- The authors detected an acceleration in the rotation rate of asteroid (25143) Itokawa through photometric observations spanning 2001 to 2013.
- By measuring rotational phase offsets between observed and modeled lightcurves, they found a YORP acceleration of 3.54 ± 0.38 × 10−8 rad day−2, equivalent to a decrease in the asteroid's rotation period of about 45 ms per year.
- Thermophysical modeling of the detailed shape model from the Hayabusa spacecraft could not reconcile the observed YORP strength unless the asteroid's center of mass is shifted by about 21 m along its long axis. This suggests Itokawa has two components with different densities that merged, either from a
Detection of solar_like_oscillations_in_relies_of_the_milk_way_asteroseismolo...Sérgio Sacani
Asteroseismic constraints on K giants make it possible to infer radii, masses and ages of tens
of thousands of field stars. Tests against independent estimates of these properties are however
scarce, especially in the metal-poor regime. Here, we report the detection of solar-like
oscillations in 8 stars belonging to the red-giant branch and red-horizontal branch of the globular
cluster M4. The detections were made in photometric observations from the K2 Mission
during its Campaign 2. Making use of independent constraints on the distance, we estimate
masses of the 8 stars by utilising different combinations of seismic and non-seismic inputs.
When introducing a correction to the Δν scaling relation as suggested by stellar models, for
RGB stars we find excellent agreement with the expected masses from isochrone fitting, and
with a distance modulus derived using independent methods. The offset with respect to independent
masses is lower, or comparable with, the uncertainties on the average RGB mass
(4 − 10%, depending on the combination of constraints used). Our results lend confidence to
asteroseismic masses in the metal poor regime. We note that a larger sample will be needed
to allow more stringent tests to be made of systematic uncertainties in all the observables
(both seismic and non-seismic), and to explore the properties of RHB stars, and of different
populations in the cluster.
The canarias einstein_ring_a_newly_discovered_optical_einstein_ringSérgio Sacani
We report the discovery of an optical Einstein Ring in the Sculptor constellation,
IAC J010127-334319, in the vicinity of the Sculptor Dwarf Spheroidal Galaxy. It is
an almost complete ring ( 300◦) with a diameter of 4.5 arcsec. The discovery was
made serendipitously from inspecting Dark Energy Camera (DECam) archive imaging
data. Confirmation of the object nature has been obtained by deriving spectroscopic
redshifts for both components, lens and source, from observations at the 10.4 m Gran
Telescopio CANARIAS (GTC) with the spectrograph OSIRIS. The lens, a massive
early-type galaxy, has a redshift of z = 0.581 while the source is a starburst galaxy
with redshift of z = 1.165. The total enclosed mass that produces the lensing effect
has been estimated to be Mtot = (1.86 ± 0.23) · 1012M⊙.
Todo mundo sabe que os raios produzidos pela Estrela da Morte em Guerra nas Estrelas não pode existir na vida real, porém no universo existem fenômenos que as vezes conseguem superar até a mais surpreendente ficção.
A galáxia Pictor A, é um desses objetos que possuem fenômenos tão espetaculares quanto aqueles exibidos no cinema. Essa galáxia localiza-se a cerca de 500 milhões de anos-luz da Terra e possui um buraco negro supermassivo no seu centro. Uma grande quantidade de energia gravitacional é lançada, à medida que o material cai em direção ao horizonte de eventos, o ponto sem volta ao redor do buraco negro. Essa energia produz um enorme jato de partículas que viajam a uma velocidade próxima da velocidade da luz no espaço intergaláctico, chamado de jato relativístico.
Para obter imagens desse jato, os cientistas usaram o Observatório de Raios-X Chandra, da NASA várias vezes durante 15 anos. Os dados do Chandra, apresentados em azul nas imagens, foram combinados com os dados obtidos em ondas de rádio a partir do Australia Telescope Compact Array, e são aparesentados em vermelho nas imagens.
Magnetic interaction of_a_super_cme_with_the_earths_magnetosphere_scenario_fo...Sérgio Sacani
Solar eruptions, known as Coronal Mass Ejections (CMEs), are
frequently observed on our Sun. Recent Kepler observations of super
ares
on G-type stars have implied that so called super-CMEs, possessing kinetic
energies 10 times of the most powerful CME event ever observed on the Sun,
could be produced with a frequency of 1 event per 800-2000 yr on solar-
like slowly rotating stars. We have performed a 3D time-dependent global
magnetohydrodynamic simulation of the magnetic interaction of such a CME
cloud with the Earth's magnetosphere. We calculated the global structure
of the perturbed magnetosphere and derive the latitude of the open-closed
magnetic eld boundary. We also estimated energy
uxes penetrating the
Earth's ionosphere and discuss the consequences of energetic particle
uxes
on biological systems on early Earth.
A nearby yoiung_m_dwarf_with_wide_possibly_planetary_m_ass_companionSérgio Sacani
O objeto de massa planetária J2126, anteriormente pensado como sendo um planeta solitário, orbita sua estrela mãe na maior órbita já descoberta até agora no universo, de acordo com uma equipe de astrônomos liderada pelo Dr. Niall Deacon, da Universidade de Hertfordshire, no Reino Unido.
O J2126, cujo nome completo é 2MASS J21265040-8140293, tem cerca de 13 vezes a massa de Júpiter.
Sua órbita é de aproximadamente 6900 Unidades Astronômicas de distância da sua estrela, a TYC 9486-927-1, uma estrela ativa, de rotação rápida e classificada como sendo do tipo Anã-M.
Essa é uma órbita 6900 vezes maior que a distância da Terra ao Sol, ou seja, aproximadamente 1 trilhão de quilômetros. Nessa sua órbita, o planeta leva 900000 anos para completar uma volta ao redor da sua estrela.
Detection of solar_like_oscillations_in_relies_of_the_milk_way_asteroseismolo...Sérgio Sacani
Asteroseismic constraints on K giants make it possible to infer radii, masses and ages of tens
of thousands of field stars. Tests against independent estimates of these properties are however
scarce, especially in the metal-poor regime. Here, we report the detection of solar-like
oscillations in 8 stars belonging to the red-giant branch and red-horizontal branch of the globular
cluster M4. The detections were made in photometric observations from the K2 Mission
during its Campaign 2. Making use of independent constraints on the distance, we estimate
masses of the 8 stars by utilising different combinations of seismic and non-seismic inputs.
When introducing a correction to the Δν scaling relation as suggested by stellar models, for
RGB stars we find excellent agreement with the expected masses from isochrone fitting, and
with a distance modulus derived using independent methods. The offset with respect to independent
masses is lower, or comparable with, the uncertainties on the average RGB mass
(4 − 10%, depending on the combination of constraints used). Our results lend confidence to
asteroseismic masses in the metal poor regime. We note that a larger sample will be needed
to allow more stringent tests to be made of systematic uncertainties in all the observables
(both seismic and non-seismic), and to explore the properties of RHB stars, and of different
populations in the cluster.
The canarias einstein_ring_a_newly_discovered_optical_einstein_ringSérgio Sacani
We report the discovery of an optical Einstein Ring in the Sculptor constellation,
IAC J010127-334319, in the vicinity of the Sculptor Dwarf Spheroidal Galaxy. It is
an almost complete ring ( 300◦) with a diameter of 4.5 arcsec. The discovery was
made serendipitously from inspecting Dark Energy Camera (DECam) archive imaging
data. Confirmation of the object nature has been obtained by deriving spectroscopic
redshifts for both components, lens and source, from observations at the 10.4 m Gran
Telescopio CANARIAS (GTC) with the spectrograph OSIRIS. The lens, a massive
early-type galaxy, has a redshift of z = 0.581 while the source is a starburst galaxy
with redshift of z = 1.165. The total enclosed mass that produces the lensing effect
has been estimated to be Mtot = (1.86 ± 0.23) · 1012M⊙.
Todo mundo sabe que os raios produzidos pela Estrela da Morte em Guerra nas Estrelas não pode existir na vida real, porém no universo existem fenômenos que as vezes conseguem superar até a mais surpreendente ficção.
A galáxia Pictor A, é um desses objetos que possuem fenômenos tão espetaculares quanto aqueles exibidos no cinema. Essa galáxia localiza-se a cerca de 500 milhões de anos-luz da Terra e possui um buraco negro supermassivo no seu centro. Uma grande quantidade de energia gravitacional é lançada, à medida que o material cai em direção ao horizonte de eventos, o ponto sem volta ao redor do buraco negro. Essa energia produz um enorme jato de partículas que viajam a uma velocidade próxima da velocidade da luz no espaço intergaláctico, chamado de jato relativístico.
Para obter imagens desse jato, os cientistas usaram o Observatório de Raios-X Chandra, da NASA várias vezes durante 15 anos. Os dados do Chandra, apresentados em azul nas imagens, foram combinados com os dados obtidos em ondas de rádio a partir do Australia Telescope Compact Array, e são aparesentados em vermelho nas imagens.
Magnetic interaction of_a_super_cme_with_the_earths_magnetosphere_scenario_fo...Sérgio Sacani
Solar eruptions, known as Coronal Mass Ejections (CMEs), are
frequently observed on our Sun. Recent Kepler observations of super
ares
on G-type stars have implied that so called super-CMEs, possessing kinetic
energies 10 times of the most powerful CME event ever observed on the Sun,
could be produced with a frequency of 1 event per 800-2000 yr on solar-
like slowly rotating stars. We have performed a 3D time-dependent global
magnetohydrodynamic simulation of the magnetic interaction of such a CME
cloud with the Earth's magnetosphere. We calculated the global structure
of the perturbed magnetosphere and derive the latitude of the open-closed
magnetic eld boundary. We also estimated energy
uxes penetrating the
Earth's ionosphere and discuss the consequences of energetic particle
uxes
on biological systems on early Earth.
A nearby yoiung_m_dwarf_with_wide_possibly_planetary_m_ass_companionSérgio Sacani
O objeto de massa planetária J2126, anteriormente pensado como sendo um planeta solitário, orbita sua estrela mãe na maior órbita já descoberta até agora no universo, de acordo com uma equipe de astrônomos liderada pelo Dr. Niall Deacon, da Universidade de Hertfordshire, no Reino Unido.
O J2126, cujo nome completo é 2MASS J21265040-8140293, tem cerca de 13 vezes a massa de Júpiter.
Sua órbita é de aproximadamente 6900 Unidades Astronômicas de distância da sua estrela, a TYC 9486-927-1, uma estrela ativa, de rotação rápida e classificada como sendo do tipo Anã-M.
Essa é uma órbita 6900 vezes maior que a distância da Terra ao Sol, ou seja, aproximadamente 1 trilhão de quilômetros. Nessa sua órbita, o planeta leva 900000 anos para completar uma volta ao redor da sua estrela.
We report the discovery of a new Kepler transiting circumbinary planet (CBP).
This latest addition to the still-small family of CBPs defies the current trend of known
short-period planets orbiting near the stability limit of binary stars. Unlike the previous
discoveries, the planet revolving around the eclipsing binary system Kepler-1647 has
a very long orbital period ( 1100 days) and was at conjunction only twice during
the Kepler mission lifetime. Due to the singular configuration of the system, Kepler-
1647b is not only the longest-period transiting CBP at the time of writing, but also one
of the longest-period transiting planets. With a radius of 1:060:01 RJup it is also the
largest CBP to date. The planet produced three transits in the light-curve of Kepler-
1647 (one of them during an eclipse, creating a syzygy) and measurably perturbed the
times of the stellar eclipses, allowing us to measure its mass to be 1:520:65 MJup.
The planet revolves around an 11-day period eclipsing binary consisting of two Solarmass
stars on a slightly inclined, mildly eccentric (ebin = 0:16), spin-synchronized
orbit. Despite having an orbital period three times longer than Earth’s, Kepler-1647b is
in the conservative habitable zone of the binary star throughout its orbit.
Exocometary gas in_th_hd_181327_debris_ringSérgio Sacani
An increasing number of observations have shown that gaseous debris discs are not an
exception. However, until now we only knew of cases around A stars. Here we present the first
detection of 12CO (2-1) disc emission around an F star, HD 181327, obtained with ALMA
observations at 1.3 mm. The continuum and CO emission are resolved into an axisymmetric
disc with ring-like morphology. Using a Markov chain Monte Carlo method coupled with
radiative transfer calculations we study the dust and CO mass distribution. We find the dust is
distributed in a ring with a radius of 86:0 0:4 AU and a radial width of 23:2 1:0 AU. At
this frequency the ring radius is smaller than in the optical, revealing grain size segregation
expected due to radiation pressure. We also report on the detection of low level continuum
emission beyond the main ring out to 200 AU. We model the CO emission in the non-LTE
regime and we find that the CO is co-located with the dust, with a total CO gas mass ranging
between 1:2 10 6 M and 2:9 10 6 M, depending on the gas kinetic temperature and
collisional partners densities. The CO densities and location suggest a secondary origin, i.e.
released from icy planetesimals in the ring. We derive a CO cometary composition that is
consistent with Solar system comets. Due to the low gas densities it is unlikely that the gas is
shaping the dust distribution.
