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Minor Planet Evidence for Water in the Rocky Debris of a Disrupted Extrasolar MInor Planet

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  • 1. DOI: 10.1126/science.1239447 , 218 (2013);342Science et al.J. Farihi Minor Planet Evidence for Water in the Rocky Debris of a Disrupted Extrasolar This copy is for your personal, non-commercial use only. clicking here.colleagues, clients, or customers by , you can order high-quality copies for yourIf you wish to distribute this article to others here.following the guidelines can be obtained byPermission to republish or repurpose articles or portions of articles ):October 11, 2013www.sciencemag.org (this information is current as of The following resources related to this article are available online at http://www.sciencemag.org/content/342/6155/218.full.html version of this article at: including high-resolution figures, can be found in the onlineUpdated information and services, http://www.sciencemag.org/content/suppl/2013/10/09/342.6155.218.DC1.html can be found at:Supporting Online Material http://www.sciencemag.org/content/342/6155/218.full.html#ref-list-1 , 5 of which can be accessed free:cites 35 articlesThis article http://www.sciencemag.org/cgi/collection/astronomy Astronomy subject collections:This article appears in the following registered trademark of AAAS. is aScience2013 by the American Association for the Advancement of Science; all rights reserved. The title CopyrightAmerican Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by theScience onOctober11,2013www.sciencemag.orgDownloadedfromonOctober11,2013www.sciencemag.orgDownloadedfromonOctober11,2013www.sciencemag.orgDownloadedfromonOctober11,2013www.sciencemag.orgDownloadedfrom
  • 2. and morphometric measurements of the tissues in the developing mouse gut (Fig. 6C). Using these measurements as inputs in our model suf- fices to quantitatively predict the formation of villi (supplementary materials, Fig. 6D, and movie S3). Compared with the chick, where the endo- derm is more than 10 times stiffer than the ad- jacent mesenchyme, the mouse endoderm is only about 1.5 times as stiff as the mesenchyme (fig. S3). Our simulations show that the soft endoderm in mouse is essential for the initial folding that oc- curs in endoderm alone and for the direct formation of an array of previllous bumps, rather than zig- zags, which are qualitatively similar to sulcus for- mation on biaxially compressed gel surfaces that lack a stiff top layer (24). The spacing of bumps and, consequently, the spacing of villi are compa- rable to the thickness of the whole endoderm- mesenchyme composite (Fig. 6C), similar to chick. The process of villification occurs before the differentiation of the gut endoderm into various epithelial cell types (25–27) and well before the postnatal process of crypt formation. In vitro cul- ture of intestinal stem cells results in the forma- tion of intestinal organoids that reproduce crypt structure (28). These organoids consist of an inner epithelium with villuslike cell types and outwardly projecting cryptlike structures. How- ever, no morphological structures are present in these in vitro cultures resembling the physical villi. These results suggest that crypt formation likely does not require the same muscle-driven compression that is necessary for villi to form. Additionally, further study is needed to un- derstand whether structural differences in the lumen of different regions of the gut are attrib- utable to distinctions in the parameters we have measured. For example, the short, wide villi that coat large longitudinal folds of the chick colon may be attributable to the thicker muscle layers of the colon. Consistent with the muscle playing such a role, studies have shown that transposi- tion of a ring containing all radial layers of the colon into regions of the small intestine preserve villi morphology (29). Our previous work provided a mechanical basis for the diversity of macroscopic looping patterns of the gut based on geometry, differen- tial growth, and tissue mechanics (30), and our present results demonstrate that the same phys- ical principles drive morphological variation on the