156.8+011745 (henceforth
ets (Debes & Sigurdsson 2002; Debes et al. 2012), is the GALEX 1931+0117) were observed as part of our snapshot
most likely origin of photospheric metals in many, if not survey, with exposure times of 1420 s, 1424 s, and 800 s,
most polluted white dwarfs. respectively. We used the G130M grating with a central
Because of the need for high-resolution, high-quality wavelength of 1291 ˚, which covers the wavelength range
A
spectroscopy, detailed abundance studies have so far been 1130 − 1435 ˚, with a gap at 1278 − 1288 A due to the
A ˚
limited to a handful of white dwarfs (Klein et al. 2010, 2011; space between the two detector segments. To mitigate the
Vennes et al. 2011b; Melis et al. 2011; Zuckerman et al. fixed pattern noise that is affecting the COS far-ultraviolet
2011; Dufour et al. 2012; Jura et al. 2012). For a given abun- detector, we split the exposure time equally between
dance and white dwarf temperature, metal lines are stronger two FP-POS positions (1 & 4, the limited duration of the
in a helium-dominated (DB) atmosphere than in a hydrogen- snapshot visits did not allow to use the full set of four
dominated (DA) atmosphere, as the opacity of helium is different FP-POS positions).
much lower than that of hydrogen. Therefore, the small sam- We also report COS observations of three PCEBs
ple of well-studied metal polluted white dwarfs is heavily observed within this snapshot survey, that will be
biased towards DB white dwarfs, which have diffusion time used as “abundances standards”: GD 448 (HR Cam,
scales of ∼ 105 − 106 yr. These long diffusion time scales Maxted et al. 1998), GD 245 (MS Peg, Schmidt et al. 1995),
introduce a significant caveat in the interpretation, as the and PG 2257+162 (KUV 22573+1613, Wachter et al. 2003),
abundances of the circumstellar debris may substantially with exposure times of 900 s, 600 s, and 1070 s, respectively.
differ from those in the white dwarf photosphere if the accre- SDSS J122859.93+104032.9 (henceforth
tion rate varies on shorter time scales (Koester 2009). While SDSS 1228+1040) was observed in Cycle 17 as part of
the life times of the debris discs are subject to large uncer- a regular Guest Observer programme. We obtained two
tainties, there are theoretical (Rafikov 2011; Metzger et al. sets of spectroscopy with the G130M grating with central
2012) and observational (Girven et al. 2012, Farihi et al. wavelengths of 1291 ˚ and 1327 ˚, and both observations
A A
2012 in press) arguments that suggest that the accretion were again split among two FP-POS positions (1 & 4). In
rates onto the white dwarfs may vary significantly over pe- addition, we obtained G160M spectroscopy with central
riods that are short compared to the diffusion time scales. wavelengths of 1577 ˚ and 1623 A. The total exposure time
A ˚
In fact, some of the most heavily polluted white dwarfs have of the G130M and G160M observations were 2821 s and
no infrared excess (Farihi et al. 2009; Klein et al. 2011), and 4899 s, respectively, seamlessly covering the wavelength
may have accreted all the circumstellar debris a few diffusion range 1130 − 1795 ˚.
A
time scales ago (Farihi et al. 2009; Girven et al. 2012). The data retrieved from the HST archive were pro-
We are currently carrying out an ultraviolet spectro- cessed and calibrated with CALCOS 2.15.6. The COS spec-
scopic survey of young DA white dwarfs that have cooling tra of the four white dwarfs shown in Fig. 1 reveal the
ages of 20 to 200 Myr, metal sinking time scales of a few broad Lyα profile typical of DA white dwarfs, plus a mul-
days, and are hence guaranteed to be in accretion-diffusion titude of narrow absorption lines from a range of metals.
equilibrium. The aim of this survey is to determine the frac- The peak signal-to-noise ratio (S/N) of the COS spectra
tion of white dwarfs that are presently accreting planetary is reached in a line-free region near 1320 ˚, and ranges
A
debris, and to determine accurate abundances for a subset. from ≃ 25 for PG 0843+516 and PG 1015+161 to ≃ 40 for
Here we present the analysis of four heavily polluted white SDSS 1228+1040 and GALEX 1931+0117. However, these
dwarfs that are known to also host planetary debris discs. values only include photon count statistics, and do not ac-
count for the residual fixed-pattern noise related to the use
of only two FP-POS positions. The resolving power of the
COS spectra, as measured from on-orbit data ranges from
2 OBSERVATIONS
∼ 15 000 at 1150 ˚ to ∼ 20 000 at 1430 ˚.
A A
The targets for our ongoing far-ultraviolet spectroscopic
survey of young and correspondingly warm (17 000 K <
2.2 Optical observations
Teff < 25 000 K) DA white dwarfs were drawn from the
compilations of Liebert et al. (2005) and Koester et al. The wavelength spanned by our COS observations does not
(2009), supplemented with a few recent discoveries (e.g. cover any strong line of either Ca (traditionally the most
c 2006 RAS, MNRAS 000, 1–16
3. The chemical diversity of exo-terrestrial planetary debris around white dwarfs 3
Figure 1. COS/G130M spectra of four white dwarfs known to have circumstellar discs, scaled to a peak flux of unity, offset by 1.4 units,
and sorted from top to bottom by increasing metal abundances. For warm white dwarfs with pure hydrogen atmospheres, the broad
Lyα line is the only spectral feature in this wavelength range. These four stars accrete from the circumstellar debris, and their spectra
are riddled with absorption lines of C, O, Al, Si, P, S, Cr, Fe, and Ni. In addition, Mg and Ca can be detected in their optical spectra
(Fig. 2).
important tracer of metal pollution in white dwarfs, and Table 1. Atmospheric parameters from spectroscopy
an important refractory element) or Mg (one of the major
constituents of rocky material in the solar system, includ- Object Teff [K] log g [cgs units]
ing the Earth). Ground-based abundance studies using the
Ca ii H/K doublet and the Mg ii 4482 ˚ line are already pub-
A PG 0843+516 = WD 0843+516
lished for GALEX 1931+0117 (Vennes et al. 2010, 2011b; optical, Liebert et al. (2005) 23 870 ± 392 7.90 ± 0.05
HST, this paper 23 095 ± 230 8.17 ± 0.06
Melis et al. 2011). Two short (10 min) VLT/UVES spectra
PG 1015+161 = WD 1015+161
of PG 1015+161 were obtained as part of the SPY project optical, Liebert et al. (2005) 19 540 ± 305 8.04 ± 0.05
(Napiwotzki et al. 2001), which Koester et al. (2005) anal- optical, Koester et al. (2009) 19 948 ± 33 7.925 ± 0.006
ysed to determine the Ca abundance of PG 1015+161 HST, this paper 19 200 ± 180 8.22 ± 0.06
(Sect. 4.2). Here we use the same spectra to determine in SDSS J122859.93+104032.9 = WD1226+110
addition the abundance of Mg. optical, Eisenstein et al. (2006) 22 125 ± 136 8.22 ± 0.02
optical, G¨nsicke et al. (2007)
a 22 292 ± 296 8.29 ± 0.05
We observed PG 0843+516 for a total of 2 h on the optical, our fit to SDSS spectrum 22 410 ± 175 8.12 ± 0.02
WHT using ISIS with the R600B grating and a 1′′ slit, cov- HST, this paper 20 565 ± 82 8.19 ± 0.03
ering the Ca and Mg lines at a resolving power of ≃ 2500 adopted, this paper (Sect. 3.1) 20900 ± 900 8.15 ± 0.04
and a S/N of ≈ 90. The data were reduced and calibrated GALEX J193156.8+011745 = WD 1929+012
as described in Pyrzas et al. (2012). optical, Vennes et al. (2010) 20 890 ± 120 7.90 ± 0.03
optical, Melis et al. (2011) 23 470 ± 300 7.99 ± 0.05
We also obtained a total of 9 h VLT/UVES spec- HST, this paper 21 200 ± 50 7.91 ± 0.02
troscopy of SDSS 1228+1040 between 2007 and 2009 us-
ing the Blue390 and Blue437 setup with a 0.9′′ slit, cov-
ering both the Ca and Mg features with a resolving power Mg ii 4482 ˚ becomes a more sensitive probe of metal pol-
A
of ≃ 40 000. The data were reduced in Gasgano using the lution (e.g. G¨nsicke et al. 2007; Farihi et al. 2012).
a
UVES pipeline. The individual spectra were of relatively low
S/N, and we analysed only the error-weighted average spec-
trum, binned to 0.05 ˚, with S/N ≃ 35.