Detection of lyman_alpha_emission_from_a_triply_imaged_z_6_85_galaxy_behind_m...Sérgio Sacani
We report the detection of Ly emission at 9538A
in the Keck/DEIMOS and HST WFC3
G102 grism data from a triply-imaged galaxy at z = 6:846 0:001 behind galaxy cluster MACS
J2129.4 0741. Combining the emission line wavelength with broadband photometry, line ratio upper
limits, and lens modeling, we rule out the scenario that this emission line is [O II] at z = 1:57. After
accounting for magnication, we calculate the weighted average of the intrinsic Ly luminosity to be
1:31042 erg s 1 and Ly equivalent width to be 7415A. Its intrinsic UV absolute magnitude at
1600A
is 18:60:2 mag and stellar mass (1:50:3)107 M, making it one of the faintest (intrinsic
LUV 0:14 L
UV) galaxies with Ly detection at z 7 to date. Its stellar mass is in the typical range
for the galaxies thought to dominate the reionization photon budget at z & 7; the inferred Ly escape
fraction is high (& 10%), which could be common for sub-L z & 7 galaxies with Ly emission. This
galaxy oers a glimpse of the galaxy population that is thought to drive reionization, and it shows
that gravitational lensing is an important avenue to probe the sub-L galaxy population.
Quase 900 galáxias próximas, porém escondidas, têm sido estudadas por uma equipe internacional de astrônomos, levando uma nova luz sobre o entendimento do Grande Atrator - uma concentração difusa de massa a 250 milhões de anos-luz de distância, que está puxando a nossa Via Láctea, e milhares de outras galáxias em sua direção.
Usando o Multibeam Receiver, instalado no rádio telescópio Parkes de 64 m, pertencente à instituição CSIRO na Austrália, a equipe foi capaz de ver através das estrelas e da poeira da nossa galáxia, vasculhando assim uma região inexplorada do espaço, conhecida pelos astrônomos como Zone of Avoidance (Zona de Anulação).
“Nós descobrimos 883 galáxias, um terço das quais nunca tinham sido vistas anteriormente”, disse o Professor Lister Staveley-Smith, membro da equipe, do ARC Centre of Excellence for All-sky Astrophysics, e da University of Western Australia, um dos nós do International Centre for Radio Astronomy Research.
First identification of_direct_collapse_black_holes_candidates_in_the_early_u...Sérgio Sacani
The first black hole seeds, formed when the Universe was younger than ⇠ 500Myr, are recognized
to play an important role for the growth of early (z ⇠ 7) super-massive black holes.
While progresses have been made in understanding their formation and growth, their observational
signatures remain largely unexplored. As a result, no detection of such sources has been
confirmed so far. Supported by numerical simulations, we present a novel photometric method
to identify black hole seed candidates in deep multi-wavelength surveys.We predict that these
highly-obscured sources are characterized by a steep spectrum in the infrared (1.6−4.5μm),
i.e. by very red colors. The method selects the only 2 objects with a robust X-ray detection
found in the CANDELS/GOODS-S survey with a photometric redshift z & 6. Fitting their
infrared spectra only with a stellar component would require unrealistic star formation rates
(& 2000M# yr−1). To date, the selected objects represent the most promising black hole seed
candidates, possibly formed via the direct collapse black hole scenario, with predicted mass
> 105M#. While this result is based on the best photometric observations of high-z sources
available to date, additional progress is expected from spectroscopic and deeper X-ray data.
Upcoming observatories, like the JWST, will greatly expand the scope of this work.
A 2 4_determination_of_the_local_value_of_the_hubble_constantSérgio Sacani
We use the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST) to
reduce the uncertainty in the local value of the Hubble constant from 3.3% to 2.4%.
The bulk of this improvement comes from new, near-infrared observations of Cepheid
variables in 11 host galaxies of recent type Ia supernovae (SNe Ia), more than doubling
the sample of reliable SNe Ia having a Cepheid-calibrated distance to a total of 19; these
in turn leverage the magnitude-redshift relation based on 300 SNe Ia at z <0.15. All
19 hosts as well as the megamaser system NGC4258 have been observed with WFC3
in the optical and near-infrared, thus nullifying cross-instrument zeropoint errors in the
relative distance estimates from Cepheids. Other noteworthy improvements include a
33% reduction in the systematic uncertainty in the maser distance to NGC4258, a larger
sample of Cepheids in the Large Magellanic Cloud (LMC), a more robust distance to
the LMC based on late-type detached eclipsing binaries (DEBs), HST observations of
Cepheids in M31, and new HST-based trigonometric parallaxes for Milky Way (MW)
Cepheids.
EXTINCTION AND THE DIMMING OF KIC 8462852Sérgio Sacani
To test alternative hypotheses for the behavior of KIC 8462852, we obtained measurements of the star
over a wide wavelength range from the UV to the mid-infrared from October 2015 through December
2016, using Swift, Spitzer and at AstroLAB IRIS. The star faded in a manner similar to the longterm
fading seen in Kepler data about 1400 days previously. The dimming rate for the entire period
reported is 22.1 ± 9.7 milli-mag yr−1
in the Swift wavebands, with amounts of 21.0 ± 4.5 mmag in
the groundbased B measurements, 14.0 ± 4.5 mmag in V , and 13.0 ± 4.5 in R, and a rate of 5.0 ± 1.2
mmag yr−1 averaged over the two warm Spitzer bands. Although the dimming is small, it is seen at
& 3 σ by three different observatories operating from the UV to the IR. The presence of long-term
secular dimming means that previous SED models of the star based on photometric measurements
taken years apart may not be accurate. We find that stellar models with Tef f = 7000 - 7100 K and
AV ∼ 0.73 best fit the Swift data from UV to optical. These models also show no excess in the
near-simultaneous Spitzer photometry at 3.6 and 4.5 µm, although a longer wavelength excess from
a substantial debris disk is still possible (e.g., as around Fomalhaut). The wavelength dependence of
the fading favors a relatively neutral color (i.e., RV & 5, but not flat across all the bands) compared
with the extinction law for the general ISM (RV = 3.1), suggesting that the dimming arises from
circumstellar material
The ASTRODEEP Frontier Fields catalogues II. Photometric redshifts and rest f...Sérgio Sacani
Aims. We present the first public release of photometric redshifts, galaxy rest frame properties and associated magnification values
in the cluster and parallel pointings of the first two Frontier Fields, Abell-2744 and MACS-J0416. The released catalogues aim to
provide a reference for future investigations of extragalactic populations in these legacy fields: from lensed high-redshift galaxies to
cluster members themselves.
Methods.We exploit a multiwavelength catalogue, ranging from Hubble Space Telescope (HST) to ground-based K and Spitzer IRAC,
which is specifically designed to enable detection and measurement of accurate fluxes in crowded cluster regions. The multiband
information is used to derive photometric redshifts and physical properties of sources detected either in the H-band image alone, or
from a stack of four WFC3 bands. To minimize systematics, median photometric redshifts are assembled from six dierent approaches
to photo-z estimates. Their reliability is assessed through a comparison with available spectroscopic samples. State-of-the-art lensing
models are used to derive magnification values on an object-by-object basis by taking into account sources positions and redshifts.
Results. We show that photometric redshifts reach a remarkable 3–5% accuracy. After accounting for magnification, the H-band
number counts are found to be in agreement at bright magnitudes with number counts from the CANDELS fields, while extending
the presently available samples to galaxies that, intrinsically, are as faint as H 32 33, thanks to strong gravitational lensing. The
Frontier Fields allow the galaxy stellar mass distribution to be probed, depending on magnification, at 0.5–1.5 dex lower masses with
respect to extragalactic wide fields, including sources at Mstar 107–108 M at z > 5. Similarly, they allow the detection of objects
with intrinsic star formation rates (SFRs) >1 dex lower than in the CANDELS fields reaching 0.1–1 M=yr at z 6–10.
The shadow _of_the_flying_saucer_a_very_low_temperature_for_large_dust_grainsSérgio Sacani
Os astrónomos usaram o ALMA e os telescópios do IRAM para fazer a primeira medição direta da temperatura dos grãos de poeira grandes situados nas regiões periféricas de um disco de formação planetária que se encontra em torno de uma estrela jovem. Ao observar de forma inovadora um objeto cujo nome informal é Disco Voador, os astrónomos descobriram que os grãos de poeira são muito mais frios do que o esperado: -266º Celsius. Este resultado surpreendente sugere que os modelos teóricos destes discos precisam de ser revistos.
Uma equipa internacional liderada por Stephane Guilloteau do Laboratoire d´Astrophysique de Bordeaux, França, mediu a temperatura de enormes grãos de poeira que se encontram em torno da jovem estrela 2MASS J16281370-2431391 na região de formação estelar Rho Ophiuchi, a cerca de 400 anos-luz de distância da Terra.
Esta estrela encontra-se rodeada por um disco de gás e poeira — chamado disco protoplanetário, uma vez que se encontra na fase inicial da formação de um sistema planetário. Este disco é visto de perfil quando observado a partir da Terra e a sua aparência em imagens no visível levou a que se lhe desse o nome informal de Disco Voador.
Os astrónomos utilizaram o ALMA para observar o brilho emitido pelas moléculas de monóxido de carbono no disco da 2MASS J16281370-2431391. As imagens revelaram-se extremamente nítidas e descobriu-se algo estranho — em alguns casos o sinal recebido era negativo. Normalmente um sinal negativo é fisicamente impossível, mas neste caso existe uma explicação, que leva a uma conclusão surpreendente.
Probing the innermost_regions_of_agn_jets_and_their_magnetic_fields_with_radi...Sérgio Sacani
Desde 1974, observações feitas com o chamado Long Baseline Interferometry, ou VLBI, combinaram sinais de um objeto cósmico recebidos em diferentes rádio telescópios espalhados pelo globo para criar uma antena com o tamanho equivalente à maior separação entre elas. Isso fez com que fosse possível fazer imagens com uma nitidez sem precedentes, com uma resolução 1000 vezes melhor do que Hubble consegue na luz visível. Agora, uma equipe internacional de astrônomos quebrou todos os recordes combinando 15 rádio telescópios na Terra e a antena de rádio da missão RadioAstron, da agência espacial russa, na órbita da Terra. O trabalho, liderado pelo Instituto de Astrofísica de Andalucía, o IAA-CSIC, forneceu novas ideias sobre a natureza das galáxias ativas, onde um buraco negro extremamente massivo engole a matéria ao redor enquanto simultaneamente emite um par de jatos de partículas de alta energia e campos magnéticos a velocidades próximas da velocidade da luz.
Observações feitas no comprimento de onda das micro-ondas são essenciais para explorar esses jatos, já que os elétrons de alta energia se movendo em campos magnéticos são mais proficientes em produzir micro-ondas. Mas a maioria das galáxias ativas com jatos brilhantes estão a bilhões de anos-luz de distância da Terra, de modo que esses jatos são minúsculos no céu. Desse modo a alta resolução é essencial para observar esses jatos em ação e então revelar fenômenos como as ondas de choque e a turbulência que controla o quanto de luz é produzida num dado tempo. “Combinando pela primeira vez rádio telescópios na Terra com rádio telescópios no espaço, operando na máxima resolução, tem permitido que a nossa equipe crie uma antena que tem um tamanho equivalente a 8 vezes o diâmetro da Terra, correspondendo a 20 micro arcos de segundo”, disse José L; Gómez, o líder da equipe no Instituto de Astrofísica de Andalucía, IAA-CSIC.
The atacama cosmology_telescope_measuring_radio_galaxy_bias_through_cross_cor...Sérgio Sacani
A radiação cósmica de micro-ondas aponta para a matéria escura invisível, marcando o ponto onde jatos de material viajam a velocidades próximas da velocidade da luz, de acordo com uma equipe internacional de astrônomos. O principal autor do estudo, Rupert Allison da Universidade de Oxford apresentou os resultados no dia 6 de Julho de 2015 no National Astronomy Meeting em Venue Cymru, em Llandudno em Wales.
Atualmente, ninguém sabe ao certo do que a matéria escura é feita, mas ela é responsável por cerca de 26% do conteúdo de energia do universo, com galáxias massivas se formando em densas regiões de matéria escura. Embora invisível, a matéria escura se mostra através do efeito gravitacional – uma grande bolha de matéria escura puxa a matéria normal (como elétrons, prótons e nêutrons) através de sua própria gravidade, eventualmente se empacotando conjuntamente para criar as estrelas e galáxias inteiras.