luminal surface of the gut. Further, we see that relatively minor changes in the geometry, growth, and physical properties of the develop- ing tissue in the guts of various species can substantially alter both the process and the form of villus patterning. A deep understanding of how patterns vary requires us to combine our knowl- edge of biophysical mechanisms with the genetic control of cell proliferation and growth; indeed this variation can occur in an organism as a func- tion of its diet, across species, and over evolu- tionary time scales via natural selection. References and Notes 1. V. A. McLin, S. J. Henning, M. Jamrich, Gastroenterology 136, 2074–2091 (2009). 2. T. K. Noah, B. Donahue, N. F. Shroyer, Exp. Cell Res. 317, 2702–2710 (2011). 3. W. J. Krause, Anat. Histol. Embryol. 40, 352–359 (2011). 4. J. W. McAvoy, K. E. Dixon, J. Anat. 125, 155–169 (1978). 5. S. Ferri, L. C. U. Junqueira, L. F. Medeiros, L. O. Mederios, J. Anat. 121, 291–301 (1976). 6. D. R. Burgess, Embryol Exp. Morph. 34, 723–740 (1975). 7. W. His, Anatomie Menschlicher Embryonen (Vogel, Leipzig, Germany, 1880). 8. D. E. Moulton, A. Goriely, J. Mech. Phys. Solids 59, 525–537 (2011). 9. L. Bell, L. Williams, Anat. Embryol. 165, 437–455 (1982). 10. M. Kurahashi et al., Neurogastroenterol. Motil. 20, 521–531 (2008). 11. K. Fukuda, Y. Tanigawa, G. Fujii, S. Yasugi, S. Hirohashi, Development 125, 3535–3542 (1998). 12. H. Benabdallah, D. Messaoudi, K. Gharzouli, Pharmacol. Res. 57, 132–141 (2008). 13. N. Harada, Y. Chijiiwa, T. Misawa, M. Yoshinaga, H. Nawata, Life Sci. 51, 1381–1387 (1992). 14. M. L. Lovett, C. M. Cannizzaro, G. Vunjak-Novakovic, D. L. Kaplan, Biomaterials 29, 4650–4657 (2008). 15. N. Bowden, S. Brittain, A. G. Evans, J. W. Hutchinson, G. W. Whitesides, Nature 393, 146–149 (1998). 16. L. Mahadevan, S. Rica, Science 307, 1740 (2005). 17. B. Audoly, A. Boudaoud, J. Mech. Phys. Solids 56, 2444–2458 (2008). 18. E. Hannezo, J. Prost, J.-F. Joanny, Phys. Rev. Lett. 107, 078104 (2011). 19. M. Ben Amar, F. Jia, Proc. Natl. Acad. Sci. U.S.A. 110, 10525–10530 (2013). 20. R. Sbarbati, J. Anat. 135, 477–499 (1982). 21. K. D. Walton et al., Proc. Natl. Acad. Sci. U.S.A. 109, 15817–15822 (2012). 22. A. Sukegawa et al., Development 127, 1971–1980 (2000). 23. M. Ramalho-Santos, D. A. Melton, A. P. McMahon, Development 127, 2763–2772 (2000). 24. T. Tallinen, J. S. Biggins, L. Mahadevan, Phys. Rev. Lett. 110, 024302 (2013). 25. M. Dauça et al., Int. J. Dev. Biol. 34, 205–218 (1990). 26. Z. Uni, A. Smirnov, D. Sklan, Poult. Sci. 82, 320–327 (2003). 27. F. T. Bellware, T. W. Betz, J. Embryol. Exp. Morphol. 24, 335–355 (1970). 28. T. Sato et al., Nature 459, 262–265 (2009). 29. W. H. St. Clair, C. A. Stahlberg, J. W. Osborne, Virchows Arch. B Cell Pathol. Incl. Mol. Pathol. 47, 27–33 (1984). 30. T. Savin et al., Nature 476, 57–62 (2011). Acknowledgments: We thank M. Kirschner for providing Xenopus tadpoles and O. Pourquie for providing snake embryos. D.L.K. and Tufts University hold a series of patents that cover the processing of silk into material structures, including those used in the research reported here. T.T. acknowledges the Academy of Finland for support. Computations were run at CSC–IT Center for Science, Finland. C.J.T. acknowledges the support of a grant from NIH RO1 HD047360. L.M. acknowledges the support of the MacArthur Foundation. Supplementary Materials www.sciencemag.org/content/342/6155/212/suppl/DC1 Materials and Methods Supplementary Text Figs. S1 to S11 Movies S1 to S3 8 April 2013; accepted 13 August 2013 Published online 29 August 2013; 10.1126/science.1238842 REPORTS Evidence for Water in the Rocky Debris of a Disrupted Extrasolar Minor Planet J. Farihi,1 * B. T. Gänsicke,2 D. Koester3 The existence of water in extrasolar planetary systems is of great interest because it constrains the potential for habitable planets and life. We have identified a circumstellar disk that resulted from the destruction of a water-rich and rocky extrasolar minor planet. The parent body formed and evolved around a star somewhat more massive than the Sun, and the debris now closely orbits the white dwarf remnant of the star. The stellar atmosphere is polluted with metals accreted from the