A
3 ATMOSPHERE MODELS
The optical spectra around the Ca ii K and Mg ii 4482 ˚
A
3.1 Effective temperature and surface gravity
lines are shown in Fig. 2. We note that while most previ-
ous studies of metal-polluted white dwarfs have focused on All observed HST /COS and optical spectra were analysed
the Ca ii H/K lines, their strength for a given abundance with theoretical model atmospheres using input physics as
decreases strongly with increasing temperature, as Ca ii is described in Koester (2010), and including the Lyman and
ionised to Ca iii. For temperatures Teff ≃ 20 000 − 25 000 K, Balmer line profiles of Tremblay & Bergeron (2009). We
c 2006 RAS, MNRAS 000, 1–16
4. 4 B.T. G¨nsicke et al.
a
Figure 3. The normalised COS spectra (black) of PG 0843+516 (left) and PG 1015+161 (right), along with our best-fit models (red).
Interstellar absorption features are indicated by vertical gray dashed lines. The interstellar lines in PG 1015+161 are blue-shifted with
respect to the photospheric features by 57 km s−1 , in PG 0843+516 this shift is 7 km s−1 . Airglow of O i can cause some contamination
of the 1302 − 1306 ˚ region. An illustrative airglow emission spectrum (arbitrarily scaled in flux) is shown. The strong Si iv 1394,1403 ˚
A A
doublet seen in the COS spectrum of PG 0843+516 is not of photospheric, but circumstellar origin (Sect. 7).
c 2006 RAS, MNRAS 000, 1–16
5. The chemical diversity of exo-terrestrial planetary debris around white dwarfs 5
Figure 4. Same as Fig. 3, but for SDSS 1228+1040 (left) and GALEX 1931+0117 (right). The interstellar lines in their spectra are are
blue-shifted with respect to the photospheric features by 36 km s−1 and 61 km s−1 , respectively. The strong absorption band seen near
1410 ˚ in the COS spectrum of GALEX 1931+0117 is thought to be related to an autoionisation line of Si ii or to a resonance feature in
A
the photoionisation cross section (Sect. 3.2.2). The same features is seen, though much weaker, in SDSS 1228+1040. The Si iv 1394,1403 ˚
A
doublet in SDSS 1228+1040 shows additional absorption, blue-shifted with respect to the photospheric features, which is of circumstellar
origin (Sect. 7).
c 2006 RAS, MNRAS 000, 1–16
6. 6 B.T. G¨nsicke et al.
a
Table 2. List of major line features used for the abundance deter-
minations and upper limits. Because of the different wavelength
ranges of the available spectra not all lines could be used for all
four stars.
Ion Vacuum wavelengths [˚]
A
C ii 1334.530,1335.660,1335.708
C iii 1174.930,1175.260,1175.590,1175.710,1175.987,1176.370
Ni 1199.550,1200.220,1200.710
Oi 1152.150,1302.170,1304.860,1306.030
Mg ii 1239.925,1240.395,1367.257,1367.708,1369.423,
4482.383,4482.407,4482.583
Al ii 1670.787,1719.442,1724.922,1724.982,1760.106,1761.977,
1763.869,1763.952,1765.816
Al iii 1379.670,1384.132,1605.766,1611.873
Figure 2. The normalised optical spectra (gray) of PG 0843+516 Si ii 1190.416,1193.292,1194.500,1197.394,1246.740,1248.426,
(WHT), PG 1015+161 (VLT/UVES, binned to 0.2 ˚), and A 1250.091,1250.436,1251.164,1260.422,1264.738,1265.002,
SDSS 1228+1040 (VLT/UVES, binned to 0.05 ˚), along with
A 1304.370,1305.592,1309.276,1309.453,1311.256,1346.884,
the best-fit models (black). The width of the Mg ii line in 1348.543,1350.072,1350.516,1350.656,1352.635,1353.721,
PG 0843+516 is due to the low resolution of the WHT data. In 1526.707,1533.431,3854.758,3857.112,3863.690,4129.219,
the spectrum of SDSS 1228+1040, the photospheric absorption of 4132.059,5042.430,5057.394,6348.864,6373.132
Ca ii K is embedded in a double-peaked emission line from the Si iii 1140.546,1141.579,1142.285,1144.309,1144.959,1154.998,
gaseous debris disc, which is, however, so broad that it does not 1155.959,1156.782,1158.101,1160.252,1161.579,1206.500,
affect the measurement of the Ca abundance. 1206.555,1294.545,1296.726,1298.892,1301.149,1303.323,
1312.591,1341.458,1342.389,1365.253,1417.237
Si iv 1393.775,1402.770
used a fine grid of models spanning the range of temper-
P ii 1149.958,1152.818,1153.995,1155.014,1156.970,1159.086,
atures and surface gravities found for the four targets by 1249.830,1452.900,1532.533,1535.923,1536.416,1542.304,
previous studies (Table 1) and determined the best-fit pa- 1543.133,1543.631
rameter by minimising χ2 , using the very good relative flux P iii 1334.813,1344.326
calibration as an additional constraint. The errors reported S ii 1250.584,1253.811,1259.519
in Sect. 4 are statistical only and do not include systematic S iii 1194.041,1194.433
effects of observation, reduction, or models. More realistic Ca ii 1169.029, 1169.198,1341.890,3737.965,3934.777
errors can be estimated from a comparison with the other Sc ii 1418.773,1418.793
measurements in the literature, which used similar models, Ti iii 1298.633,1298.697,1298.996,1327.603
but optical spectra. Table 1 suggests a systematic trend for V iii 1148.465,1149.945,1149.945
Cr iii 1136.669,1146.342,1247.846,1252.616,1259.018,1261.865,
somewhat lower temperatures derived from the ultraviolet
1263.611
data when compared to the values based on optical spec- Mn ii 1162.015,1188.505,1192.316,1192.330,1197.184,1199.391,
troscopy. A similar trend is seen for DA white dwarfs with 1201.118,1233.956,1254.410
Teff ∼ 20 000 K in Lajoie & Bergeron (2007), who compared Mn iii 1174.809,1177.478,1179.851,1183.308,1183.863,1183.880
the effective temperatures derived from optical and (Inter- Fe ii/iii many weak lines, individually recognisable 1140-1152
national Ultraviolet Explorer ) ultraviolet spectroscopy. We Ni ii 1317.217,1335.201,1370.123,1381.286,1393.324,1411.065
carried out a range of test calculations to explore the effect
of these systematic uncertainties in Teff and log g on the
derived metal abundances (Sect. 3.2). The abundances and
mass fluxes do not change by more than ≃ 0.1 dex, which is GALEX 1931+0117 has the richest absorption spectrum, in
less than the typical uncertainty of our fits, and the abun- which we securely identified transitions of nine elements (C,
dance ratios vary by much less. Hence, the discussion in O, Al, Si, P, S, Cr, Fe, Ni), and we included those metals in
Sect. 5 and 6 is not affected by the systematic uncertainties the abundance analysis of all four targets. We also include in
in Teff and log g. the analysis N, Na, Ti, V, Mn, which have moderately strong
Finally, to assess the possible effect that the presence of transitions in the wavelength range covered by the COS ob-
metals has on the effective temperature and surface gravity, servations, but that were not detected. All metals were fully
we computed a small grid of models for the two most metal- included in the calculation of the equation of state.