Muitas das maiores dessas são galáxias ativas com buracos negros supermassivos em seus centros. Alguma parte do gás caindo diretamente na direção do buraco negro é ejetada como jatos de partículas e radiação. As observações feitas com rádio telescópios mostram que esses jatos as vezes se espalham por milhões de anos-luz desde a galáxia – mais distante até mesmo do que a extensão da própria galáxia.
Os cientistas esperam que os jatos possam viver em regiões onde existe um excesso de concentração da matéria escura, maior do que o da média. Mas como a matéria escura é invisível, testar essa ideia não é algo tão direto.
WHERE IS THE FLUX GOING? THE LONG-TERM PHOTOMETRIC VARIABILITY OF BOYAJIAN’S ...Sérgio Sacani
We present ∼ 800 days of photometric monitoring of Boyajian’s Star (KIC 8462852) from the AllSky
Automated Survey for Supernovae (ASAS-SN) and ∼ 4000 days of monitoring from the All Sky
Automated Survey (ASAS). We show that from 2015 to the present the brightness of Boyajian’s Star
has steadily decreased at a rate of 6.3 ± 1.4 mmag yr−1
, such that the star is now 1.5% fainter than it
was in February 2015. Moreover, the longer time baseline afforded by ASAS suggests that Boyajian’s
Star has also undergone two brightening episodes in the past 11 years, rather than only exhibiting a
monotonic decline. We analyze a sample of ∼ 1000 comparison stars of similar brightness located in
the same ASAS-SN field and demonstrate that the recent fading is significant at & 99.4% confidence.
The 2015 − 2017 dimming rate is consistent with that measured with Kepler data for the time period
from 2009 to 2013. This long-term variability is difficult to explain with any of the physical models
for the star’s behavior proposed to date
We present spectroscopic observations of the nearby dwarf galaxy AGC 198691. This object is part
of the Survey of H I in Extremely Low-Mass Dwarfs (SHIELD) project, which is a multi-wavelength
study of galaxies with H I masses in the range of 106-107:2 M discovered by the ALFALFA survey.
We have obtained spectra of the lone H II region in AGC 198691 with the new high-throughput
KPNO Ohio State Multi-Object Spectrograph (KOSMOS) on the Mayall 4-m as well as with the Blue
Channel spectrograph on the MMT 6.5-m telescope. These observations enable the measurement of the
temperature-sensitive [O III]4363 line and hence the determination of a \direct" oxygen abundance
for AGC 198691. We nd this system to be an extremely metal-decient (XMD) system with an
oxygen abundance of 12+log(O/H) = 7.02 0.03, making AGC 198691 the lowest-abundance starforming
galaxy known in the local universe. Two of the ve lowest-abundance galaxies known have
been discovered by the ALFALFA blind H I survey; this high yield of XMD galaxies represents a
paradigm shift in the search for extremely metal-poor galaxies.
Imaging the dust_sublimation_front_of_a_circumbinary_diskSérgio Sacani
Aims. We present the first near-IR milli-arcsecond-scale image of a post-AGB binary that is surrounded by hot circumbinary dust.
Methods. A very rich interferometric data set in six spectral channels was acquired of IRAS 08544-4431 with the new RAPID camera
on the PIONIER beam combiner at the Very Large Telescope Interferometer (VLTI). A broadband image in the H-band was reconstructed
by combining the data of all spectral channels using the SPARCO method.
Results. We spatially separate all the building blocks of the IRAS 08544-4431 system in our milliarcsecond-resolution image. Our
dissection reveals a dust sublimation front that is strikingly similar to that expected in early-stage protoplanetary disks, as well as an
unexpected flux signal of 4% from the secondary star. The energy output from this companion indicates the presence of a compact
circum-companion accretion disk, which is likely the origin of the fast outflow detected in H.
Conclusions. Our image provides the most detailed view into the heart of a dusty circumstellar disk to date. Our results demonstrate
that binary evolution processes and circumstellar disk evolution can be studied in detail in space and over time.
A giant galaxy in the young Universe with a massive ringSérgio Sacani
In the local (redshift z ≈ 0) Universe, collisional ring galaxies make up only ~0.01% of galaxies1 and are formed by head-on galactic collisions that trigger radially propagating density waves2–4. These striking systems provide key snapshots for dissecting galactic disks and are studied extensively in the local Universe5–9. However, not much is known about distant (z > 0.1) collisional rings10–14. Here we present a detailed study of a ring galaxy at a look-back time of 10.8 Gyr (z = 2.19). Compared with our Milky Way, this galaxy has a similar stellar mass, but has a stellar half-light radius that is 1.5–2.2 times larger and is forming stars 50 times faster. The extended, dif- fuse stellar light outside the star-forming ring, combined with a radial velocity on the ring and an intruder galaxy nearby, provides evidence for this galaxy hosting a collisional ring. If the ring is secularly evolved15,16, the implied large bar in a giant disk would be inconsistent with the current understand- ing of the earliest formation of barred spirals17–21. Contrary to previous predictions10–12, this work suggests that massive col- lisional rings were as rare 11 Gyr ago as they are today. Our discovery offers a unique pathway for studying density waves in young galaxies, as well as constraining the cosmic evolution of spiral disks and galaxy groups.
The completeness-corrected rate of stellar encounters with the Sun from the f...Sérgio Sacani
I report on close encounters of stars to the Sun found in the first Gaia data release (GDR1). Combining Gaia astrometry with radial
velocities of around 320 000 stars drawn from various catalogues, I integrate orbits in a Galactic potential to identify those stars which
come within a few parsecs. Such encounters could influence the solar system, for example through gravitational perturbations of the
Oort cloud. 16 stars are found to come within 2 pc (although a few of these have dubious data). This is fewer than were found in a
similar study based on Hipparcos data, even though the present study has many more candidates. This is partly because I reject stars
with large radial velocity uncertainties (>10 km s−1
), and partly because of missing stars in GDR1 (especially at the bright end). The
closest encounter found is Gl 710, a K dwarf long-known to come close to the Sun in about 1.3 Myr. The Gaia astrometry predict
a much closer passage than pre-Gaia estimates, however: just 16 000 AU (90% confidence interval: 10 000–21 000 AU), which will
bring this star well within the Oort cloud. Using a simple model for the spatial, velocity, and luminosity distributions of stars, together
with an approximation of the observational selection function, I model the incompleteness of this Gaia-based search as a function
of the time and distance of closest approach. Applying this to a subset of the observed encounters (excluding duplicates and stars
with implausibly large velocities), I estimate the rate of stellar encounters within 5 pc averaged over the past and future 5 Myr to be
545±59 Myr−1
. Assuming a quadratic scaling of the rate within some encounter distance (which my model predicts), this corresponds
to 87 ± 9 Myr−1 within 2 pc. A more accurate analysis and assessment will be possible with future Gaia data releases.
We report the discovery of a new Kepler transiting circumbinary planet (CBP).
This latest addition to the still-small family of CBPs defies the current trend of known
short-period planets orbiting near the stability limit of binary stars. Unlike the previous
discoveries, the planet revolving around the eclipsing binary system Kepler-1647 has
a very long orbital period ( 1100 days) and was at conjunction only twice during
the Kepler mission lifetime. Due to the singular configuration of the system, Kepler-
1647b is not only the longest-period transiting CBP at the time of writing, but also one
of the longest-period transiting planets. With a radius of 1:060:01 RJup it is also the
largest CBP to date. The planet produced three transits in the light-curve of Kepler-
1647 (one of them during an eclipse, creating a syzygy) and measurably perturbed the
times of the stellar eclipses, allowing us to measure its mass to be 1:520:65 MJup.
The planet revolves around an 11-day period eclipsing binary consisting of two Solarmass
stars on a slightly inclined, mildly eccentric (ebin = 0:16), spin-synchronized
orbit. Despite having an orbital period three times longer than Earth’s, Kepler-1647b is
in the conservative habitable zone of the binary star throughout its orbit.
Exocometary gas in_th_hd_181327_debris_ringSérgio Sacani
An increasing number of observations have shown that gaseous debris discs are not an
exception. However, until now we only knew of cases around A stars. Here we present the first
detection of 12CO (2-1) disc emission around an F star, HD 181327, obtained with ALMA
observations at 1.3 mm. The continuum and CO emission are resolved into an axisymmetric
disc with ring-like morphology. Using a Markov chain Monte Carlo method coupled with
radiative transfer calculations we study the dust and CO mass distribution. We find the dust is
distributed in a ring with a radius of 86:0 0:4 AU and a radial width of 23:2 1:0 AU. At
this frequency the ring radius is smaller than in the optical, revealing grain size segregation
expected due to radiation pressure. We also report on the detection of low level continuum
emission beyond the main ring out to 200 AU. We model the CO emission in the non-LTE
regime and we find that the CO is co-located with the dust, with a total CO gas mass ranging
between 1:2 10 6 M and 2:9 10 6 M, depending on the gas kinetic temperature and
collisional partners densities. The CO densities and location suggest a secondary origin, i.e.
released from icy planetesimals in the ring. We derive a CO cometary composition that is
consistent with Solar system comets. Due to the low gas densities it is unlikely that the gas is
shaping the dust distribution.
Detection of lyman_alpha_emission_from_a_triply_imaged_z_6_85_galaxy_behind_m...Sérgio Sacani
We report the detection of Ly emission at 9538A
in the Keck/DEIMOS and HST WFC3
G102 grism data from a triply-imaged galaxy at z = 6:846 0:001 behind galaxy cluster MACS
J2129.4 0741. Combining the emission line wavelength with broadband photometry, line ratio upper
limits, and lens modeling, we rule out the scenario that this emission line is [O II] at z = 1:57. After
accounting for magnication, we calculate the weighted average of the intrinsic Ly luminosity to be
1:31042 erg s 1 and Ly equivalent width to be 7415A. Its intrinsic UV absolute magnitude at
1600A
is 18:60:2 mag and stellar mass (1:50:3)107 M, making it one of the faintest (intrinsic
LUV 0:14 L
UV) galaxies with Ly detection at z 7 to date. Its stellar mass is in the typical range
for the galaxies thought to dominate the reionization photon budget at z & 7; the inferred Ly escape
fraction is high (& 10%), which could be common for sub-L z & 7 galaxies with Ly emission. This
galaxy oers a glimpse of the galaxy population that is thought to drive reionization, and it shows
that gravitational lensing is an important avenue to probe the sub-L galaxy population.
Quase 900 galáxias próximas, porém escondidas, têm sido estudadas por uma equipe internacional de astrônomos, levando uma nova luz sobre o entendimento do Grande Atrator - uma concentração difusa de massa a 250 milhões de anos-luz de distância, que está puxando a nossa Via Láctea, e milhares de outras galáxias em sua direção.
Usando o Multibeam Receiver, instalado no rádio telescópio Parkes de 64 m, pertencente à instituição CSIRO na Austrália, a equipe foi capaz de ver através das estrelas e da poeira da nossa galáxia, vasculhando assim uma região inexplorada do espaço, conhecida pelos astrônomos como Zone of Avoidance (Zona de Anulação).
“Nós descobrimos 883 galáxias, um terço das quais nunca tinham sido vistas anteriormente”, disse o Professor Lister Staveley-Smith, membro da equipe, do ARC Centre of Excellence for All-sky Astrophysics, e da University of Western Australia, um dos nós do International Centre for Radio Astronomy Research.
First identification of_direct_collapse_black_holes_candidates_in_the_early_u...Sérgio Sacani
The first black hole seeds, formed when the Universe was younger than ⇠ 500Myr, are recognized
to play an important role for the growth of early (z ⇠ 7) super-massive black holes.
While progresses have been made in understanding their formation and growth, their observational
signatures remain largely unexplored. As a result, no detection of such sources has been
confirmed so far. Supported by numerical simulations, we present a novel photometric method
to identify black hole seed candidates in deep multi-wavelength surveys.We predict that these
highly-obscured sources are characterized by a steep spectrum in the infrared (1.6−4.5μm),
i.e. by very red colors. The method selects the only 2 objects with a robust X-ray detection
found in the CANDELS/GOODS-S survey with a photometric redshift z & 6. Fitting their
infrared spectra only with a stellar component would require unrealistic star formation rates
(& 2000M# yr−1). To date, the selected objects represent the most promising black hole seed
candidates, possibly formed via the direct collapse black hole scenario, with predicted mass
> 105M#. While this result is based on the best photometric observations of high-z sources
available to date, additional progress is expected from spectroscopic and deeper X-ray data.