disk, including oxygen in excess of that expected for oxide minerals, indicating that the parent body was originally composed of 26% water by mass. This finding demonstrates that water-bearing planetesimals exist around A- and F-type stars that end their lives as white dwarfs. T he enormous recent progress in the dis- covery of exoplanetary systems provides a growing understanding of their frequency and nature, but our knowledge is still limited in many respects. There is now observational evi- dence of rocky exoplanets (1, 2), and the mass and radius (and hence density) of these planets can be calculated from transit depth and radial velocity amplitude; however, estimates of their bulk composition remain degenerate and model- dependent. Transit spectroscopy offers some in- formation on giant exoplanet atmospheres (3), and planetesimal debris disks often reveal the signa- ture of emitting dust and gas species (4), yet both techniques only scratch the surface of planets, as- teroids, and comets. Interestingly, white dwarfs— the Earth-sized embers of stars like the Sun—offer a unique window onto terrestrial exoplanetary sys- tems: These stellar remnants can distill entire 1 Institute of Astronomy, University of Cambridge, Cambridge CB3 0HA, UK. 2 Department of Physics, University of Warwick, Coventry CV5 7AL, UK. 3 Institut für Theoretische Physik und Astrophysik, University of Kiel, 24098 Kiel, Germany. *Corresponding author. E-mail: jfarihi@ast.cam.ac.uk 11 OCTOBER 2013 VOL 342 SCIENCE www.sciencemag.org218
  • 3. planetesimals into their constituent elements, thus providing the bulk chemical composition for the building blocks of solid exoplanets. Owing to high surface gravities, any atmo- spheric heavy elements sink rapidly as white dwarfs cool below 25,000 K (5), leaving be- hind only hydrogen and helium in their outer- most layers—a prediction that is corroborated by observation (6). Those white dwarfs with rocky planetary system remnants can become con- taminated by the accretion of small, but spec- troscopically detectable, amounts of metals (7). Heavy element absorption lines in cool white dwarfs are a telltale of external pollution, often implying either ongoing mass accretion rates above 108 g s−1 (8) or large asteroid-sized masses of metals within the convection zone of the star (9). In recent years, metal-rich dust (10, 11) and gas (12) disks, likely produced by the tidal disruption of a large asteroid (13), have been observed to be closely orbiting 30 cool white dwarfs [e.g., (14–19)] and provide a ready explanation for the metal absorption features seen in their atmo- spheres (20). The circumstellar material being gradually accreted by the white dwarf can be directly observed in the stellar photosphere to reveal its elemental abundances (21). These plan- etary system remnants offer empirical insight into the assembly and chemistry of terrestrial exo- planets that is unavailable for any exoplanet or- biting a main-sequence star. Until now, no white dwarf has shown re- liable evidence for the accretion of water-rich, rocky planetary material. Unambiguous signa- tures of icy asteroids at white dwarfs should include (i) atmospheric metal pollution rich in refractory elements; (ii) trace oxygen in excess of that expected for metal oxides; (iii) circum- stellar debris from which these elements are ac- creted; and, where applicable, (iv) trace hydrogen (in a helium-dominated atmosphere) sufficient to account for the excess oxygen as H2O. The presence of a circumstellar disk signals that ac- cretion is ongoing, identifies the source material, and enables a confident quantitative assessment of the accreted elemental abundances, which in turn allows a calculation of the water fraction of the disrupted parent body. The metal-enriched white dwarfs GD 362 and GD 16 both have circumstellar disks and relatively large trace hydrogen abundances in helium- dominated atmospheres (22), but as yet no as- sessment of photospheric oxygen is available (21, 23). These two stars have effective temper- atures below 12,000 K, and their trace hydrogen could potentially be the result of helium dredge- up in a previously hydrogen-rich atmosphere (24). The warmer, metal-lined white dwarfs