polluted stars (PG 0843+516, GALEX 1931+0117), includ- Synthetic spectra were calculated adopting the atmo-
ing metals at the abundances determined in Sect. 3.2, and spheric parameters determined in Sect. 3.1, and includ-
re-fitted the HST /COS spectra. For both stars, the best-fit ing approximately 2500 metal lines. The basic source of
Teff and log g did not change significantly, and we there- atomic line data (wavelengths, excitation energies, transi-
fore adopted the atmospheric parameters from the pure- tion probabilities log gf, Stark broadening constant Γ4 ) was
hydrogen fits for all four targets. VALD (Vienna Atomic Line Database), which is described
in Piskunov et al. (1995), Ryabchikova et al. (1997), and
Kupka et al. (1999, 2000). The ion Si ii has a large number
3.2 Metal abundances
of lines in the ultraviolet, and we noted a significant scatter
The COS spectra of the four white dwarfs contain a in the abundances derived from different lines. Replacing the
multitude of absorption lines from a range of elements. log gf values from VALD values with those from the NIST
c 2006 RAS, MNRAS 000, 1–16
7. The chemical diversity of exo-terrestrial planetary debris around white dwarfs 7
(National Institute of Standards) database, which differ for SDSS 1228+1040 and GALEX 1931+0117, for the latter also
some lines by up to 0.3 dex, leads to more consistent results. the oxygen abundances show this difference. There are at
Nevertheless, the situation for this ion is not satisfactory least three possible explanations:
(Sect. 3.2.2), and we have consulted a number of original Uncertain atomic data. This is a perennial prob-
sources in the literature (Lanz & Artru 1985; Nahar 1998; lem, as there are many, and large differences in various
Bautista et al. 2009) during the compilation of the most re- compilations of atomic data. The O i resonance lines in
liable atomic data. GALEX 1931+0117 are perturbed by airglow, interstellar
The abundances were varied until a satisfactory fit, as absorption and overlapping Si ii lines (see above), and the
judged by visual inspection, was achieved for each element. ultraviolet abundance determination rests largely on one ex-
We then changed the abundances in several steps of 0.1 - 0.2 ˚
cited line at 1152.1 A. Similarly, the optical O abundance
dex, until the fit was clearly worse. The resulting difference is measured only from the O i 7777 ˚ triplet (Vennes et al.
A
was used as a conservative estimate for the abundance error, 2010; Melis et al. 2011). However, our abundance measure-
or for an upper limit if no line was identified. Table 2 lists the ments for Si use many lines in the ultraviolet. In the recent
lines used in this procedure, although not all lines could be compilation by Bautista et al. (2009) the authors combined
used for all four stars. The best-fit models to the COS obser- several different computational methods, previous theoret-
vations are illustrated in Figs. 3 and 4, and the metal abun- ical calculations by other authors, and experimental data
dances of the four white dwarfs are given in Table 3 (along into a “recommended” value for log gf. These values agree
with the previous abundance studies were carried out for fairly well with the ultraviolet data from NIST that we have
GALEX 1931+0117, Vennes et al. 2011b; Melis et al. 2011). used. However, for the five optical lines they consider, the
Notably, upper limits for N were always larger than solar values are 0.25−0.30 dex smaller, though with errors as large
relative to C. For Na, Ti, V, Mn (and additionally Ca in as 0.3 dex. Using these values would increase the abundance
PG 0843+516 as well as Ca, Al, P, S, Ni in PG 1015+161) determined from optical spectra, contrary to what would be
the upper limits were larger than solar relative to Si. We needed for a more consistent solution. In addition, in a re-
have used these (solar) values in the models, but it did not cent analysis of ultraviolet spectra for the DBZ star GD 40,
change the atmosphere structure and the results for the de- Jura et al. (2012) find a discrepancy between optical and
tected elements. ultraviolet abundances for Si of the same size, but in oppo-
site direction - the abundances are smaller for the optical
determinations. Since that study used the same models and
3.2.1 Interstellar line absorption and airglow atomic data as the one presented here, there is no indication
that the atomic data are behind this discrepancy.
In all objects interstellar absorption is visible in the reso-
Abundance stratification. Contrary to DB stars like
nance lines of C ii, N i, O i, Si ii, and S ii. In SDSS 1228+1040,
GD 40 at similar temperatures, there are no convection zones
PG 1015+161, and GALEX 1931+0117 the interstellar ab-
in the atmospheres and envelopes of our four objects, which
sorption lines are shifted blue-wards with respect to the
would act as a homogeneously mixed reservoir in the accre-
photospheric lines by velocities of v = 57, 36, and
tion/diffusion scenario. Assuming a steady state between the
61 km s−1 , respectively. In PG 0843+516, |v| < 7 km s−1 ,
two processes, we thus expect a stratified abundance config-
and the interstellar lines are not fully separated from
uration. Whether this can explain the observations will be
the photospheric features. However, the presence of some
studied in Sect. 3.2.3.
interstellar absorption is obvious from the line ratio of
Genuine variation of the accretion rates. As will also
C ii 1334.5 ˚/C ii 1335.7 ˚ (Fig. 3 & 4). Because the lat-
A A
be discussed in the next section, the time scales for dif-
ter line originates from a level only 0.008 eV above the real
fusion in these atmospheres are of the order of days. If
ground state, it is equally populated in a stellar photosphere,
the accretion rate is not constant the observed abundances
but not in the interstellar medium, where the blue compo-
may change on the same short time scales. Given that the
nent is much stronger in spite of a lower transition probabil-
COS and ground-based observations that we analysed were
ity. Nevertheless, the abundances of C, O, Si, and S are ro-
taken months to years apart, such variations can not be ex-
bust, as a sufficient number of excited transitions are present
cluded. Noticeable variations of the Ca ii equivalent widths
in the photospheric spectrum (Table 2).
in the debris disc white dwarf G29-38 were reported by
The COS pipeline does not correct for airglow emission.
von Hippel & Thompson (2007). However, a similar study
Therefore, the reduced COS spectra can contain geocoronal
on the same star by Debes & L´pez-Morales (2008) did not
o
lines of O i 1302, 1305, 1306 ˚ whose intensity, and, to a
A
find any variations in the line strengths. Thus, the cur-
lesser extent, profile shape, vary as a function of HST ’s or-
rent evidence for accretion rate variations on time scales
bital day/night, and weakly with the Earth-limb angle. Air-
of months to years is ambiguous, and a second-epoch COS
glow is clearly seen in the spectrum of GALEX 1931+0117
observations of the stars studied here would be desirable.
(Fig. 4, right panel), which affects the fit to the photospheric
We also noticed an unidentified absorption feature be-
O i and Si ii lines in this region. For Si, this is a minor prob-
tween 1400 and 1410 ˚, with a strength roughly correlated
A
lem as there are many additional lines of Si ii-iv. For O,
another strong line in the COS spectra is O i 1152 ˚. A with the Si abundances. Such a feature has been discussed
in the literature and related to an autoionisation line of Si ii
or to a resonance feature in the photoionisation cross sec-
tion (Artru & Lanz 1987; Lanz et al. 1996). We have tested
3.2.2 Silicon
such a hypothetical line with their data for the oscillator
We notice relatively large differences of the silicon abun- strength and line width data. However, the width (≈ 80 ˚) A
dance determined from optical versus ultraviolet spectra in is much too broad to lead to visible features in the spectrum.
c 2006 RAS, MNRAS 000, 1–16
8. 8 B.T. G¨nsicke et al.
a
Table 3. Element number abundances log[Z/H] and limits determined from the analysis of our HST /COS and optical spectra. For Si,
we separately report the abundances determined from the optical data alone (“opt”, see Sect. 3.2.2), and for Ca and Mg, we report both
the abundances from a homogenous and a stratified atmosphere (“strat”, see Sect. 3.2.3), the latter ones are preferred as the strongest
lines for both elements are observed are in the optical. For GALEX 1931+0117, we also list the results of Vennes et al. (2011b) and
Melis et al. (2011).