Upcoming observatories, like the JWST, will greatly expand the scope of this work.
A 2 4_determination_of_the_local_value_of_the_hubble_constantSérgio Sacani
We use the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST) to
reduce the uncertainty in the local value of the Hubble constant from 3.3% to 2.4%.
The bulk of this improvement comes from new, near-infrared observations of Cepheid
variables in 11 host galaxies of recent type Ia supernovae (SNe Ia), more than doubling
the sample of reliable SNe Ia having a Cepheid-calibrated distance to a total of 19; these
in turn leverage the magnitude-redshift relation based on 300 SNe Ia at z <0.15. All
19 hosts as well as the megamaser system NGC4258 have been observed with WFC3
in the optical and near-infrared, thus nullifying cross-instrument zeropoint errors in the
relative distance estimates from Cepheids. Other noteworthy improvements include a
33% reduction in the systematic uncertainty in the maser distance to NGC4258, a larger
sample of Cepheids in the Large Magellanic Cloud (LMC), a more robust distance to
the LMC based on late-type detached eclipsing binaries (DEBs), HST observations of
Cepheids in M31, and new HST-based trigonometric parallaxes for Milky Way (MW)
Cepheids.
EXTINCTION AND THE DIMMING OF KIC 8462852Sérgio Sacani
To test alternative hypotheses for the behavior of KIC 8462852, we obtained measurements of the star
over a wide wavelength range from the UV to the mid-infrared from October 2015 through December
2016, using Swift, Spitzer and at AstroLAB IRIS. The star faded in a manner similar to the longterm
fading seen in Kepler data about 1400 days previously. The dimming rate for the entire period
reported is 22.1 ± 9.7 milli-mag yr−1
in the Swift wavebands, with amounts of 21.0 ± 4.5 mmag in
the groundbased B measurements, 14.0 ± 4.5 mmag in V , and 13.0 ± 4.5 in R, and a rate of 5.0 ± 1.2
mmag yr−1 averaged over the two warm Spitzer bands. Although the dimming is small, it is seen at
& 3 σ by three different observatories operating from the UV to the IR. The presence of long-term
secular dimming means that previous SED models of the star based on photometric measurements
taken years apart may not be accurate. We find that stellar models with Tef f = 7000 - 7100 K and
AV ∼ 0.73 best fit the Swift data from UV to optical. These models also show no excess in the
near-simultaneous Spitzer photometry at 3.6 and 4.5 µm, although a longer wavelength excess from
a substantial debris disk is still possible (e.g., as around Fomalhaut). The wavelength dependence of
the fading favors a relatively neutral color (i.e., RV & 5, but not flat across all the bands) compared
with the extinction law for the general ISM (RV = 3.1), suggesting that the dimming arises from
circumstellar material
The ASTRODEEP Frontier Fields catalogues II. Photometric redshifts and rest f...Sérgio Sacani
Aims. We present the first public release of photometric redshifts, galaxy rest frame properties and associated magnification values
in the cluster and parallel pointings of the first two Frontier Fields, Abell-2744 and MACS-J0416. The released catalogues aim to
provide a reference for future investigations of extragalactic populations in these legacy fields: from lensed high-redshift galaxies to
cluster members themselves.
Methods.We exploit a multiwavelength catalogue, ranging from Hubble Space Telescope (HST) to ground-based K and Spitzer IRAC,
which is specifically designed to enable detection and measurement of accurate fluxes in crowded cluster regions. The multiband
information is used to derive photometric redshifts and physical properties of sources detected either in the H-band image alone, or
from a stack of four WFC3 bands. To minimize systematics, median photometric redshifts are assembled from six dierent approaches
to photo-z estimates. Their reliability is assessed through a comparison with available spectroscopic samples. State-of-the-art lensing
models are used to derive magnification values on an object-by-object basis by taking into account sources positions and redshifts.
Results. We show that photometric redshifts reach a remarkable 3–5% accuracy. After accounting for magnification, the H-band
number counts are found to be in agreement at bright magnitudes with number counts from the CANDELS fields, while extending
the presently available samples to galaxies that, intrinsically, are as faint as H 32 33, thanks to strong gravitational lensing. The
Frontier Fields allow the galaxy stellar mass distribution to be probed, depending on magnification, at 0.5–1.5 dex lower masses with
respect to extragalactic wide fields, including sources at Mstar 107–108 M at z > 5. Similarly, they allow the detection of objects
with intrinsic star formation rates (SFRs) >1 dex lower than in the CANDELS fields reaching 0.1–1 M=yr at z 6–10.
The shadow _of_the_flying_saucer_a_very_low_temperature_for_large_dust_grainsSérgio Sacani
Os astrónomos usaram o ALMA e os telescópios do IRAM para fazer a primeira medição direta da temperatura dos grãos de poeira grandes situados nas regiões periféricas de um disco de formação planetária que se encontra em torno de uma estrela jovem. Ao observar de forma inovadora um objeto cujo nome informal é Disco Voador, os astrónomos descobriram que os grãos de poeira são muito mais frios do que o esperado: -266º Celsius. Este resultado surpreendente sugere que os modelos teóricos destes discos precisam de ser revistos.
Uma equipa internacional liderada por Stephane Guilloteau do Laboratoire d´Astrophysique de Bordeaux, França, mediu a temperatura de enormes grãos de poeira que se encontram em torno da jovem estrela 2MASS J16281370-2431391 na região de formação estelar Rho Ophiuchi, a cerca de 400 anos-luz de distância da Terra.
Esta estrela encontra-se rodeada por um disco de gás e poeira — chamado disco protoplanetário, uma vez que se encontra na fase inicial da formação de um sistema planetário. Este disco é visto de perfil quando observado a partir da Terra e a sua aparência em imagens no visível levou a que se lhe desse o nome informal de Disco Voador.
Os astrónomos utilizaram o ALMA para observar o brilho emitido pelas moléculas de monóxido de carbono no disco da 2MASS J16281370-2431391. As imagens revelaram-se extremamente nítidas e descobriu-se algo estranho — em alguns casos o sinal recebido era negativo. Normalmente um sinal negativo é fisicamente impossível, mas neste caso existe uma explicação, que leva a uma conclusão surpreendente.
Probing the innermost_regions_of_agn_jets_and_their_magnetic_fields_with_radi...Sérgio Sacani
Desde 1974, observações feitas com o chamado Long Baseline Interferometry, ou VLBI, combinaram sinais de um objeto cósmico recebidos em diferentes rádio telescópios espalhados pelo globo para criar uma antena com o tamanho equivalente à maior separação entre elas. Isso fez com que fosse possível fazer imagens com uma nitidez sem precedentes, com uma resolução 1000 vezes melhor do que Hubble consegue na luz visível. Agora, uma equipe internacional de astrônomos quebrou todos os recordes combinando 15 rádio telescópios na Terra e a antena de rádio da missão RadioAstron, da agência espacial russa, na órbita da Terra. O trabalho, liderado pelo Instituto de Astrofísica de Andalucía, o IAA-CSIC, forneceu novas ideias sobre a natureza das galáxias ativas, onde um buraco negro extremamente massivo engole a matéria ao redor enquanto simultaneamente emite um par de jatos de partículas de alta energia e campos magnéticos a velocidades próximas da velocidade da luz.
Observações feitas no comprimento de onda das micro-ondas são essenciais para explorar esses jatos, já que os elétrons de alta energia se movendo em campos magnéticos são mais proficientes em produzir micro-ondas. Mas a maioria das galáxias ativas com jatos brilhantes estão a bilhões de anos-luz de distância da Terra, de modo que esses jatos são minúsculos no céu. Desse modo a alta resolução é essencial para observar esses jatos em ação e então revelar fenômenos como as ondas de choque e a turbulência que controla o quanto de luz é produzida num dado tempo. “Combinando pela primeira vez rádio telescópios na Terra com rádio telescópios no espaço, operando na máxima resolução, tem permitido que a nossa equipe crie uma antena que tem um tamanho equivalente a 8 vezes o diâmetro da Terra, correspondendo a 20 micro arcos de segundo”, disse José L; Gómez, o líder da equipe no Instituto de Astrofísica de Andalucía, IAA-CSIC.
The atacama cosmology_telescope_measuring_radio_galaxy_bias_through_cross_cor...Sérgio Sacani
A radiação cósmica de micro-ondas aponta para a matéria escura invisível, marcando o ponto onde jatos de material viajam a velocidades próximas da velocidade da luz, de acordo com uma equipe internacional de astrônomos. O principal autor do estudo, Rupert Allison da Universidade de Oxford apresentou os resultados no dia 6 de Julho de 2015 no National Astronomy Meeting em Venue Cymru, em Llandudno em Wales.
Atualmente, ninguém sabe ao certo do que a matéria escura é feita, mas ela é responsável por cerca de 26% do conteúdo de energia do universo, com galáxias massivas se formando em densas regiões de matéria escura. Embora invisível, a matéria escura se mostra através do efeito gravitacional – uma grande bolha de matéria escura puxa a matéria normal (como elétrons, prótons e nêutrons) através de sua própria gravidade, eventualmente se empacotando conjuntamente para criar as estrelas e galáxias inteiras.
Muitas das maiores dessas são galáxias ativas com buracos negros supermassivos em seus centros. Alguma parte do gás caindo diretamente na direção do buraco negro é ejetada como jatos de partículas e radiação. As observações feitas com rádio telescópios mostram que esses jatos as vezes se espalham por milhões de anos-luz desde a galáxia – mais distante até mesmo do que a extensão da própria galáxia.
Os cientistas esperam que os jatos possam viver em regiões onde existe um excesso de concentração da matéria escura, maior do que o da média. Mas como a matéria escura é invisível, testar essa ideia não é algo tão direto.
WHERE IS THE FLUX GOING? THE LONG-TERM PHOTOMETRIC VARIABILITY OF BOYAJIAN’S ...Sérgio Sacani
We present ∼ 800 days of photometric monitoring of Boyajian’s Star (KIC 8462852) from the AllSky
Automated Survey for Supernovae (ASAS-SN) and ∼ 4000 days of monitoring from the All Sky
Automated Survey (ASAS). We show that from 2015 to the present the brightness of Boyajian’s Star
has steadily decreased at a rate of 6.3 ± 1.4 mmag yr−1
, such that the star is now 1.5% fainter than it
was in February 2015. Moreover, the longer time baseline afforded by ASAS suggests that Boyajian’s
Star has also undergone two brightening episodes in the past 11 years, rather than only exhibiting a
monotonic decline. We analyze a sample of ∼ 1000 comparison stars of similar brightness located in
the same ASAS-SN field and demonstrate that the recent fading is significant at & 99.4% confidence.
The 2015 − 2017 dimming rate is consistent with that measured with Kepler data for the time period
from 2009 to 2013. This long-term variability is difficult to explain with any of the physical models
for the star’s behavior proposed to date
We present spectroscopic observations of the nearby dwarf galaxy AGC 198691. This object is part
of the Survey of H I in Extremely Low-Mass Dwarfs (SHIELD) project, which is a multi-wavelength
study of galaxies with H I masses in the range of 106-107:2 M discovered by the ALFALFA survey.
We have obtained spectra of the lone H II region in AGC 198691 with the new high-throughput
KPNO Ohio State Multi-Object Spectrograph (KOSMOS) on the Mayall 4-m as well as with the Blue
Channel spectrograph on the MMT 6.5-m telescope. These observations enable the measurement of the
temperature-sensitive [O III]4363 line and hence the determination of a \direct" oxygen abundance
for AGC 198691. We nd this system to be an extremely metal-decient (XMD) system with an
oxygen abundance of 12+log(O/H) = 7.02 0.03, making AGC 198691 the lowest-abundance starforming
galaxy known in the local universe. Two of the ve lowest-abundance galaxies known have
been discovered by the ALFALFA blind H I survey; this high yield of XMD galaxies represents a
paradigm shift in the search for extremely metal-poor galaxies.
Imaging the dust_sublimation_front_of_a_circumbinary_diskSérgio Sacani
Aims. We present the first near-IR milli-arcsecond-scale image of a post-AGB binary that is surrounded by hot circumbinary dust.
Methods. A very rich interferometric data set in six spectral channels was acquired of IRAS 08544-4431 with the new RAPID camera
on the PIONIER beam combiner at the Very Large Telescope Interferometer (VLTI). A broadband image in the H-band was reconstructed
by combining the data of all spectral channels using the SPARCO method.
Results. We spatially separate all the building blocks of the IRAS 08544-4431 system in our milliarcsecond-resolution image. Our
dissection reveals a dust sublimation front that is strikingly similar to that expected in early-stage protoplanetary disks, as well as an
unexpected flux signal of 4% from the secondary star. The energy output from this companion indicates the presence of a compact
circum-companion accretion disk, which is likely the origin of the fast outflow detected in H.