GD 61 and GD 378 have photospheric oxygen (25), but the accretion history of GD 378 is unconstrained (i.e., it does not have a detectable disk), and without this information, the atmospheric oxygen could be consistent with that contained in dry min- erals common in the inner solar system (26). In the case of GD 61, elemental abundance uncer- tainties have previously prevented a formally sig- nificant detection of oxygen excess (27). We used the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope to obtain ultraviolet spectroscopy of the white dwarf GD 61, and, together with supporting ground- based observations, we derived detections or lim- its for all the major rock-forming elements (O, Mg, Al, Si, Ca, Fe). These data permit a con- fident evaluation of the total oxygen fraction present in common silicates within the parent body of the infalling material, and we identified excess oxygen attributable to H2O as follows. (i) The observed carbon deficiency indicates that this element has no impact on the total oxygen budget, even if every atom is delivered as CO2. (ii) The elements Mg, Al, Si, and Ca are as- sumed to be carried as MgO, Al2O3, SiO2, and CaO at the measured or upper-limit abundance. (iii) The remaining oxygen exceeds that which can be bound in FeO, and the debris is interpreted to be water-rich. By this reasoning, we found oxy- gen in excess of that expected for anhydrous min- erals in the material at an H2O mass fraction of 0.26 (Table 1 and Fig. 1). Because we have assumed the maximum al- lowed FeO, and because some fraction of metal- lic iron is possible, the inferred water fraction of the debris is actually bound between 0.26 and 0.28. Although this makes little difference in the case of GD 61, where the parent body material appears distinctly mantle-like (27), there are at least two cases where metallic iron is a major (and even dominant) mass carrier within the parent bodies of circumstellar debris observed at white dwarfs (28). Overall, these data strongly suggest that the material observed in and around polluted white dwarfs had an origin in relatively massive and differentiated planetary bodies. We have assumed a steady state between ac- cretion and diffusion in GD 61. However, a typ- ical metal sinking time scale for this star is 105 years, and thus the infalling disk material could potentially be in an early phase of accretion where material accumulates in the outer layers, prior to appreciable sinking (27). In this early-phase scenario, the oxygen excess and water fraction would increase relative to those derived from the steady-state assumption, and hence we confi- dently conclude that the debris around GD 61 originated in a water-rich parent body. Although the lifetimes of disks at white dwarfs are not robustly constrained, the best estimates imply Table 1. Oxide and water mass fractions in the planetary debris at GD 61. We adopt the steady-state values, which assume accretion-diffusion equilibrium. Oxygen carrier Steady state Early phase CO2 <0.002 <0.002 MgO 0.17 0.18 Al2O3 <0.02 <0.02 SiO2 0.32 0.27 CaO 0.02 0.01 FeO* 0.05 0.02 Excess 0.42 0.50 H2O in debris 0.26 0.33 *All iron is assumed to be contained in FeO; some metallic Fe will modestly increase the excess oxygen. Fig. 1. Oxygen budget in GD 61 and terrestrial bodies. The first two columns are the early phase (EP) and steady-state (SS) fractions of oxygen carried by all the major rock-forming elements in GD 61, assuming that all iron is carried as FeO. Additional columns show the oxide compositions of the bulk silicate (crust plus mantle) Earth, Moon, Mars, and Vesta (35). Their totals do not reach 1.0 because trace oxides have been omitted. The overall chemistry of GD 61 is consistent with a body composed almost entirely of silicates, and thus appears relatively mantle-like but with substantial water. In contrast, Earth is relatively water-poor and contains approximately 0.023% H2O (1.4 × 1024 g). www.sciencemag.org SCIENCE VOL 342 11 OCTOBER 2013 219 REPORTS