Element PG 0843+516 PG 1015+161 SDSS 1228+1040 ........ GALEX 1931+0117 ........
........................... this paper ........................... Vennes et al. Melis et al.
C −7.30 ± 0.30 < −8.00 −7.50 ± 0.20 −6.80 ± 0.30 < −4.15 < −4.85
O −5.00 ± 0.30 −5.50 ± 0.20 −4.55 ± 0.20 −4.10 ± 0.30 −3.62 ± 0.05 −3.68 ± 0.10
Mg −4.90 ± 0.20 −5.30 ± 0.20 −5.10 ± 0.20 −4.42 ± 0.06 −4.10 ± 0.10
Mg (strat) −5.00 ± 0.20 −5.30 ± 0.20 −5.20 ± 0.20
Al −6.50 ± 0.20 −5.75 ± 0.20 −6.20 ± 0.20
Si −5.20 ± 0.20 −6.40 ± 0.20 −5.20 ± 0.20 −4.75 ± 0.20
Si (opt) −4.70 ± 0.20 −4.24 ± 0.07 −4.35 ± 0.11
P −6.60 ± 0.20 < −7.30 −7.00 ± 0.30
S −5.50 ± 0.30 < −6.20 −6.60 ± 0.20
Ca −6.30 ± 0.20 −5.70 ± 0.20 −6.11 ± 0.04 −5.83 ± 0.10
Ca (strat) −6.45 ± 0.20 −5.94 ± 0.20
Cr −5.80 ± 0.30 < −5.80 < −6.00 −6.10 ± 0.30 −5.92 ± 0.14
Mn −6.26 ± 0.15
Fe −4.60 ± 0.20 −5.50 ± 0.30 −5.20 ± 0.30 −4.50 ± 0.30 −4.43 ± 0.09 −4.10 ± 0.10
Ni −6.30 ± 0.30 < −6.50 −6.70 ± 0.30 < −5.60
We have also included the Si ii photoionisation cross sec- the “standard” layer, assuming that no trace of any heavy
tions from the Opacity Project (Seaton et al. 1994), which element below this would be seen in a spectrum.
indeed show a resonance maximum in this spectral region. However, a more consistent way to determine the abun-
But again, the Si abundance is too small to let this feature dances in the accreted material, which is the quantity ulti-
show up in the spectrum. mately desired, is the assumption of a steady state between
Our model uses the six Si ii lines at 1403.8, 1404.2, accretion and diffusion throughout the whole atmosphere.
1404.5, 1409.1, 1409.9, and 1410.2 ˚ in this range (Table 2).
A At Rosseland optical depth τ = 2/3, and typical conditions
The first two have the source “guess” in VALD, the first for the observed ultraviolet spectra, the diffusion times in
three have no entry in NIST, and the log gf values of the the four white dwarfs analysed here are ≃ 0.4 to four days.
strongest line (1410.2 ˚) differ by ≈ 0.8 dex between the two
A Assuming that the accretion rate does not vary over such
databases. The upper levels of the transitions have a par- time scales, we can use the condition of constant flow of an
ent configuration belonging to the second ionisation limit of element with mass fraction X(τ )
Si ii. They are still ≈ 0.7 eV below the first ionisation limit
and thus not strictly auto-ionising. However, the broaden- ρXv = const (1)
ing may well be underestimated by our simple approxima-
tion formulae. In summary, the atomic data of the lines in
with ρ and v the mass density and the diffusion velocity
the region are very uncertain and may be the explanation
of this element. ρ and v are known from the atmosphere
for the broad feature. However, with the present data we
model and diffusion calculations, and X(τ = 2/3) is derived
cannot prove that hypothesis.
from the spectral analysis. This determines the diffusion flux
Finally, we note that the Si iv 1394,1403 ˚ doublet in
A
at τ = 2/3. In steady state, as it is the case for the DAZ
PG 0843+516 is very poorly fit by our atmosphere model
analysed here, the diffusion flux is constant throughout the
˚
(Fig. 3). A weaker additional Si iv 1394,1403 A absorption is
atmosphere, and is equal to the accretion rate polluting the
also seen in the spectrum of SDSS 1228+1040 (Fig. 4). We
atmosphere. The constant diffusion flux then in turn allows
interpret this as evidence for absorption by hot gas close to
the determination of the abundance stratification X(τ ) (see
the white dwarf, see the discussion in Sect. 7.
also Vennes et al. 2011b, for a thorough discussion).
We calculated new stratified models and synthetic spec-
tra for all objects, using the steady state condition and the
abundances (at τ = 2/3) from Table 3. The resulting spec-
3.2.3 Diffusion and stratified atmosphere models
tra are almost indistinguishable from those of the homoge-
In the absence of a convection zone there is no deep homoge- neous atmospheres; the only exception are small increases of
nous reservoir in our DAZ sample, and therefore there is no the optical Mg ii and Ca ii line strengths. The small change
straightforward definition of diffusion time scales. Adopting can easily be explained by the structure of the stratified
the usual definition, i.e. dividing the mass of some element atmosphere. In these models ρ v increases with depth, and
above a layer in the envelope or atmosphere of the star by the consequently the abundance decreases. On the other hand a
diffusion flux, results in diffusion time scales that strongly monochromatic optical depth of ≈ 2/3 is reached in the ul-
depend on the chosen layer. Koester & Wilken (2006) and traviolet near Rosseland optical depth of τRoss ≃ 2/3, while
Koester (2009) defined the Rosseland optical depth τ = 5 as it is reached at τRoss ≈ 0.15 for λ = 4480 ˚, i.e. higher
A
c 2006 RAS, MNRAS 000, 1–16
9. The chemical diversity of exo-terrestrial planetary debris around white dwarfs 9
Table 4. Diffusion fluxes ρXv [g s−1 ] within the white dwarf atmospheres, which are equal to the rates at which planetary debris
material is accreted. Σ gives the sum of the accretions rates of all detected elements (i.e. not including those with upper limits). For
GALEX 1931+0117, we adopt in our calculation the Mg and Ca abundances of Vennes et al. (2011b) and the Mn abundance of Melis et al.
(2011) .
Element PG 0843+516 PG 1015+161 SDSS 1228+1040 GALEX 1931+0117
C 1.66 × 105 4.65 × 104 1.25 × 105 4.57 × 105
O 9.27 × 107 3.78 × 107 2.70 × 108 5.61 × 108
Mg 4.47 × 107 2.66 × 107 3.21 × 107 1.47 × 108
Al 2.09 × 106 1.18 × 107 3.08 × 106
Si 4.77 × 107 3.64 × 106 4.80 × 107 9.93 × 107
P 2.44 × 106 < 5.24 × 105 7.57 × 105
S 3.92 × 107 < 9.46 × 106 2.64 × 106
Ca 4.84 × 106 1.57 × 107 8.10 × 106
Cr 3.81 × 107 < 3.85 × 107 < 2.29 × 107 1.37 × 107
Mn 1.06 × 107
Fe 7.11 × 108 9.50 × 107 1.72 × 108 6.45 × 108
Ni 1.66 × 107 < 9.98 × 106 4.71 × 106
Σ 1.02 × 109 1.68 × 108 5.61 × 108 1.50 × 109
in the atmosphere, where the abundance is correspondingly the ionisation stages. This procedure was already used in
higher. Koester & Wilken (2006) and Koester (2009) for the models
For PG 0843+516, PG 1015+161, and SDSS 1228+1040, without or with only a shallow convection zone, although not
the Ca and Mg abundances were obtained from the optical explicitly stated in those papers. New in our present calcu-
data (Sect. 2.2) and our models. We have iterated them by lation is the consideration of neutral particles, following the
fitting to stratified models (denoted with “strat” in Table 3). discussion and methods outlined in Vennes et al. (2011a).