Conclusions. Our image provides the most detailed view into the heart of a dusty circumstellar disk to date. Our results demonstrate
that binary evolution processes and circumstellar disk evolution can be studied in detail in space and over time.
A giant galaxy in the young Universe with a massive ringSérgio Sacani
In the local (redshift z ≈ 0) Universe, collisional ring galaxies make up only ~0.01% of galaxies1 and are formed by head-on galactic collisions that trigger radially propagating density waves2–4. These striking systems provide key snapshots for dissecting galactic disks and are studied extensively in the local Universe5–9. However, not much is known about distant (z > 0.1) collisional rings10–14. Here we present a detailed study of a ring galaxy at a look-back time of 10.8 Gyr (z = 2.19). Compared with our Milky Way, this galaxy has a similar stellar mass, but has a stellar half-light radius that is 1.5–2.2 times larger and is forming stars 50 times faster. The extended, dif- fuse stellar light outside the star-forming ring, combined with a radial velocity on the ring and an intruder galaxy nearby, provides evidence for this galaxy hosting a collisional ring. If the ring is secularly evolved15,16, the implied large bar in a giant disk would be inconsistent with the current understand- ing of the earliest formation of barred spirals17–21. Contrary to previous predictions10–12, this work suggests that massive col- lisional rings were as rare 11 Gyr ago as they are today. Our discovery offers a unique pathway for studying density waves in young galaxies, as well as constraining the cosmic evolution of spiral disks and galaxy groups.
The completeness-corrected rate of stellar encounters with the Sun from the f...Sérgio Sacani
I report on close encounters of stars to the Sun found in the first Gaia data release (GDR1). Combining Gaia astrometry with radial
velocities of around 320 000 stars drawn from various catalogues, I integrate orbits in a Galactic potential to identify those stars which
come within a few parsecs. Such encounters could influence the solar system, for example through gravitational perturbations of the
Oort cloud. 16 stars are found to come within 2 pc (although a few of these have dubious data). This is fewer than were found in a
similar study based on Hipparcos data, even though the present study has many more candidates. This is partly because I reject stars
with large radial velocity uncertainties (>10 km s−1
), and partly because of missing stars in GDR1 (especially at the bright end). The
closest encounter found is Gl 710, a K dwarf long-known to come close to the Sun in about 1.3 Myr. The Gaia astrometry predict
a much closer passage than pre-Gaia estimates, however: just 16 000 AU (90% confidence interval: 10 000–21 000 AU), which will
bring this star well within the Oort cloud. Using a simple model for the spatial, velocity, and luminosity distributions of stars, together
with an approximation of the observational selection function, I model the incompleteness of this Gaia-based search as a function
of the time and distance of closest approach. Applying this to a subset of the observed encounters (excluding duplicates and stars
with implausibly large velocities), I estimate the rate of stellar encounters within 5 pc averaged over the past and future 5 Myr to be
545±59 Myr−1
. Assuming a quadratic scaling of the rate within some encounter distance (which my model predicts), this corresponds
to 87 ± 9 Myr−1 within 2 pc. A more accurate analysis and assessment will be possible with future Gaia data releases.
Direct Measure of Radiative And Dynamical Properties Of An Exoplanet AtmosphereSérgio Sacani
Two decades after the discovery of 51Pegb, the formation processes and atmospheres of short-period gas giants
remain poorly understood. Observations of eccentric systems provide key insights on those topics as they can
illuminate how a planet’s atmosphere responds to changes in incident flux. We report here the analysis of multi-day
multi-channel photometry of the eccentric (e ~ 0.93) hot Jupiter HD80606b obtained with the Spitzer Space
Telescope. The planet’s extreme eccentricity combined with the long coverage and exquisite precision of new
periastron-passage observations allow us to break the degeneracy between the radiative and dynamical timescales
of HD80606b’s atmosphere and constrain its global thermal response. Our analysis reveals that the atmospheric
layers probed heat rapidly (∼4 hr radiative timescale) from<500 to 1400 K as they absorb ~20% of the incoming
stellar flux during the periastron passage, while the planet’s rotation period is 93 35
85
-
+ hr, which exceeds the predicted
pseudo-synchronous period (40 hr).
Key words: methods: numerical – planet–star interactions – planets and satellites: atmospheres – planets and
satellites: dynamical evolution and stability – planets and satellites: individual (HD 80606 b) – techniques:
photometric
An Earth-sized exoplanet with a Mercury-like compositionSérgio Sacani
Earth, Venus, Mars and some extrasolar terrestrial planets1
have a mass and radius that is consistent with a mass fraction
of about 30% metallic core and 70% silicate mantle2
. At the
inner frontier of the Solar System, Mercury has a completely
different composition, with a mass fraction of about 70%
metallic core and 30% silicate mantle3
. Several formation or
evolution scenarios are proposed to explain this metal-rich
composition, such as a giant impact4, mantle evaporation5
or the depletion of silicate at the inner edge of the protoplanetary
disk6. These scenarios are still strongly debated.
Here, we report the discovery of a multiple transiting planetary
system (K2-229) in which the inner planet has a radius
of 1.165 ± 0.066 Earth radii and a mass of 2.59 ± 0.43 Earth
masses. This Earth-sized planet thus has a core-mass fraction
that is compatible with that of Mercury, although it was
expected to be similar to that of Earth based on host-star
chemistry7
. This larger Mercury analogue either formed with
a very peculiar composition or has evolved, for example, by
losing part of its mantle. Further characterization of Mercurylike
exoplanets such as K2-229 b will help to put the detailed
in situ observations of Mercury (with MESSENGER and
BepiColombo8) into the global context of the formation and
evolution of solar and extrasolar terrestrial planets.
Confirmation of the_ogle_planet_signature_and_its_characteristics_with_lens_s...Sérgio Sacani
O Telescópio Espacial Hubble e o Observatório W. M. Keck, no Havaí, fizeram confirmações independentes de um exoplaneta orbitando sua estrela central de uma distância bem grande. O planeta foi descoberto através de uma técnica chamada de microlente gravitacional.
Essa descoberta traz uma nova peça para o processo de caçada de exoplanetas: para descobrir planetas longe de suas estrelas, como Júpiter e Saturno estão do Sol. Os resultados obtidos pelo Hubble e pelo Keck apareceram em dois artigos da edição de 30 de Julho de 2015 do The Astrophysical Journal.
A grande maioria dos exoplanetas catalogados são aqueles localizados bem perto de suas estrelas, isso acontece porque as técnicas atuais de se caçar exoplanetas favorecem a descoberta de planetas com curtos períodos orbitais. Mas esse não é o caso da técnica de microlente gravitacional, que pode encontrar planetas mais frios e mais distantes com órbitas de longo período que outros métodos não são capazes de detectar.
Matter ejections behind the highs and lows of the transitional millisecond pu...Sérgio Sacani
Transitional millisecond pulsars are an emerging class of sources linking low-mass X-ray binaries to millisecond radio pulsars in
binary systems. These pulsars alternate between a radio pulsar state and an active low-luminosity X-ray disc state. During the active
state, these sources exhibit two distinct emission modes (high and low) that alternate unpredictably, abruptly, and incessantly. X-ray
to optical pulsations are observed only during the high mode. Knowledge of the root reason for this puzzling behaviour remains
elusive. This paper presents the results of the most extensive multi-wavelength campaign ever conducted on the transitional pulsar
prototype, PSR J1023+0038, covering from radio to X-rays. The campaign was carried out over two nights in June 2021, and involved
12 different telescopes and instruments including XMM-Newton, HST, VLT/FORS2 (in polarimetric mode), ALMA, VLA and FAST.
By modelling the broadband spectral energy distributions in both emission modes, we show that the mode switches are caused by
changes in the innermost region of the accretion disc. These changes trigger the emission of discrete mass ejections, which occur on
top of a compact jet, as testified by the detection of at least one short-duration millimetre flare with A
The Possible Tidal Demise of Kepler’s First Planetary SystemSérgio Sacani
We present evidence of tidally-driven inspiral in the Kepler-1658 (KOI-4) system, which consists of a giant planet
(1.1RJ, 5.9MJ) orbiting an evolved host star (2.9Re, 1.5Me). Using transit timing measurements from Kepler,
Palomar/WIRC, and TESS, we show that the orbital period of Kepler-1658b appears to be decreasing at a rate = -
+ P 131 22
20 ms yr−1
, corresponding to an infall timescale P P » 2.5 Myr. We consider other explanations for the
data including line-of-sight acceleration and orbital precession, but find them to be implausible. The observed
period derivative implies a tidal quality factor
¢ = ´ -
+ Q 2.50 10 0.62
0.85 4, in good agreement with theoretical
predictions for inertial wave dissipation in subgiant stars. Additionally, while it probably cannot explain the entire
inspiral rate, a small amount of planetary dissipation could naturally explain the deep optical eclipse observed for
the planet via enhanced thermal emission. As the first evolved system with detected inspiral, Kepler-1658 is a new
benchmark for understanding tidal physics at the end of the planetary life cycle
Confirmation of the_planetary_microlensing_signal_and_star_and_planet_mass_de...Sérgio Sacani
O Telescópio Espacial Hubble e o Observatório W. M. Keck, no Havaí, fizeram confirmações independentes de um exoplaneta orbitando sua estrela central de uma distância bem grande. O planeta foi descoberto através de uma técnica chamada de microlente gravitacional.
Essa descoberta traz uma nova peça para o processo de caçada de exoplanetas: para descobrir planetas longe de suas estrelas, como Júpiter e Saturno estão do Sol. Os resultados obtidos pelo Hubble e pelo Keck apareceram em dois artigos da edição de 30 de Julho de 2015 do The Astrophysical Journal.
A grande maioria dos exoplanetas catalogados são aqueles localizados bem perto de suas estrelas, isso acontece porque as técnicas atuais de se caçar exoplanetas favorecem a descoberta de planetas com curtos períodos orbitais. Mas esse não é o caso da técnica de microlente gravitacional, que pode encontrar planetas mais frios e mais distantes com órbitas de longo período que outros métodos não são capazes de detectar.
The independent pulsations of Jupiter’s northern and southern X-ray aurorasSérgio Sacani
Auroral hot spots are observed across the Universe at different
scales1
and mark the coupling between a surrounding
plasma environment and an atmosphere. Within our own
Solar System, Jupiter possesses the only resolvable example
of this large-scale energy transfer. Jupiter’s northern X-ray
aurora is concentrated into a hot spot, which is located at the
most poleward regions of the planet’s aurora and pulses either
periodically2,3
or irregularly4,5
. X-ray emission line spectra
demonstrate that Jupiter’s northern hot spot is produced by
high charge-state oxygen, sulfur and/or carbon ions with an
energy of tens of MeV (refs 4–6) that are undergoing charge
exchange. Observations instead failed to reveal a similar
feature in the south2,3,7,8. Here, we report the existence of a
persistent southern X-ray hot spot. Surprisingly, this largescale
southern auroral structure behaves independently of its
northern counterpart. Using XMM-Newton and Chandra X-ray
campaigns, performed in May–June 2016 and March 2007, we
show that Jupiter’s northern and southern spots each exhibit
different characteristics, such as different periodic pulsations
and uncorrelated changes in brightness. These observations
imply that highly energetic, non-conjugate magnetospheric
processes sometimes drive the polar regions of Jupiter’s dayside
magnetosphere. This is in contrast to current models of
X-ray generation for Jupiter9,10. Understanding the behaviour
and drivers of Jupiter’s pair of hot spots is critical to the use
of X-rays as diagnostics of the wide range of rapidly rotating
celestial bodies that exhibit these auroral phenomena.
Spirals and clumps in V960 Mon: signs of planet formation via gravitational i...Sérgio Sacani
The formation of giant planets has traditionally been divided into two pathways: core accretion and gravitational instability. However, in recent years, gravitational instability has become less favored, primarily due
to the scarcity of observations of fragmented protoplanetary disks around young stars and low occurrence rate
of massive planets on very wide orbits. In this study, we present a SPHERE/IRDIS polarized light observation
of the young outbursting object V960 Mon. The image reveals a vast structure of intricately shaped scattered
light with several spiral arms. This finding motivated a re-analysis of archival ALMA 1.3 mm data acquired
just two years after the onset of the outburst of V960 Mon. In these data, we discover several clumps of continuum emission aligned along a spiral arm that coincides with the scattered light structure. We interpret the
localized emission as fragments formed from a spiral arm under gravitational collapse. Estimating the mass of
solids within these clumps to be of several Earth masses, we suggest this observation to be the first evidence of
gravitational instability occurring on planetary scales. This study discusses the significance of this finding for
planet formation and its potential connection with the outbursting state of V960 Mon.