  • 4. that the chance of catching GD 61 in an early phase is less than 1% (17, 29–31). The helium-rich nature of GD 61 permits an assessment of its trace hydrogen content and total asteroid mass for a single parent body. The total metal mass within the stellar convection zone is 1.3 × 1021 g, roughly equivalent to that of an asteroid 90 km in diameter. However, be- cause metals continuously sink, it is expected that the destroyed parent body was substantially more massive, unless the star is being observed shortly after the disruption event. In contrast, hy- drogen floats and accumulates, and thus places an upper limit on the total mass of accreted water- rich debris. If all the trace hydrogen were deliv- ered as H2O from a single planetesimal, the total accreted water mass would be 5.2 × 1022 g, and a 26% H2O mass fraction would imply a parent body mass of 2 × 1023 g, which is similar to that of the main-belt asteroid 4 Vesta (32). These data imply that water in planetesi- mals can survive post–main sequence evolution. One possibility is that solid or liquid water is retained beneath the surface of a sufficiently large (diameter >100 km) parent body (26), and is thus protected from heating and vaporization by the outermost layers. Upon shattering during a close approach with a white dwarf, any ex- posed water ice (and volatiles) should rapidly sublimate but will eventually fall onto the star; the feeble luminosities of white dwarfs are in- capable of removing even light gases by radia- tion pressure (31). Another possibility is that a substantial mass of water is contained in hydrated minerals (e.g., phyllosilicates), as observed in main- belt asteroids via spectroscopy and inferred from the analysis of meteorites (33). In this case, the H2O equivalent is not removed until much higher temperatures are attained, and such water-bearing asteroids may remain essentially unaffected by the giant phases of the host star. The white dwarf GD 61 contains the unmis- takable signature of a rocky minor planet anal- ogous to the asteroid 1 Ceres in water content (34) and probably analogous to Vesta in mass. The absence of detectable carbon indicates that the parent body of the circumstellar debris was not an icy planetesimal analogous to comets, but was instead similar in overall composition to asteroids in the outer main belt. This exoplan- etary system originated around an early A-type star that formed large planetesimals similar to those in the inner solar system that were the building blocks for Earth and other terrestrial planets. References and Notes 1. N. M. Batalha et al., Astrophys. J. 729, 27 (2011). 2. F. Fressin et al., Nature 482, 195–198 (2012). 3. D. K. Sing et al., Mon. Not. R. Astron. Soc. 416, 1443–1455 (2011). 4. C. M. Lisse et al., Astrophys. J. 747, 93 (2012). 5. D. Koester, Astron. Astrophys. 498, 517–525 (2009). 6. B. Zuckerman, D. Koester, I. N. Reid, M. Hünsch, Astrophys. J. 596, 477–495 (2003). 7. Astronomers use the term “metal” when referring to elements heavier than helium. 8. D. Koester, D. Wilken, Astron. Astrophys. 453, 1051–1057 (2006). 9. J. Farihi, M. A. Barstow, S. Redfield, P. Dufour, N. C. Hambly, Mon. Not. R. Astron. Soc. 404, 2123 (2010). 10. M. Jura, J. Farihi, B. Zuckerman, Astron. J. 137, 3191–3197 (2009). 11. W. T. Reach et al., Astrophys. J. 635, L161–L164 (2005). 12. B. T. Gänsicke, T. R. Marsh, J. Southworth, A. Rebassa-Mansergas, Science 314, 1908–1910 (2006). 13. J. H. Debes, K. J. Walsh, C. Stark, Astrophys. J. 747, 148 (2012). 14. J. Farihi et al., Mon. Not. R. Astron. Soc. 421, 1635–1643 (2012). 15. J. Farihi, M. Jura, J. E. Lee, B. Zuckerman, Astrophys. J. 714, 1386–1397 (2010). 16. S. Xu, M. Jura, Astrophys. J. 745, 88 (2012). 17. J. Girven et al., Astrophys. J. 749, 154 (2012). 18. J. Farihi, M. Jura, B. Zuckerman, Astrophys. J. 694, 805–819 (2009). 19. M. Jura, J. Farihi, B. Zuckerman, Astrophys. J. 663, 1285–1290 (2007). 20. M. Jura, Astrophys. J. 584, L91–L94 (2003). 21. B. Zuckerman, D. Koester, C. Melis, B. M. S. Hansen, M. Jura, Astrophys. J. 671, 872–877 (2007). 22. M. Jura, M. Muno, J. Farihi, B. Zuckerman, Astrophys. J. 699, 1473–1479 (2009). 23. D. Koester, R. Napiwotzki, B. Voss, D. Homeier, D. Reimers, Astron. Astrophys. 439, 317–321 (2005). 24. P. E. Tremblay, P. Bergeron, Astrophys. J. 672, 1144–1152 (2008). 25. S. Desharnais, F. Wesemael, P. Chayer, J. W. Kruk, R. A. Saffer, Astrophys. J. 672, 540–552 (2008). 