For GALEX 1931+0117, we adopted the photospheric Mg an The main results of our calculations are the diffusion
Ca abundances of Vennes et al. (2011b) and the Mn abun- fluxes, Xρv, for each element, which are assumed (in steady
dance of Melis et al. (2011) to calculate the corresponding state) to be the abundances of the accreted matter. These
diffusion fluxes. are summarised for the four objects in Table 4. The total
As a result we have to conclude that diffusion and a diffusion fluxes ( = accretion rates) are obtained by multi-
2
stratified abundance structure lead only to minor adjust- plying these fluxes with 4πRwd , where we used the cooling
ments of the abundances that cannot explain the large dis- tracks of Wood (1995) to obtain the white dwarf radii from
crepancy between optical and ultraviolet determinations for Teff and log g. The mass fluxes ( = accretion rates) of the in-
silicon. There is, however, an important caveat to this con- dividual elements, as well as their sum, are shown in Fig. 5
clusion. Our diffusion calculations use only the surface grav- and discussed in Sect. 6. The number abundances of the
ity (and as a minor effect the temperature gradient for circumstellar debris are then calculated from the diffusion
thermal diffusion) as driving force. Chayer & Dupuis (2010) fluxes via
have recently demonstrated that for silicon, radiative levi- N(X) ˙
M(X) A(Si)
tation can lead to a negative effective gravity and support = (2)
N(Si) ˙
M(Si) A(X)
the atoms in the outer layers of the atmosphere against dif-
fusion. They only published detailed data for a DAZ model where A is the atomic mass. The implications that these
with 20 000 K and log g = 8.00, and in their model only abundances have on the nature and origin of the circumstel-
abundances smaller than log [Si/H] = −8.0 are really sup- lar debris are discussed in detail in Sect. 5.
ported, because the lines saturate at higher abundances, ef-
fectively reducing the radiative support. However, it is quite
feasible that even if the atoms are not totally supported, the 4 NOTES ON INDIVIDUAL WHITE DWARFS
diffusion velocity would be smaller, changing the abundance
gradient. The answer to this puzzle will have to await sim- In the following sections, we give a brief overview of previous
ilar, detailed models for a variety of stellar parameters and work on the four white dwarfs that we have analysed, as well
heavy elements that can be tested against the large range as a summary of the key results of our observations.
of Si abundances found in our snapshot survey (G¨nsicke et
a
al. in prep).
4.1 PG 0843+516
Other points worth mentioning are that the determi-
nation of an effective ion charge with the simple pressure PG 0843+516 was identified as a DA white dwarf
ionisation description of Paquette et al. (1986) is not ap- in the Palomar-Green Survey (Green et al. 1986), and
propriate in the absence of deep convection. We have used Liebert et al. (2005) obtained Teff = 23 870 ± 392 K, log g =
the usual Saha equation (with a small lowering of the ion- 7.90 ± 0.05 from the analysis of a high-quality optical spec-
isation potential from non-ideal interactions) to determine trum. The best fit to our HST data was Teff = 23 095 ±
the abundances of different ions from an element. The dif- 230 K, log g = 8.17 ± 0.06. Our COS spectrum reveals
fusion velocity is then calculated as a weighted average of PG 0843+516 to be an extremely polluted DAZ white dwarf
c 2006 RAS, MNRAS 000, 1–16
10. 10 B.T. G¨nsicke et al.
a
Teff = 22 125 ± 136 K, log g = 8.22 ± 0.02 from a fit
to the SDSS spectrum. G¨nsicke et al. (2006) discovered
a
double-peaked emission lines of Ca ii 8498,8542,8662 ˚ as A
˚
well as weak Fe ii emission lines and Mg ii 4482 A absorp-
tion, and concluded that SDSS 1228+1040 accretes from a
volatile-depleted gaseous circumstellar disc. The Ca ii lines
form in a region extending in radius from a few tenths
R⊙ to ≃ 1.2 R⊙ , no emission is detected from closer in
to the white dwarf (but see Sect. 7). Spitzer observations
showed that SDSS 1228+1040 also exhibits an infrared ex-
cess (Brinkworth et al. 2009), and that there is a large ra-
dial overlap between the gaseous and dusty components of
the disc. Yet, the strong Ca ii emission lines require a gas
temperature of T ∼ 4000 − 6000 K (e.g. Hartmann et al.
2011), substantially exceeding the sublimation temperature
of the dust. This implies the thermal decoupling of the
gas and dust, most likely in the form of a complex verti-
Figure 5. Accretion rates of the elements detected in our four
cal temperature structure, with hotter, optically thin gas
targets. Their sum is given in the right-most column.
on top cooler, probably optically thick dust (Kinnear 2011;
Melis et al. 2010). Irradiation from the white dwarf is suf-
(Fig. 1 & 3), with an accretion rate of ≃ 109 g s−1 , placing ficient to explain this temperature inversion (Kinnear 2011;
it head-to-head with GALEX 1931+0117 (Sect. 6). We iden- Melis et al. 2010), but the origin of the gas found at radii
tified in the COS spectrum photospheric absorption lines larger than the sublimation radius is unclear, and may be
of C, O, Al, Si, P, S, Fe, Cr, and Ni, plus Mg in the op- related to relatively fresh disruption events (G¨nsicke et al.
a
tical WHT spectrum, the second largest set of elements 2008; Melis et al. 2010) or the intrinsic evolution of the de-
detected in a DAZ white dwarf. The fact that the metal bris disc (Bochkarev & Rafikov 2011; Metzger et al. 2012).
pollution of PG 0843+516 went unnoticed in the published Among the four white dwarfs studied here, SDSS 1228+1040
high-quality intermediate resolution spectroscopy underlines is the only one that exhibits emission lines from a gaseous
the strength of our ultraviolet survey for young and rel- disc.
atively warm white dwarfs accreting planetary debris. We The COS spectrum of SDSS 1228+1040 contains ab-
note that Xu & Jura (2012) recently detected infrared flux sorption lines of C, O, Al, Si, Cr, and Ni. SDSS 1228+1040
excess at PG 0843+516 in an analysis of archival Spitzer was observed outside the snapshot program described in
data, making this the second white dwarf (after G29-38, Sect. 2.1, and our COS spectroscopy extends up to 1790 ˚, A
Zuckerman & Becklin 1987; Koester et al. 1997) where cir- i.e. 360 ˚ further than that obtained for the other three
A
cumstellar dust was found without prior knowledge of pho- white dwarfs. This extended wavelength range includes ad-
tospheric metal pollution. ditional strong lines of Si ii, Al ii, and Al iii, but no further
elements. Our high-quality average UVES spectrum is used
to determine the abundances of Mg and Ca, bringing the
4.2 PG 1015+161 total number of detected elements in SDSS 1228+1040 to
eight.