Detection of an atmosphere around the super earth 55 cancri eSérgio Sacani
We report the analysis of two new spectroscopic observations of the super-Earth 55 Cancri e, in the near
infrared, obtained with the WFC3 camera onboard the HST. 55 Cancri e orbits so close to its parent
star, that temperatures much higher than 2000 K are expected on its surface. Given the brightness
of 55 Cancri, the observations were obtained in scanning mode, adopting a very long scanning length
and a very high scanning speed. We use our specialized pipeline to take into account systematics
introduced by these observational parameters when coupled with the geometrical distortions of the
instrument. We measure the transit depth per wavelength channel with an average relative uncertainty
of 22 ppm per visit and nd modulations that depart from a straight line model with a 6 condence
level. These results suggest that 55 Cancri e is surrounded by an atmosphere, which is probably
hydrogen-rich. Our fully Bayesian spectral retrieval code, T -REx, has identied HCN to be the
most likely molecular candidate able to explain the features at 1.42 and 1.54 m. While additional
spectroscopic observations in a broader wavelength range in the infrared will be needed to conrm
the HCN detection, we discuss here the implications of such result. Our chemical model, developed
with combustion specialists, indicates that relatively high mixing ratios of HCN may be caused by a
high C/O ratio. This result suggests this super-Earth is a carbon-rich environment even more exotic
than previously thought.
Magnetic field and_wind_of_kappa_ceti_towards_the_planetary_habitability_of_t...Sérgio Sacani
We report magnetic field measurements for κ
1 Cet, a proxy of the young Sun when life arose on Earth. We carry out an analysis
of the magnetic properties determined from spectropolarimetric observations and reconstruct its large-scale surface magnetic
field to derive the magnetic environment, stellar winds and particle flux permeating the interplanetary medium around κ
1 Cet.
Our results show a closer magnetosphere and mass-loss rate of M˙ = 9.7 × 10−13 M yr−1
, i.e., a factor 50 times larger than the
current solar wind mass-loss rate, resulting in a larger interaction via space weather disturbances between the stellar wind and
a hypothetical young-Earth analogue, potentially affecting the planet’s habitability. Interaction of the wind from the young Sun
with the planetary ancient magnetic field may have affected the young Earth and its life conditions.
Artigo descreve a descoberta de um sistema de anéis 200 vezes maior do que o sistema de anéis de Saturno num exoplaneta orbitando a jovem estrela J1407
A higher efficiency_of_converting_gas_to_stars_push_galaxies_at_z_1_6_well_ab...Sérgio Sacani
Galáxias formando estrelas em taxas extremas a nove bilhões de anos atrás eram mais eficientes do que a média das galáxias atuais, descobriram os pesquisadores.
A maioria das estrelas acredita-se localizam-se na sequência principal onde quanto maior a massa da galáxia, mais eficiente ela é na formação de novas estrelas. Contudo, de vez em quando uma galáxia apresentará uma explosão de novas estrelas que brilham mais do que o resto. Uma colisão entre duas grandes galáxias é normalmente a causa dessas fases de explosões de formação de estrelas, onde o gás frio que reside nas grandes nuvens moleculares torna-se o combustível para sustentar essas altas taxas de formação de estrelas.
A questão que os astrônomos têm feito é se essas explosões de estrelas no início o universo foram o resultado de se ter um suprimento de gás abundante, ou se as galáxias convertiam o gás de maneira mais eficiente.
Um novo estudo, publicado no Astrophysical Journal Letters de 15 de Outubro, liderado por John Silverman, do Kavli Institute for Physics and Mathematics of the Universe, estudou o conteúdo do gás monóxido de carbono (CO) em sete galáxias de explosão de estrelas muito distantes, quando o universo tinha apenas 4 bilhões de anos de vida. Isso foi possível devido a capacidade do Atacama Large Millimiter/Submillimiter Array (ALMA), localizado no platô no topo da montanha no Chile, que trabalha para detectar as ondas eletromagnéticas no comprimento de onda milimétrico (importante para se estudar o gás molecular) e um nível de sensibilidade que só agora começa a ser explorado pelos astrônomos.
Os pesquisadores descobriram que a quantidade de gás CO emitido já tinha diminuído, mesmo apesar da galáxia continuar a formar estrelas em altas taxas. Essas observações são similares àquelas registradas para as galáxias de explosões de estrelas próximas da Terra atualmente, mas a quantidade da depleção de gás não foi tão rápida quanto se esperava. Isso levou os pesquisadores a concluírem que poderia haver um contínuo aumento na eficiência, dependendo em de quanto acima da taxa de se formar estrelas ela está da sequência principal.
A surge of light at the birth of a supernovaSérgio Sacani
It is difficult to establish the properties of massive stars that explode
as supernovae1,2
. The electromagnetic emission during the first
minutes to hours after the emergence of the shock from the stellar
surface conveys important information about the final evolution
and structure of the exploding star3–6. However, the unpredictable
nature of supernova events hinders the detection of this brief initial
phase7–9. Here we report the serendipitous discovery of a newly
born, normal type IIb supernova (SN 2016gkg)10, which reveals a
rapid brightening at optical wavelengths of about 40 magnitudes
per day. The very frequent sampling of the observations allowed
us to study in detail the outermost structure of the progenitor of
the supernova and the physics of the emergence of the shock. We
develop hydrodynamical models of the explosion that naturally
account for the complete evolution of the supernova over distinct
phases regulated by different physical processes. This result
suggests that it is appropriate to decouple the treatment of the
shock propagation from the unknown mechanism that triggers
the explosion.
Artigo que descreve a descoberta do exoplaneta Kepler-432b, um exoplaneta mais massivo que Júpiter que orbita uma estrela gigante vermelha bem próximo e numa órbita extremamente alongada.
Similar to The Internal Structure of Asteroid (25143) Itokawa as Revealed by Detection of YORP Spin-up (20)
Operation “Blue Star” is the only event in the history of Independent India where the state went into war with its own people. Even after about 40 years it is not clear if it was culmination of states anger over people of the region, a political game of power or start of dictatorial chapter in the democratic setup.
The people of Punjab felt alienated from main stream due to denial of their just demands during a long democratic struggle since independence. As it happen all over the word, it led to militant struggle with great loss of lives of military, police and civilian personnel. Killing of Indira Gandhi and massacre of innocent Sikhs in Delhi and other India cities was also associated with this movement.
The French Revolution, which began in 1789, was a period of radical social and political upheaval in France. It marked the decline of absolute monarchies, the rise of secular and democratic republics, and the eventual rise of Napoleon Bonaparte. This revolutionary period is crucial in understanding the transition from feudalism to modernity in Europe.
For more information, visit-www.vavaclasses.com
June 3, 2024 Anti-Semitism Letter Sent to MIT President Kornbluth and MIT Cor...Levi Shapiro
Letter from the Congress of the United States regarding Anti-Semitism sent June 3rd to MIT President Sally Kornbluth, MIT Corp Chair, Mark Gorenberg
Dear Dr. Kornbluth and Mr. Gorenberg,
The US House of Representatives is deeply concerned by ongoing and pervasive acts of antisemitic
harassment and intimidation at the Massachusetts Institute of Technology (MIT). Failing to act decisively to ensure a safe learning environment for all students would be a grave dereliction of your responsibilities as President of MIT and Chair of the MIT Corporation.
This Congress will not stand idly by and allow an environment hostile to Jewish students to persist. The House believes that your institution is in violation of Title VI of the Civil Rights Act, and the inability or
unwillingness to rectify this violation through action requires accountability.
Postsecondary education is a unique opportunity for students to learn and have their ideas and beliefs challenged. However, universities receiving hundreds of millions of federal funds annually have denied
students that opportunity and have been hijacked to become venues for the promotion of terrorism, antisemitic harassment and intimidation, unlawful encampments, and in some cases, assaults and riots.
The House of Representatives will not countenance the use of federal funds to indoctrinate students into hateful, antisemitic, anti-American supporters of terrorism. Investigations into campus antisemitism by the Committee on Education and the Workforce and the Committee on Ways and Means have been expanded into a Congress-wide probe across all relevant jurisdictions to address this national crisis. The undersigned Committees will conduct oversight into the use of federal funds at MIT and its learning environment under authorities granted to each Committee.
• The Committee on Education and the Workforce has been investigating your institution since December 7, 2023. The Committee has broad jurisdiction over postsecondary education, including its compliance with Title VI of the Civil Rights Act, campus safety concerns over disruptions to the learning environment, and the awarding of federal student aid under the Higher Education Act.
• The Committee on Oversight and Accountability is investigating the sources of funding and other support flowing to groups espousing pro-Hamas propaganda and engaged in antisemitic harassment and intimidation of students. The Committee on Oversight and Accountability is the principal oversight committee of the US House of Representatives and has broad authority to investigate “any matter” at “any time” under House Rule X.
• The Committee on Ways and Means has been investigating several universities since November 15, 2023, when the Committee held a hearing entitled From Ivory Towers to Dark Corners: Investigating the Nexus Between Antisemitism, Tax-Exempt Universities, and Terror Financing. The Committee followed the hearing with letters to those institutions on January 10, 202
Honest Reviews of Tim Han LMA Course Program.pptxtimhan337
Personal development courses are widely available today, with each one promising life-changing outcomes. Tim Han’s Life Mastery Achievers (LMA) Course has drawn a lot of interest. In addition to offering my frank assessment of Success Insider’s LMA Course, this piece examines the course’s effects via a variety of Tim Han LMA course reviews and Success Insider comments.
Welcome to TechSoup New Member Orientation and Q&A (May 2024).pdfTechSoup
In this webinar you will learn how your organization can access TechSoup's wide variety of product discount and donation programs. From hardware to software, we'll give you a tour of the tools available to help your nonprofit with productivity, collaboration, financial management, donor tracking, security, and more.
Francesca Gottschalk - How can education support child empowerment.pptxEduSkills OECD
Francesca Gottschalk from the OECD’s Centre for Educational Research and Innovation presents at the Ask an Expert Webinar: How can education support child empowerment?
Macroeconomics- Movie Location
This will be used as part of your Personal Professional Portfolio once graded.
Objective:
Prepare a presentation or a paper using research, basic comparative analysis, data organization and application of economic information. You will make an informed assessment of an economic climate outside of the United States to accomplish an entertainment industry objective.
Palestine last event orientationfvgnh .pptxRaedMohamed3
An EFL lesson about the current events in Palestine. It is intended to be for intermediate students who wish to increase their listening skills through a short lesson in power point.
Synthetic Fiber Construction in lab .pptxPavel ( NSTU)
Synthetic fiber production is a fascinating and complex field that blends chemistry, engineering, and environmental science. By understanding these aspects, students can gain a comprehensive view of synthetic fiber production, its impact on society and the environment, and the potential for future innovations. Synthetic fibers play a crucial role in modern society, impacting various aspects of daily life, industry, and the environment. ynthetic fibers are integral to modern life, offering a range of benefits from cost-effectiveness and versatility to innovative applications and performance characteristics. While they pose environmental challenges, ongoing research and development aim to create more sustainable and eco-friendly alternatives. Understanding the importance of synthetic fibers helps in appreciating their role in the economy, industry, and daily life, while also emphasizing the need for sustainable practices and innovation.
Home assignment II on Spectroscopy 2024 Answers.pdf
The Internal Structure of Asteroid (25143) Itokawa as Revealed by Detection of YORP Spin-up
1. c ESO 2014
Astronomy & Astrophysics manuscript no. Lowry-Final-1column
January 7, 2014
The Internal Structure of Asteroid (25143) Itokawa
as Revealed by Detection of YORP Spin-up
,
S. C. Lowry1 , P. R. Weissman2 , S. R. Duddy1 , B. Rozitis3 , A. Fitzsimmons4 , S. F.
Green3 , M. D. Hicks2 , C. Snodgrass5 , S. D. Wolters3 , S. R. Chesley2 , J. Pittichov´ 2 , and
a
P. van Oers6
1
Centre for Astrophysics and Planetary Science, School of Physical Sciences (SEPnet), The
University of Kent, Canterbury, CT2 7NH, UK
e-mail: s.c.lowry@kent.ac.uk
2
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
3
Planetary and Space Sciences, Department of Physical Sciences, The Open University, Milton
Keynes, MK7 6AA, UK
4
Astrophysics Research Centre, Queens University Belfast, Belfast, BT7 1NN, UK
5
Max Planck Institute for Solar System Research, Max-Planck-Str. 2, 37191 Katlenburg-Lindau,
Germany
6
Isaac Newton Group of Telescopes, E-38700 Santa Cruz de la Palma, Canary Islands, Spain
Received 03 Sep 2013 / Accepted 16 Dec 2013
ABSTRACT
Context. Near-Earth asteroid (25143) Itokawa was visited by the Hayabusa spacecraft in 2005,
resulting in a highly detailed shape and surface topography model. This model has led to several
predictions for the expected radiative torques on this asteroid, suggesting that its spin rate should
be decelerating.