26. M. Jura, S. Xu, Astron. J. 140, 1129–1136 (2010). 27. J. Farihi et al., Astrophys. J. 728, L8 (2011). 28. B. T. Gänsicke et al., Mon. Not. R. Astron. Soc. 424, 333–347 (2012). 29. B. Klein, M. Jura, D. Koester, B. Zuckerman, C. Melis, Astrophys. J. 709, 950–962 (2010). 30. M. Jura, Astron. J. 135, 1785–1792 (2008). 31. J. Farihi, B. Zuckerman, E. E. Becklin, Astrophys. J. 674, 431–446 (2008). 32. C. T. Russell et al., Science 336, 684–686 (2012). 33. A.S.Rivkin, E.S.Howell, F.Vilas, L.A. Lebofsky, in Asteroids III, W. F. Bottke Jr., A. Cellino, P. Paolicchi, R. P. Binzel, Eds. (Univ. of Arizona Press, Tucson, AZ, 2002), pp. 235–253. 34. P. C. Thomas et al., Nature 437, 224–226 (2005). 35. C. Visscher, B. Fegley Jr., Astrophys. J. 767, L12 (2013). Acknowledgments: This work is based on observations made with the Hubble Space Telescope, which is operated by the Association of Universities for Research in Astronomy under NASA contract NAS 5-26555. These observations are associated with program programs 12169 and 12474. Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. J.F. acknowledges support from the UK Science and Technology Facilities Council in the form of an Ernest Rutherford Fellowship (ST/J003344/1). The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement no. 267697 (WDTracer). B.T.G. was supported in part by the UK Science and Technology Facilities Council (ST/I001719/1). Keck telescope time for program 2011B-0554 was granted by NOAO through the Telescope System Instrumentation Program, funded by NSF. Supplementary Materials www.sciencemag.org/content/342/6155/218/suppl/DC1 Materials and Methods Fig. S1 Tables S1 and S2 References (36, 37) 22 April 2013; accepted 15 August 2013 10.1126/science.1239447 Femtosecond Visualization of Lattice Dynamics in Shock-Compressed Matter D. Milathianaki,1 * S. Boutet,1 G. J. Williams,1 A. Higginbotham,2 D. Ratner,1 A. E. Gleason,3 M. Messerschmidt,1 M. M. Seibert,1,4 D. C. Swift,5 P. Hering,1 J. Robinson,1 W. E. White,1 J. S. Wark2 The ultrafast evolution of microstructure is key to understanding high-pressure and strain-rate phenomena. However, the visualization of lattice dynamics at scales commensurate with those of atomistic simulations has been challenging. Here, we report femtosecond x-ray diffraction measurements unveiling the response of copper to laser shock-compression at peak normal elastic stresses of ~73 gigapascals (GPa) and strain rates of 109 per second. We capture the evolution of the lattice from a one-dimensional (1D) elastic to a 3D plastically relaxed state within a few tens of picoseconds, after reaching shear stresses of 18 GPa. Our in situ high-precision measurement of material strength at spatial (<1 micrometer) and temporal (<50 picoseconds) scales provides a direct comparison with multimillion-atom molecular dynamics simulations. T he distinct properties of materials at high- pressure and/or strain-rate conditions lead to a broad range of phenomena in fields such as high-energy-density physics (1), Earth and planetary sciences (2, 3), aerospace engi- neering (4), and materials science (5, 6). For the latter, a predictive understanding and control of mechanical properties, enabled by the di- rect comparison of experiments with large-scale atomistic simulations, is the ultimate goal. Where- as the bulk material behavior can be inferred by macroscopic measurements (7, 8), key infor- mation on the mechanical properties requires knowledge of the physics embedded at the lattice level. Such knowledge has traditionally been obtained via nanosecond-resolution x-ray 11 OCTOBER 2013 VOL 342 SCIENCE www.sciencemag.org220 REPORTS
  • 5. www.sciencemag.org/content/342/6155/218/suppl/DC1 Supplementary Materials for Evidence for Water in the Rocky Debris of a Disrupted Extrasolar Minor Planet J. Farihi,* B. T. Gänsicke, D. Koester *Corresponding author. E-mail: jfarihi@ast.cam.ac.uk Published 11 October 2013, Science 342, 218 (2013) DOI: 10.1126/science.1239447 This PDF file includes: Materials and Methods Fig. S1 Tables S1 and S2 References