PG 1015+161 is another DA white dwarf discovered in the We fitted the SDSS spectrum, finding Teff = 22 410 ±
Palomar-Green Survey (Green et al. 1986). Liebert et al. 175 K, log g = 8.12 ± 0.03, whereas a fit to the ultraviolet
(2005) determined Teff = 19 540 ± 305 K, log g = 8.04 ± 0.05 spectrum gives Teff = 20 565 ± 82 K, log g = 8.19 ± 0.03.
from optical spectroscopy. High-resolution spectroscopy of This discrepancy underlines that, for high-quality data,
PG 1015+161 was obtained as part of the SPY project the uncertainties are dominated by systematic rather than
(Napiwotzki et al. 2001), from which Koester et al. (2009) statistical errors. As a compromise we take the weighted
measured Teff = 19 948 ± 33 K and log g = 7.925 ± 0.006. mean of the latter two results with increased errors, Teff =
Our fit to the HST spectrum gives in Teff = 19 200 ± 180 K, 20 900 ± 900 K, log g = 8.15 ± 0.04.
log g = 8.22 ± 0.06. Koester et al. (2005) detected of a pho-
tospheric Ca ii K absorption line in the SPY data, with
a number abundance log [Ca/H] = −6.3, which triggered
follow-up observations with Spitzer that revealed the pres-
ence of circumstellar dust (Jura et al. 2007). The COS spec- 4.4 GALEX 1931+0117
trum contains absorption lines of O, Si, and Fe. In addition
As part of a spectroscopic identification program of
˚
to Ca ii K, we detected Mg ii 4482 A in the SPY spectrum.
ultraviolet-excess objects Vennes et al. (2010) recently iden-
PG 1015+161 has the lowest accretion rate among the four
tified GALEX 1931+0117 as a nearby (≃ 55 pc) DAZ white
stars discussed in this paper.
dwarf. Vennes et al. (2010) and Melis et al. (2011) analysed
optical spectroscopy, and obtained Teff = 20 890 ± 120 K,
log g = 7.90 +0.03 and Teff = 23 470 ± 300 K, log g =
4.3 SDSS 1228+1040 −0.06
7.99 ± 0.05, respectively. Our best-fit parameters from the
Eisenstein et al. (2006) identified this DA white dwarf in HST /COS spectrum are Teff = 21 200 ± 50 K, log g =
Data Release 4 of the Sloan Digital Sky Survey, and found 7.91 ± 0.02, consistent with that of Vennes et al. (2010)
c 2006 RAS, MNRAS 000, 1–16
11. The chemical diversity of exo-terrestrial planetary debris around white dwarfs 11
but somewhat lower than that of Melis et al. (2011)1 . The model. While the C abundance of the bulk Earth is subject
VLT/UVES spectroscopy obtained by Vennes et al. (2010, to some model-dependent assumptions (see the left panel
2011b) revealed strong metal lines of O, Mg, Si, Ca, and of Fig. 6 for an alternative chemical model of the Earth by
Fe, indicating ongoing accretion. Vennes et al. (2010) also All`gre et al. 2001), these uncertainties are comparable to
e
showed that the 2MASS H- and K-band fluxes exceeded the errors in our abundance determinations.
those expected from the white dwarf, and suggested a close For comparison, we include in Fig. 6 the abundance ra-
brown dwarf or a dusty debris disc as origin of the accret- tios of three white dwarfs that accrete from the wind of a
ing material. Debes et al. (2011) ruled out the presence of close M-dwarf companion, that were also observed as part of
a sub-stellar companion based on the infrared fluxes de- our COS snapshot programme2 . The only elements detected
tected by WISE, and argued that the white dwarf accretes in the COS spectra of these three stars are C, O, Si and S,
from a dusty disc. This was independently confirmed by and they exhibit high abundances in C and S, as expected
VLT/ISAAC near-IR observations obtained by Melis et al. for the accretion of solar-like material. The extremely low
(2011), who also measured abundances for Cr and Mn. abundances of the volatile C found for the debris around
Our HST /COS spectroscopy provides independent the four white dwarfs strongly underlines its rocky nature.
measurements for O, Si, Cr, and Fe, as well as the first de- This corroborates the previous studies of Jura (2006) and
tection of C, Al, P, S, and Ni, bringing the total number of Jura et al. (2012), who found strong evidence for substan-
elements observed in the photosphere of GALEX 1931+0117 tial depletion of C around three DB white dwarfs.
to 11 (Table 3). As discussed in Sect. 3.2, the O, Si, Cr, and However, Fig. 6 also shows that there is a significant
Fe abundances that we derive from the COS spectroscopy scatter among the individual abundances for a given ele-
are lower than those determined by Vennes et al. (2011b) ment. Among the four targets, the abundances of the debris
and Melis et al. (2011). However, the discussion of the na- in SDSS 1228+1040 most closely resembles those of the bulk
ture of the planetary material is usually based on rela- Earth. PG 1015+161 stands out by having all detected ele-
tive metal-to-metal abundance ratios (Nittler et al. 2004), ments over-abundant with respect to Si, when compared to
which are more robust than absolute abundances measure- the bulk Earth. An interesting trend is seen in PG 0843+516,
ments. Figure 6 compares the metal abundances determined where Fe, Ni, and S are significantly over-abundant, and, in
for GALEX 1931+0117 normalised with respect to Si, and fact, broadly consistent with the abundance ratios of the
relative to the corresponding ratios for the chemical compo- core Earth model. In particular, the volatile S is extremely
sition of the bulk Earth. It is evident that our metal-to-Si ra- overabundant with respect to C, compared to the bulk sili-
tios are consistent with those of Melis et al. (2011), whereas cate Earth. In melts, S will form FeS, and hence be depleted
the Mg/Si, Fe/Si, and Ca/Si ratios of Vennes et al. (2011b) from remaining minerals. The affinity of S to Fe is thought
are systematically lower. to be the reason for the depletion of S in the silicate mantle
of the Earth, as it will have settled into the Earth’s core
in the form of iron sulfide (Ahrens 1979; Dreibus & Palme
5 THE NATURE AND ORIGIN OF THE 1996). Similarly, also Cr is significantly over-abundant in
CIRCUMSTELLAR MATERIAL PG 0843+516 with respect to the bulk Earth. While Cr is
a moderately volatile element, the depletion of Cr in the
The four white dwarfs studied here have diffusion time scales silicate Earth is thought to be due to partitioning into the
of a few days (Sect. 3.2.3), and we can therefore safely as- Earth’s core (Moynier et al. 2011). Finally, the refractory
sume that we observe them in accretion-diffusion equilib- lithophile Al is under-abundant compared to the silicate
rium. In other words, the abundances of the circumstellar Earth. Thus, the abundance pattern seen in PG 0843+516
debris can be determined from the photospheric analysis suggests that the planetary debris is rich in material that
without any additional assumptions regarding the history has undergone at least partial melting, and possibly differ-
of the accretion rate that are necessary for stars with very entiation. A possible test of this hypothesis would be a mea-
long diffusion time scales (e.g. Klein et al. 2011). In what surement of the abundance of Zn, a lithophile element with
follows, we discuss the abundances of the circumstellar de- a similar volatility as S that is not depleted into iron melt
bris normalised to Si, the main rock-forming element, as is (Lodders 2003), and it will be important to test whether the
common use for solar-system objects (e.g. Lodders & Fegley refractory lithophile Ca is depleted at a similar level as Al.