Aims. To detect changes in rotation rate that may be due to YORP-induced radiative torques,
which in turn may be used to investigate the interior structure of the asteroid.
Methods. Through an observational survey spanning 2001 to 2013 we obtained rotational
lightcurve data at various times over the last five close Earth-approaches of the asteroid. We
applied a polyhedron-shape-modelling technique to assess the spin-state of the asteroid and its
long term evolution. We also applied a detailed thermophysical analysis to the shape model determined from the Hayabusa spacecraft.
Results. We have successfully measured an acceleration in Itokawa’s spin rate of dω/dt = (3.54 ±
0.38) × 10−8 rad day−2 , equivalent to a decrease of its rotation period of ∼ 45 ms year−1 . From the
thermophysical analysis we find that the center-of-mass for Itokawa must be shifted by ∼ 21 m
along the long-axis of the asteroid to reconcile the observed YORP strength with theory.
Conclusions. This can be explained if Itokawa is composed of two separate bodies with very
different bulk densities of 1750 ± 110 kg m−3 and 2850 ± 500 kg m−3 , and was formed from
the merger of two separate bodies, either in the aftermath of a catastrophic disruption of a larger
differentiated body, or from the collapse of a binary system. We therefore demonstrate that an
observational measurement of radiative torques, when combined with a detailed shape model,
1
Article published by EDP Sciences, to be cited as http://dx.doi.org/10.1051/0004-6361/201322602
2. S. C. Lowry et al.: YORP Spin-up Detected on Asteroid Itokawa
can provide insight into the interior structure of an asteroid. Futhermore, this is the first measurement of density inhomogeneity within an asteroidal body, that reveals significant internal
structure variation. A specialised spacecraft is normally required for this.
Key words. near-Earth asteroid – photometry – lightcurve inversion – YORP – ATPM
1. Introduction
Asteroid (25143) Itokawa is a relatively small near-Earth asteroid and its physical evolution is likely
to be strongly affected by the Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect, which is a
torque that can modify the rotation rates and spin-axis orientations of small bodies in the solar
system. It is caused by the recoil effect from the anisotropic reflection and emission of solar radiation and thermal photons, respectively (Rubincam 2000). This process is responsible for many
observed phenomena in asteroid science (Slivan 2002; Vokrouhlick´ et al. 2003; Ostro et al. 2006;
y
Vokrouhlick´ & Nesvorn´ 2008; Pravec et al. 2010), and was detected on the very small, fast spiny
y
ning near-Earth asteroid (54509) YORP (Lowry et al. 2007; Taylor et al. 2007). The effect has also
ˇ
been detected on asteroids (1862) Apollo and (1620) Geographos (Kaasalainen et al. 2007; Durech
ˇ
et al. 2008a), and with a tentative detection on asteroid (3103) Eger (Durech et al. 2012).
Itokawa is an important target for the study of the YORP effect as we can apply state-of-the-art
thermophysical modelling to the detailed spacecraft shape model (Saito et al. 2006), to determine
the expected YORP strength for the asteroid given its current orbital and spin-state properties.
If the observed angular acceleration cannot be reconciled with theoretical predictions, then we
can begin to explore other causes for the discrepancy. This may include inhomogeneous mass
distributions within the body or non-uniform surface roughness, thus placing valuable constraints
thereon (Scheeres & Gaskell 2008). The potential for using an observed measurement of YORP to
reliably probe the interior structure of an asteroid is unique among remote-observing techniques
and analysis methods.
Here we present results and analysis from a long-term photometric monitoring programme
designed to detect changes in rotation rate that may be due to YORP. The structure of the paper
is as follows: Sections 2 and 3 describe the observational data that was acquired and how the
rotational lightcurves were extracted and analysed to detect YORP accelerations. In Section 4 we
present our thermophysical analysis as applied to the detailed spacecraft shape model, in order to
provide a comparision of our results with theory. Section 5 provides a general discussion of the
results, and their implications.
2. Optical Observations, and Lightcurve Extraction & Modelling
We monitored Itokawa between August 2001 and January 2013, using ground-based optical telescopes in Chile, the US and Europe (Table 1). The asteroid was observed at 10 different epochs for
1-3 nights each time (LC1-10). On each occasion time-series optical CCD imaging was obtained
in either the broadband V or R filters. The aspect angle (angle between observer line-of-sight and
Based in part on observations collected at the European Southern Observatory, Chile, under programme
ID: 185.C-1033.
Table 2 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr
(130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/
2
3. S. C. Lowry et al.: YORP Spin-up Detected on Asteroid Itokawa
the known rotation axis of the asteroid) changed little during the entire monitoring period, which
can help to reduce measurement uncertainties in any detection of YORP. LC1-4 were included in a
ˇ
previous inconclusive attempt to detect YORP on Itokawa (Durech et al. 2008b). We followed up
with new observations from 2009-2013 (LC5-10).
Bias subtractions and flat-fielding were performed in the usual manner. After this initial processing the images were then co-added to increase the signal-to-noise ratio and thus the quality
of the extracted lightcurves. The images were aligned so that the background stars in each image
appeared stationary in the field of view. These aligned images were then shifted according to the
apparent rates of motion of the asteroid in order to produce a series of images in which the asteroid appeared stationary in the field. The images in each set were then co-added in groups. In the
case of the data from the Isaac Newton Telescope (DS9 in Table 1), groups of twelve images were
combined, while the group size was just four for the New Technology Telescope data (DS8). In
general, the group size was kept small enough so that the total exposure time was less than 5% of
the rotation period of the asteroid or ∼2100 s, but large enough to produce an appreciable increase
in the quality of the extracted lightcurve.
Aperture photometry was then performed to measure the brightness of the asteroid relative to
background stars of constant brightness. The FWHM of the seeing profile in each image was used
to set the radius of the photometry aperture for the asteroid. This was normally measured using
the background stars, although in some cases these were significantly trailed due to long exposure times and the use of non-sidereal tracking. In this case, the FWHM of the seeing profile was
measured directly from the asteroid. Where trailing of background stars was evident, rectangular
photometry apertures were preferred for the comparison stars. This minimizes any sky-background
contribution, which can reduce the quality of the photometry. The rectangular apertures were centred on the middle of the star trail, and their length and direction in the image were calculated from
the exposure time and rates of motion of the asteroid. The width of the aperture is set to be equal
to the FWHM of the seeing profile measured from the asteroid. This method improves the quality
of the extracted lightcurves, especially in those cases where the background stars might be faint.
Instrumental magnitudes for the background comparison stars were measured and a weighted average taken to ensure that variations in stellar brightness for the fainter stars had minimal affect on
the asteroid rotational lightcurves.
2.1. Model-Lightcurve Generation from Polyhedron Shape Models
Upon extraction of the rotational lightcurves, each datapoint was light-time corrected. In the case
of Itokawa this was typically on the order of several minutes. This step is crucial to ensure accurate
measurements of the rotational phase offset between the artificial and observed lightcurves, as an
error of one minute can introduce an uncertainty of 0.5 degrees in rotation phase for this asteroid.
The light-travel time, the direction vectors of the asteroid from the sun and the observer, and the
topocentric positions, were calculated using the JPL HORIZONS online system.
Artificial lightcurves were generated using a convex hull of the Itokawa shape model developed
by Gaskell (2008). The Gaskell model consists of several hundred thousand facets and is highly
detailed. However, since the rotational lightcurve is the result of changes in the area projected
towards the observer, the extreme detail of this model is unnecessary. Therefore the model was
3
4. S. C. Lowry et al.: YORP Spin-up Detected on Asteroid Itokawa
ˇ
scaled down to a convex hull of only 2436 facets as has been done in previous studies (Durech
et al. 2008a), and is quite sufficient for our purposes. This has the advantage that it significantly
speeds up the generation of the artificial lightcurves.
The pole orientation of the asteroid as measured by the spacecraft (Demura et al. 2006) was
used to model its rotation in space. The rotational phase θ of the shape model is calculated for each
data point using,
θ = ω0 (t − T 0 ) + (ν/2)(t − T 0 )2
(1)
where ω0 is the initial angular velocity of the asteroid, t is the time of the observation, T 0 is
a fixed arbitrary time related to the initial orientation of the asteroid, and ν (= dω/dt) is the rate
of change of angular velocity with time or the YORP strength or acceleration. A ray-tracing algorithm was used to determine the angles between each facet normal and both the Sun and observer.
The scattering model employed was a simple combination of a Lambert surface and the LommelSeeliger model (Kaasalainen et al. 2001). The flux from each facet was then summed to produce
the expected brightness of the asteroid for each datapoint and converted to a magnitude.
The artificial and observed lightcurves are then placed on the same relative scale. This was
achieved by first subtracting the average brightness from the artificial and observed lightcurves
so that the amplitudes oscillated about zero magnitudes. Secondly, a range of small vertical shifts
was applied to the artificial lightcurves and the χ2 difference between the artificial and observed
lightcurves calculated. This metric was employed throughout the analysis to determine the quality
of the fit between the artificial and observed lightcurves. The vertical shift in brightness corresponding to the lowest χ2 value was then applied to ensure that the lightcurves had the same brightness
scale.
We then determine the initial orientation of the asteroid in space. As described earlier this
is related to the parameter T 0 . T 0 can be assigned arbitrarily and an additional rotation applied
to the shape model. However, for the purposes of our analysis, T 0 was assigned such that the
artificial lightcurves were perfectly aligned with the lightcurves observed in August and September
of 2001 (i.e. LC1+2 in Table 1). This was done by creating artificial lightcurves for the August and
September 2001 data using a range of T 0 values separated by approximately half-degree intervals.
The best T 0 was found to be 2452143.4815 (on August 21st, 2001 UT). All subsequent models were
advanced from this initial T 0 . We can then incorporate a constant rotation period for the model, or
a rotation period that is varying linearly with time.
3. Measuring the Observed YORP Strength
We adopted two different strategies for detecting and measuring YORP from the observational
lightcurve data. The first involves the measurement of rotation phase offsets between the observed
lightcurves and artificial lightcurves generated using the Hayabusa shape model with a fixed sidereal rotation period. A linear increase in rotation rate due to YORP causes a quadratic increase in
rotational phase offset φ with time t,
φ = (ν/2)(t − T 0 )2 + (ω0 + )(t − T 0 )
(2)
where is the difference between the estimated rotation rate and the actual rotation rate ω0 , at
time zero T 0 . Any uncertainty in the initial rotation rate used to advance the model - that is any
4
5. S. C. Lowry et al.: YORP Spin-up Detected on Asteroid Itokawa
non-zero value for
- introduces a linear increase in phase offset with time, easily distinguished
from the quadratic phase offset progression due to YORP. This is an iterative process that also
allows us to determine any YORP acceleration or deceleration that may be present. We chose
as our starting conditions an initial rotation period of 12.13245 hours, consistent with previous
ˇ
studies (Durech et al. 2008b). Once the artificial lightcurves were generated for each observed
lightcurve with this starting period, we measured the rotational phase offsets between them. This
was done for each lightcurve separately, by applying an incremental phase offset to the artificial
lightcurve and calculating the χ2 value. We sweep through a suitable range of phase offsets until
the χ2 value is minimized. The error-bars, as listed in column 9 of Table 1, are the formal 1σ uncertainties from the χ2 fitting process. Measurement of the phase offsets indicated a YORP
strength of (3.28±0.49) × 10−8 rad day−2 and an initial rotation period of 12.13237 hours. Repeating
this procedure with the new input rotation period produced a YORP strength of (3.19±0.41) × 10−8
rad day−2 and an initial rotation period of 12.132369 hours. We repeated the procedure until no
further significant variation was observed. The final YORP strength measured via this method was
(3.27 ± 0.29) × 10−8 rad day−2 with an initial rotation period at T 0 of 12.132371 ± (6 × 10−6 )
hours (Fig. 1). Previous analysis had suggested that a fixed rotation-period model fitted all data
ˇ
between 2001 and 2008 (Durech et al. 2008b). The fixed-period model fit the data reasonably well
until 2009 when a significant offset in phase between the artificial and observed lightcurves became
very clear (Fig. 1 and 2). This offset in phase increased further between 2009 and 2013 indicating
that the rotation rate was not fixed but was changing linearly with time, completely consistent with
YORP.