  • 6. Supporting Online Material for Evidence for Water in the Rocky Debris of Disrupted Extrasolar Minor Planets J. Farihi1,4∗ , B. T. G¨ansicke2 , D. Koester3 1 Institute of Astronomy, University of Cambridge, Cambridge CB3 0HA, UK 2 Department of Physics, University of Warwick, Coventry CV5 7AL, UK 3 Institut f¨ur Theoretische Physik und Astrophysik, University of Kiel, 24098 Kiel, Germany 4 STFC Ernest Rutherford Fellow ∗ To whom correspondence should be addressed; E-mail: jfarihi@ast.cam.ac.uk We describe here in detail the observations and analyses supporting the main paper, specifically the spectroscopy of the metal-enriched white dwarf atmosphere and the analytical link to the elemental abundances of the infalling planetary debris. 1 Summary of the Observations and Datasets GD 61 exhibits infrared excess consistent with circumstellar dust orbiting within its Roche limit (26), and bears the unambiguous signature of debris accretion via its metal-polluted atmosphere. The white dwarf was observed with the Cosmic Origins Spectrograph (COS) during Hubble Space Telescope Cycle 19 on 2012 January 28. The ultraviolet spectra were obtained with a total exposure time of 1600 s (split between two FP-POS positions) using the G130M grating and a central wavelength setting at 1291 ˚A, covering 1130−1435 ˚A at R ≈ 18 000. The COS data were processed and calibrated with CALCOS 2.15.6, and are shown in Figure S1. Optical 1
  • 7. spectroscopy of GD61 was obtained on 2011 October 24 with the Keck II Telescope and the Echelle Spectrograph and Imager (36, ESI) in echelle mode, effectively covering 3900−9200 Å at R 13 000. The spectra were obtained in a series of 16 exposures of 900 s each, for a total exposure time of 4 hr, and reduced using standard tasks in IRAF1 . 2 Derivation of Photospheric and Debris Abundances Elemental abundances for GD61 were derived from the COS and ESI data by fitting white dwarf atmospheric models (37) to the observed spectra. For these calculations, Teff = 17 280 K and log g = 8.20 are adopted, based on a published analysis of low-resolution optical spectra (24). The resulting photospheric abundances and upper limits are listed in Table S1 together with previous measurements from the Far Ultraviolet Spectroscopic Explorer (24, FUSE) and Keck I HIRES (26). Notably, all heavy element abundances agree well, despite being derived using separate instruments and with multiple absorption lines across distinct wavelength regimes. The transformation between the heavy element abundances in the white dwarf atmospheres and those within the infalling planetary debris are calculated assuming a steady state balance between accretion and diffusion. An early (or build-up) phase of accretion is theoretically possible in GD61, but this is unlikely (see main paper). Importantly, in this case an early phase would imply a larger oxygen excess and H2O fraction, and therefore the more conservative, and most probable, assumption is made. For white dwarfs with significant convection zones like GD61, the atmospheric mass fraction Xz of heavy element z is related to its accretion rate zM via z cvz zz tX MM (S1) ___________________________________ 1 IRAF is distributed by the National Optical Astronomy Observatories, which are operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation. 2
  • 8. where tz is the sinking timescale for the element and Mcvz is the mass of the stellar convection zone. The mass fraction is determined from the model atmosphere fits and the sinking timescale is known from white dwarf diffusion calculations (5). In essence, Equation S1 states that the accretion rate of element z equals its rate of depletion as it settles below the mixing layer. The ratio of two heavy elements within the debris (and hence parent body) is either the ratio of their respective accretion rates in the steady state, or the ratio of their atmospheric mass fractions in the early phase, and related by ˙Mz1 ˙Mz2 = Xz1 Xz2 × tz2 tz1 (S2) Table S2 lists the relevant quantities of GD 61 for the key elements that determine the total oxygen budget of the debris. The steady state metal abundances relative to oxygen are taken from the fourth column. The sinking timescales for GD 61 have been updated following a correction in the theoretical calculations2 , and they are somewhat different than those presented in a previous analysis (26). Notably, this correction has strengthened the case for an oxygen excess in GD 61. 