2011). The most promising feature to measure the Zn abundances is
Figure 6 (right panel) illustrates the metal-to-Si ratios the Zn ii 2026,2062 ˚ resonance doublet, and Ca ii K should
A
of the planetary debris around the four white dwarfs rela- be easily detectable in high-resolution optical spectroscopy.
tive to the same abundances of the bulk Earth model by To further explore the chemical diversity of the plan-
McDonough (2000). The first striking observation is that etary debris around the four white dwarfs studied here,
the C/Si ratios of all four stars (including one upper limit) we compare pairwise a range of metal-to-Si abundance ra-
are much lower than that of CI chondrites, and in fact agree tios with those of the bulk Earth and bulk silicate Earth
within their errors with the C/Si value of the bulk Earth (McDonough 2000), as well as with those of a variety of me-
teorites (taken from Nittler et al. 2004). We inspect first the
1 Melis et al. (2011) discuss the discrepancy between their model
and the GALEX fluxes. From their Table 1, it appears that
they did not correct for the non-linearity of the GALEX detec- 2 A more detailed discussion of these binaries will be published
tors for bright targets. The corrected GALEX magnitudes given elsewhere. Here, they merely serve as “abundance standard white
by Vennes et al. (2010) are in good agreement with our best-fit dwarfs” which accrete material with abundance ratios that are
model. expected to be close to solar, i.e. rich in volatiles.
c 2006 RAS, MNRAS 000, 1–16
12. 12 B.T. G¨nsicke et al.
a
Figure 6. Heavy element abundances derived for the circumstellar debris at the four white dwarf targets (see Table 4), relative to Si,
and normalised to the same ratios of the bulk Earth (McDonough 2000). The elements are arranged, left to right, in order of increasing
sublimation temperature (Lodders 2003). Left panel: abundances for the debris around GALEX 1931+0117 (green: this paper, blue:
Melis et al. 2011, magenta: Vennes et al. 2011b). Also shown are the abundance ratios for the core Earth (which makes up ∼ 1/3 of
the Earth’s mass, open triangles) and the silicate Earth (i.e crust and mantle, which make up ∼ 2/3 of the Earth’s mass, open circles).
The bulk Earth composition of All`gre et al. (1995) is shown as solid black squares, illustrating the level of uncertainty in the (model-
e
dependent) composition of the Earth. The short-dashed line shows the abundance ratios of CI chondrites, the long-dashed line those
corresponding to solar abundances (both from Lodders 2003). Right panel: Metal-to-Si ratios for PG 0843+516 (blue), PG 1015+161
(magenta), SDSS 1228+1040 (red), and GALEX 1931+0117 (green). Shown in orange are the abundance ratios of three white dwarfs
that accrete from the wind of a close M-dwarf companion. As expected for the accretion of material with near-solar abundances, the
volatiles C and S are found to be strongly enhanced compared to the four white dwarfs that host debris discs of exo-terrestrial material.
relative abundances of Al and Ca, which are two of the three ferent stars. We note that the debris at PG 0843+516 falls
most abundant refractory lithophile elements (the third one close to the abundance ratios of Pallasites, a class of stony-
being Ti), i.e. elements that sublimate only at very high iron meteorites. This further supports our hypothesis that
temperatures, and that do not enter the core in the case of PG 0843+516 is accreting material in which iron has under-
differentiation. Therefore, the Al/Ca ratio is nearly constant gone (partial) melting.
across most classes of meteorites, and hence, the Al/Si values Another interesting pair of elements is C and O (Fig. 7,
determined from many solar-system bodies follows a linear lower left panel). The possible range of the C/O ratio
correlation with Ca/Si (Fig. 7, top right). Finding that the among exo-planets has been subject to intense discussion.
abundances for the debris discs, where Al, Ca, and Si are It is thought that for C/O > 0.8 in the proto-planetary
available, generally follow that trend is reassuring, as large discs, the ambient chemistry will favour solid “carbon plan-
variations in the relative Al and Ca abundances would cast ets”, that are dominated by carbides rather than oxides
doubts on the overall methodology using white dwarf pho- (Kuchner & Seager 2005). The possible existence of carbon
tospheres as proxies for the abundances of the circumstellar planets has gained some support by the recent report of a
material. C/O value exceeding unity in the atmosphere of the transit-
The relative abundances of O, Si, Mg, and Fe, which ing hot Jupiter WASP-12b (Madhusudhan et al. 2011), and
are the major constituents of the terrestrial planets in the by abundance studies that found a significant fraction of
solar system, show substantial variations between different exo-planet host stars having C/O > 0.8 (Petigura & Marcy
meteorite groups (Fig. 7, top left and bottom right panels), 2011; Delgado Mena et al. 2010), but see Fortney (2012) for
and at least as much scatter between the individual white a critical discussion.
dwarfs. The difficulty with these elements is that they form a Planetary debris at white dwarfs provides a unique op-
range of different minerals (metal oxides), depending on the portunity to probe the C/O ratio of exo-terrestrial ma-
prevailing pressure and temperature. Iron in particular may terial. However, measuring C abundances in white dwarfs
occur as pure metal, alloy, or mineral, and is subject to dif- is challenging, as the optical detection of carbon in cool
ferentiation into planetary cores. Oxygen, on the other hand, white dwarfs is usually related to dredge-up from the
can be be locked in a wide range of oxides (see the discus- core rather than external pollution (e.g. Dufour et al. 2005;
sion by Klein et al. 2010), or potentially water (Klein et al. Koester & Knist 2006; Desharnais et al. 2008). At higher
2010; Jura & Xu 2010; Farihi et al. 2011; Jura & Xu 2012). temperatures, where convective dredge-up can be excluded,
Therefore, the relative abundances of O, Si, Mg, and Fe will suitable lines of C are only found at ultraviolet wavelengths.
vary according to the processing that material underwent As mentioned above, the four stars studied here have very
(e.g. condensation, melting, and differentiation), and it is similar (low) C/Si ratios, but do show a range of O/Si ra-
maybe not too surprising to find that the debris around tios. Nevertheless, the debris around all four stars stud-
white dwarfs exhibits at substantial degree of diversity, as ies here, as well as GD 40 (Jura et al. 2012), have −3
it represents different planetary systems formed around dif- log(C/O) −2.3, very similar to the bulk silicate Earth,
c 2006 RAS, MNRAS 000, 1–16
13. The chemical diversity of exo-terrestrial planetary debris around white dwarfs 13
Figure 7. The chemical abundances of planetary debris, determined from the photospheric studies of polluted white dwarfs, reveal a large
degree of diversity. The four panels illustrate a range of metal-to-Si number abundance ratios for the four stars analysed in this paper,
compared to those of bulk Earth and bulk silicate Earth (BE and BSE; McDonough 2000), solar abundances and CI chondrites (S and CI;
Lodders 2003), and several meteorite classes (gray = carbonaceous Chondrites, green = Mesoderites, blue = Pallasites, red = Diogenites,
orange = Howardites, magenta = Eucrites; Nittler et al. 2004). Also shown, in light blue, are the abundance ratios for the polluted DB
white dwarfs GD 362 (Zuckerman et al. 2007), GD 40 (Jura et al. 2012), and HS 2253+8023 (Klein et al. 2011).
log(C/O) ≃ −2.5, and are hence representative of solar sys- 53 metal-polluted white dwarfs following the prescription of
tem minerals. Koester & Wilken (2006), but scaling the results by the typ-
ical gas-to-dust ratio in the interstellar medium to account
for the absence of H and He in the accreted debris, resulting
˙
in M ≃ 2 × 109 g s−1 for PG 1015+161.
6 ACCRETION RATES
The uncertainty in the estimated accretion rates can be
Estimating accretion rates for metal-polluted white dwarfs greatly reduced if photospheric abundances for the major
is notoriously difficult, as it is based on scaling from the constituents of the debris material can be measured. While
elements detected in the photosphere to an assumed bulk we do not detect all elements that are likely present in the
composition of the accreted material. In addition, in the case circumstellar debris at the four white dwarfs studied here,
of white dwarfs with significant convective envelope masses, we have determined the accretion rates of all the major el-
only the average accretion rate over the diffusion time scale ements, in particular O, Si, Mg, and Fe (Sect. 3.2.3). The
can be obtained. accretion rates of all detected elements, as well as their
Koester & Wilken (2006) calculated accretion rates for sum are given in Table 4, and are illustrated in Fig. 5.