3 Evaluation of Oxygen Excess and Uncertainties The method for calculating the overall oxygen budget is as follows. We begin with the columns in Table S2, and in particular the identify the total oxygen budget with: 1) its mass accretion rate for the steady state or 2) its mass within the stellar convection zone for the early phase. We calculate the fraction of oxygen that can be absorbed as CO2 based on the upper limit for carbon, and subtract this from the total available. Next, we perform a similar calculation for the mass of oxygen in MgO, Al2O3, Si2, CaO, and FeO based on their detections or upper limits, again subtracting these from the budget. After accounting for all the major oxygen carriers, any remaining mass is considered excess. 2 http://www1.astrophysik.uni-kiel.de/~koester/astrophysics/astrophysics.html 3
  • 9. The collective data for GD 61 is robust and comprehensive, comprising four instruments with each probing distinct wavelength regions and containing multiple transitions for each ele- ment from the far-ultraviolet to the red optical region. The uncertainties in the metal abundances of this white dwarf are given as 3σ adopted values in the last column of Table S1. Using a brute force approach, all 128 possible combinations of abundance values are calculated for C, O, Mg, Al, Si, Ca, Fe where the abundance values N(X)/N(He) take on each of the values x ± δx. Evaluating all possible permutations, the dispersion in the resulting oxygen excesses values (0.068) results in a 6.1σ confidence for the case of steady state accretion. References and Notes 1. N. M. Batalha, et al., Astrophys. J. 729, 27 (2011) 2. F. Fressin, et al., Nature 482, 195 (2012) 3. D. K. Sing, et al., Mon. Not. R. Ast. Soc. 416, 1443 (2011) 4. C. M. Lisse, et al., Astrophys. J. 747, 93 (2012) 5. D. Koester, Astron. Astrophys. 498, 517 (2009) 6. B. Zuckerman, D. Koester, I. N.Reid, M. H¨unsch, Astrophys. J. 596, 477 (2003) 7. D. Koester, D. Wilken, Astron. Astrophys. 453, 1051 (2006) 8. J. Farihi, M. A. Barstow, S. Redfield, P. Dufour, N. C. Hambly, Mon. Not. R. Ast. Soc. 404, 2123 (2010) 9. M. Jura, J. Farihi, B. Zuckerman, Astron. J. 137, 3191 (2009) 10. W. T. Reach, M. J. Kuchner, T. von Hippel, A. Burrows, F. Mullally, M. Kilic, D. E. Winget, Astrophys. J. 635, L161 (2005) 4
  • 10. Figure S1. The normalized COS spectra of GD 61 (grey), together with the best fitting model spectra (red). Interstellar absorption features are indicated by vertical grey dashed lines, and are blueshifted with respect to the photospheric features by 40 km s−1 . Geocoronal airglow of O I at 1302.2, 1304.9, and 1306.0 ˚A can contaminate COS spectra to some degree, and typical airglow line profiles are shown in the middle panel scaled to an arbitrary flux. 7
  • 11. Table S1. Elemental Abundances N(X)/N(He) in GD 61 Ultraviolet Optical Element COS FUSE ESI HIRES Adopted Detections: H −3.70 (0.10) −4.00 (0.10) −3.98 (0.10) −3.89 (0.15) O −6.00 (0.15) −5.80 (0.20) −5.75 (0.20) −5.95 (0.13) Mg −6.50 (0.30) −6.74 (0.10) −6.65 (0.18) −6.69 (0.14) Si −6.82 (0.12) −6.70 (0.20) −6.85 (0.10) −6.85 (0.09) −6.82 (0.11) S −8.00 (0.20) −8.00 (0.20) Ca −7.77 (0.06) −7.90 (0.19) −7.90 (0.19) Fe −7.60 (0.30) −7.60 (0.20) −7.60 (0.20) Upper limits: C −9.10 −8.80 N −8.00 Na −6.80 P −8.70 Al −7.80 −7.20 Ti −8.60 Sc −8.20 Cr −8.00 Fe −7.50 Ni −8.80 8
  • 12. Table S2. Atmospheric and Debris Properties for Key Trace Elements in GD 61 Early Phase Steady State Element tdiff XzMcvz a ˙Mz (105 yr) (1021 g) (108 g s−1 ) H ∞ 5.755 C 1.730 < 0.001 < 0.001 O 1.706 0.802 1.489 Mg 1.808 0.222 0.389 Al 1.735 < 0.019 < 0.035 Si 1.438 0.190 0.419 S 0.952 0.014 0.048 Ca 0.782 0.023 0.091 Fe 0.855 0.063 0.232 Total Z 1.332 2.704 Note. The metal-to-metal ratios within the planetary debris for the early phase and steady state regimes are derived directly from the values in the third and fourth columns respectively. a The third column is the mass of each element residing in the convection zone of GD 61, and their total (excluding hydrogen) represents a minimum mass for the parent body due to the continual sinking of metals. 9
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