38 DAZ white dwarfs based on the abundance of Ca, ˙
For PG 1015+161, we find M ≃ 1.7 × 108 g s−1 , which is
and adopting solar abundances for the accreting mate- strictly speaking a lower limit, however, the undetected el-
rial. For PG 1015+161, these assumptions implied M ≃ ˙ ements (e.g. Al, S, Ti, Mn, Cr) are unlikely to contribute
2 × 1011 g s−1 . Since then, it has become increasingly clear more than 10% of the total accretion rate. Similarly, we find
that many, if not most, metal-polluted (single) white dwarfs the accretion rates of PG 0843+516, SDSS 1228+1040, and
accrete volatile-depleted material from circumstellar plane- ˙
GALEX 1931+0117 to be M ≃ 1.0×109 g s−1 , 5.6×108 g s−1 ,
9 −1
tary debris. Farihi et al. (2009) estimated accretion rates for and 1.5 × 10 g s , respectively.
c 2006 RAS, MNRAS 000, 1–16
14. 14 B.T. G¨nsicke et al.
a
7 HOT CIRCUMSTELLAR GAS spheric fits match the observed Si iv lines well, i.e. there
is no evidence for any additional absorption component.
The discs around white dwarfs are passive, i.e. their emis- Given that these two stars have, respectively, the lowest
sion is solely due to the thermal reprocessing of inter- and highest accretion rate of our small sample (Sect. 6),
cepted stellar flux. The inner disc radius where typical there seems to be no clear correlation between the detec-
dust grains will rapidly sublimate is determined by the lu- tion of absorption from highly ionised gas to the mass flow
minosity of the white dwarf (von Hippel et al. 2007). The rate onto the white dwarf. A key difference between the two
gaseous material will viscously spread, both flowing in- stars where circumstellar Si iv absorption is detected is that
wards onto the white dwarf, and outwards over the dusty SDSS 1228+1040 also shows strong emission lines from cir-
disc, potentially accelerating the inwards migration of the cumstellar gas, which indicate a relatively high inclination of
dust via aerodynamic drag (Rafikov 2011). While gaseous the accretion disc. In contrast, no gaseous emission is found
material orbiting at radii coincident with circumstellar in PG 0843+516 (G¨nsicke et al. in prep). Identifying addi-
a
dust is observed in a number of systems in the form of tional absorption features from these hot layers of gas would
double-peaked emission lines (G¨nsicke et al. 2006, 2007,
a provide substantial constraints on the physical parameters
2008; Brinkworth et al. 2009, 2012; Melis et al. 2011, 2012; in the corresponding regions. The strongest line seen in cat-
Farihi et al. 2012; Dufour et al. 2012), there has yet been aclysmic variables, N v, is naturally absent in the white
no detection of gaseous material well inside the sublimation dwarfs accreting rocky debris3 , but the O vi 1032,1038 ˚ A
radius. doublet detected in U Gem (Long et al. 2006) is a promising
Inspection of Fig. 3 reveals that the strength of candidate.
the Si iv 1394,1403 ˚ doublet in PG 0843+516 is extremely
A
under-predicted by the photospheric model. These Si iv lines
correspond to the highest ionisation energy of all tran-
8 CONCLUSIONS
sitions detected in the COS spectrum. For the tempera-
ture and the Si abundance of PG 0843+516, the observed Recent years have seen a surge of interest in the evo-
strength of the Si iv lines is absolutely incompatible with lution of extra-solar planetary systems through the late
a purely photospheric origin. The most plausible explana- phases in the lifes of their host stars (e.g. Burleigh et al.
tion is that there is additional absorption along the line 2002; Debes & Sigurdsson 2002; Villaver & Livio 2007,
of sight, associated with hot gas close to the white dwarf 2009; Nordhaus et al. 2010; Di Stefano et al. 2010). While
that is optically thin except for the strong resonance lines no planet has yet been discovered orbiting a white dwarf
of high-ionisation species, such as Si iv. In fact, extremely (Hogan et al. 2009; Faedi et al. 2011), significant progress
similar features were found in the far-ultraviolet observa- has been made in the discovery and understanding of plan-
tions of cataclysmic variables, i.e. white dwarfs that ac- etary debris discs around white dwarfs.
crete from a (hydrogen-rich) accretion disc that is in turn Our COS study substantially increases the number of
fed by Roche-lobe overflow of a close M-dwarf companion. polluted white dwarfs for which a wide range of chemical
HST /GHRS and FUSE spectroscopy of the white dwarf in elements have been detected. We find that the C/Si ratio
U Gem contains very strong absorption of N v 1239,1243 A ˚ is consistent with that of the bulk Earth, which confirms
and O vi 1032,1038 ˚ that can not form in the ≃ 30 000 K
A the rocky nature of the debris at these white dwarfs, and
photosphere, as well as excess absorption in Si II 1394,1403 ˚
A their C/O values are typical of minerals dominated by Fe
(Sion et al. 1998; Long & Gilliland 1999; Long et al. 2006). and Mg silicates. There is so far no detection of planetary
All three high-ionisation doublets are red-shifted with re- debris at white dwarfs that has a large C/O ratio which
spect to the systemic velocity of the white dwarf, but some- would be indicative of silicon carbide-based minerals. The
what less so than the lower-ionisation photospheric lines, abundances of planetary material found around white dwarfs
which are subject to the gravitational redshift at the pho- show a large diversity, comparable to, or exceeding that seen
tospheric radius. These observations were interpreted as ev- among different meteorite classes in the solar system. We
idence for a hot (∼ 80 000 K) layer of gas sufficiently close find that the Al/Ca ratio follows a similar trend as observed
to the white dwarf to still experience a noticeable gravi- among solar system objects, which suggests that processing
tational redshift. Measuring the central wavelengths of the of proto- and post-planetary material follows similar under-
strong Si iv 1394, 1403 ˚ lines in PG 0843+516, we find that
A lying principles. A particularly interesting pattern is found
they are blue-shifted with respect to the photospheric fea- in PG 0843+516, where over-abundances of S, Cr, Fe, and Ni
tures by ≃ 25 km s−1 , which implies a height of ≃ 1.5 white are suggestive of the accretion of material that underwent
dwarf radii above the white dwarf surface. This assumes that melting and possibly differentiation. Extending the abun-
there is no significant flow velocity, which seems reasonably dance studies of metal-polluted white dwarfs both in de-
well justified given the symmetric shape of the Si iv profiles. tail and number will provide further insight into the diver-
A discrepancy between the best-fit white dwarf model sity of exo-terrestrial material, and guide the understand-
and the region around the Si iv doublet is also seen in ing of terrestrial exo-planet formation (Bond et al. 2010;
the COS spectrum of SDSS 1228+1040 (Fig. 4, bottom left Carter-Bond et al. 2012).
panel), however, in this star, the additional absorption is 3 For completeness, we note that circumstellar high-ionisation
rather weak. These additional absorption features are clearly
absorption lines have also been found around a number of hot
blue-shifted with respect to the photospheric lines, however,
white dwarfs (Bannister et al. 2003; Dickinson et al. 2012). How-
the relatively low signal-to-noise ratio of the spectrum pre- ever, the origin of the circumstellar material is not clear, and the
vents an accurate determination of this offset. detection of strong C lines suggests a different nature compared
For PG 1015+161 and GALEX 1931+0117, the photo- to the rocky debris found around the stars studied here.
c 2006 RAS, MNRAS 000, 1–16
15. The chemical diversity of exo-terrestrial planetary debris around white dwarfs 15
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a a
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Bond, J. C., Santos, N. C., Udry, S., Mayor, M., 2010,
teorite abundance data with us, and William Januszewski,
ApJ, 725, 2349
Charles Proffitt, and Elena Mason for their tireless efforts
Desharnais, S., Wesemael, F., Chayer, P., Kruk, J. W., Saf-
in the implementation of the HST program. D.K. wants to
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thank P.-E. Tremblay and P. Bergeron for sharing their new
Di Stefano, R., Howell, S. B., Kawaler, S. D., 2010, ApJ,
calculations of the hydrogen Lyman and Balmer line Stark
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Science Institute, which is operated by the Association of
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