The second procedure involves producing artificial lightcurves over a large grid of initial sidereal rotation periods and YORP values and measuring the χ2 value at each iteration, i.e. we allow
the shape model rotation rate to change linearly with time. In this way we can determine the relationship between the initial rotation period used to advance the model and the observed angular
acceleration. We conducted a search over the rotation period range 12.13238 ± 10−4 hours in intervals of 2.5 × 10−8 hours and with various YORP strengths in the range (5 ± 5) × 10−8 rad day−2 ,
and at intervals of 10−11 rad day−2 . Our best-fit parameters found with this method are 12.1323789
(± 4.7 × 10−6 ) hours for a YORP strength of (3.81 ± 0.24) × 10−8 rad day−2 (see Fig. 3). This is
consistent with the first method above at the 1-σ level.
For the subsequent analysis we adopt the average YORP value from the two methods of ν =
(3.54 ± 0.38) × 10−8 rad day−2 , which is equivalent to a decrease of Itokawa’s rotation period of
45.4 ± 4.9 ms year−1 .
4. Thermophysical Analysis and Measured Density Inhomogeneity
The observed rotational acceleration (i.e. YORP spin-up) is contrary to previous theoretical YORP
studies, which predict strong rotational deceleration acting on Itokawa (i.e. YORP spin-down).
In particular, studies based on the Hayabusa-derived shape models predicted rotational accelerations (Scheeres et al. 2007; Breiter, et al. 2009) of (-5.5 to -2.0) × 10−7 rad day−2 , which differ
significantly from our observed value of 3.54 × 10−8 rad day−2 . These predictions were also inconsistent with an upper limit of |ν| < 1.5 × 10−7 rad day−2 that was derived from existing light-curve
ˇ
observations in 2008 (Durech et al. 2008b). To explain the inconsistency, it was suggested that
5
6. S. C. Lowry et al.: YORP Spin-up Detected on Asteroid Itokawa
a non-uniform internal mass distribution that shifted the center-of-mass (COM) away from the
‘center-of-figure’ towards the ‘head’ of Itokawa could be a possible cause (Scheeres & Gaskell
2008). Other theoretical work indicated that the YORP effect can be extremely sensitive to unresolved shape features (Statler 2009), the shape model resolution (Breiter, et al. 2009), and surface
roughness (Rozitis & Green 2012), such that the error in any prediction could be very large.
We determined a theoretical YORP value for this asteroid by applying the Advanced
Thermophysical Model (ATPM) (Rozitis & Green 2011, 2012, 2013) to the 49,152-facet spacecraft shape model of Itokawa (Gaskell 2008). Assuming a moment of inertia, IZ , of 7.77 × 1014
kg m2 (Breiter, et al. 2009) along with a moderately rough surface at cm scales (Ostro et al. 2004;
M¨ ller et al. 2005) with a uniform spatial distribution, the ATPM predicts a rotational acceleration
u
of -1.80 × 10−7 rad day−2 , consistent with previous determinations. A COM shift can reconcile
our YORP model with the observed value, which we can determine by combining the ATPM with
the methodology used for calculating such COM offsets (Scheeres & Gaskell 2008). Possible new
locations of the COM in Itokawa’s x − y plane exist along lines defined by
TCM = TCF − FY ∆x + F X ∆y
(3)
where ∆x and ∆y are the COM offsets from the ‘center-of-figure’ in Itokawa’s body-fixed x − y
plane [n.b. distance from the centre-of-figure is ∆r = (∆x2 + ∆y2 )0.5 ], F X and FY are the overall
photon force components acting on Itokawa in the same frame of reference, TCM (i.e. TCM = IZ ν)
is the inferred YORP torque acting about Itokawa’s COM, and TCF is the calculated YORP torque
acting about Itokawa’s center-of-figure. This approach requires a minimum COM offset from the
center-of-figure, ∆r, of ∼14 m, or an offset, ∆x, of ∼21 m if the offset is just along the x-axis where
Itokawa’s ‘body’ and ‘head’ are approximately aligned (see Fig. 4).
When surface roughness is included in the predictions then the thermal-IR beaming effect it
induces has the tendency to dampen the YORP rotational acceleration on average but can add
uncertainties of the order of several tens of per cent if the roughness is allowed to vary across
the surface (Rozitis & Green 2012). In this work, the unresolved surface roughness is described
in terms of each shape facet containing a fractional coverage, fR , of hemispherical craters, with
the remaining fraction (1 − fR ), representing a smooth flat surface. The hemispherical crater is a
simple way to accurately reproduce the thermal-IR beaming effect (i.e. re-radiation of absorbed
sunlight back towards the Sun) produced by a range of surface roughness morphologies and spatial
scales, and has been verified by application to lunar data (Rozitis & Green 2011). These spatial
scales start at the thermal skin depth (∼1 cm) and range up to the facet size of the shape model used
(∼4 m in this case). Previous thermophysical modelling of thermal-infrared observations of Itokawa
indicate that the surface is rough at these spatial scales but the distribution is unknown (M¨ ller et al.
u
2005). Radar circular polarisation ratios also give an indication of an asteroids wavelength-scale
roughness (Ostro et al. 2002; Benner et al. 2008), and Itokawas disk-integrated ratio at 3.5 cm,
µC = 0.47 ± 0.04, is significantly larger than that at 12.6 cm, µC = 0.26 ± 0.04 (Ostro et al. 2004;
Nolan et al. 2013). This indicates that most of the surface roughness occurs at the cm-scale, and
won’t effectively be described in the highest resolution shape model of Itokawa (∼3 million facets)
as it has a facet size of 0.5 m (Gaskell et al. 2008). These spatial scales are also much larger than
the <1 mm photometric roughness that is inferred from optical scattering (e.g. Hapke 1981; Hapke
6
7. S. C. Lowry et al.: YORP Spin-up Detected on Asteroid Itokawa
& Wells 1981), and therefore we are unable at present to determine the real distribution of surface
roughness for Itokawa from any kind of observation.
To assess the impact of potential surface roughness variations on the range of theoretical YORP
values that may be possible, we performed a Monte Carlo analysis where the roughness was allowed to vary in a patchy way but still had the same value when averaged across the surface. To generate a patchy surface roughness distribution, the surface of Itokawa was divided into 10 randomly
assigned areas that each have their own unique degree of roughness, which was also randomly chosen from a normal distribution with a mean value and standard deviation of fR = 0.50 ± 0.08. The
model reflected and emitted photon torques from each shape facet were adjusted according to the
shape facet’s individual roughness fraction, which were then summed across the surface to give the
overall YORP torque acting on Itokawa, and hence the YORP rotational acceleration when divided
by Itokawa’s moment of inertia. Based on 1000 trials, the distribution of predicted YORP rotational
acceleration had a mean value and standard deviation of (-1.80 ± 1.96) × 10−7 rad day−2 (Fig. 5),
which also encompasses the shape sensitivity range highlighted by previous studies (Scheeres et al.
2007; Breiter, et al. 2009). In 16.5% of these trials, a YORP spin-up was predicted. However, the
roughness distributions that produce YORP rotational acceleration values similar to that observed
have an artificial appearance that maximises the YORP spin-up and does not correspond with any
geological features (Fig. 6). This shows that an asymmetric roughness distribution cannot alone
be responsible for the observed YORP spin-up. Accounting for the YORP rotational acceleration
uncertainty resulting from possible and realistic roughness distributions leads to potential COM
offsets of ∆r = 14 ± 7 m or ∆x = 21 ± 12 m (Fig. 7).
A COM offset along the x-axis towards Itokawa’s ‘head’ strongly implies that it has a higher
bulk density than the ‘body’ (Scheeres & Gaskell 2008). Approximating Itokawa’s shape as two
ellipsoids with dimensions of 490 × 310 × 260 m (i.e. ‘body’) and 230 × 200 × 180 m (i.e. ‘head’)
resting on each other (Scheeres & Gaskell 2008; Demura et al. 2006) (Fig. 6) allows the COM offset
to be calculated as a function of the densities of the ‘body’ and ‘head’ (Fig. 8). ∆x = 21 ± 12 m
results in bulk densities of 1810 ± 80 kg m−3 and 2620 ± 370 kg m−3 , for the ‘body’ and ‘head’
respectively. The overall bulk density remains at 1950 kg m−3 , in order to be consistent with the
spacecraft-derived value from Abe et al. (2006b). The mass, and hence density of Itokawa was
derived by Abe et al. from the measured acceleration of the spacecraft, determined using laser
ranger data and an assumption of uniform density. In theory, any non-uniformity of density would
have an effect on the spacecraft’s trajectory when close to the asteroid. However, Hayabusa did not
orbit Itokawa and the mass was derived from a single descent between 1.4 and 0.8 km. Significant
non-gravitational forces (from solar radiation pressure and thrusters) had also to be taken into
account, resulting in a precision in the derived mass of 5%. It is impossible to determine if a density
inhomogeneity could be determined from such limited measurements and there is no mention of
this possibility in the paper. Our measurement of a significant density inhomogeneity is therefore
not inconsistent with the Hayabusa study.
The ellipsoid approach is not an optimal representation of Itokawa so we repeat the calculations
after simply dissecting the asteroid at x = 150 m as shown. The new density values are 1750 ± 110
kg m−3 and 2850 ± 500 kg m−3 . The measured COM offsets, and thus the density difference between ‘body’ and ‘head’, are therefore not overly sensitive to the precise relative dimensions of
the ellipses or any slight offset in the relative orientation between them. The derived bulk density
7
8. S. C. Lowry et al.: YORP Spin-up Detected on Asteroid Itokawa
difference between the two components of Itokawa is comparable to that seen between the primary
(1970 ± 240 kg m−3 ) and secondary (2810 +820/-630 kg m−3 ) of the (66391) 1999 KW4 binary
system (Ostro et al. 2006).
We extended our analysis to see if the merging of two bodies of equal density, causing a localised compression around the contact interface or ‘neck’ region, could explain the apparent COM
offset (Fig. 6 and 9). Using Itokawa’s shape model and assuming a uniform density we determined
the mass distribution (which is also equal to the volume distribution) along Itokawa’s x-axis. We
find the neck to be narrowest at +150 m, which we assume to be the centre of the neck (Fig. 9, left
panel). To determine possible center-of-mass (COM) offsets we varied the width and density of the
neck, and the density of the remaining body is adjusted accordingly to ensure a constant mass for
Itokawa. We represent the neck density as a multiple of the overall bulk density (e.g. x1.7 means
1950 kg m−3 × 1.7 = 3315 kg m−3 ), and this multiple ranges from 1.1 to 1.7. (Fig. 9, right panel).
Multiples greater than 1.7 are not likely to be feasible as they give the neck a density greater than
that of meteorites associated with S-type asteroids. A neck width of ∼100 m would be required to
produce a nominal COM offset of ∼21 m when a neck density multiple of 1.7 is assumed. Neck
widths greater than ∼50 m seem unrealistic given that the head itself is merely ∼150 m in diameter.
Furthermore, typical bulk porosities for S-type asteroids are between 20-40%. Accounting for even
the conservative lower end of this range sets a more realistic density multiple upper limit of x1.36.
The COM offset for a 50 m neck with this density-multiple would be just 4.6 m. For the range of
plausible neck dimensions and densities, the neck concentration explanation can only realistically
explain COM offsets less than ∼5 m.
5. Discussion
We speculate on the various scenarios that may explain the apparent COM shift and density inhomogeneity. Such scenarios include:
a) The merging of two bodies of equal density, causing a localised compression around the
contact interface or ‘neck’ region. This scenario is consistent with findings from the Hayabusa
spacecraft data, which showed no significant difference in surface composition or regolith structure
between the two lobes (Abe et al. 2006a; Saito et al. 2006). As discussed above, we have analysed
this in detail and conclude that such a scenario can only realistically account for up to ∼5 m in the
determined COM shift (Fig. 9).
b) Two completely unrelated bodies combined in a slow collision. The uniform surface composition and regolith structure strongly precludes this. We surmise that the probability must be
negligibly low for two unrelated objects to come together at a sufficiently-low encounter velocity to ensure the survival of both lobes upon contact and preserve its ‘bi-lobed’ shape, and have
identical surface compositions and distinctive surface regolith structure.
c) The ‘head’ and ‘body’ formed from the remains of a catastrophic collision on a larger differentiated body, presumably in the main asteroid belt. While this could certainly produce two bodies
of very different bulk densities, which later came together, the same principle applies as in scenario
‘b’, at least to a certain extent as we shall discuss. If a high-density fragment from the inner core
of the original body settled on the surface of what is now the ‘body’ of Itokawa, then the fragment
would subsequently need to be completely enveloped in material identical in composition to the
8
9. S. C. Lowry et al.: YORP Spin-up Detected on Asteroid Itokawa
‘body’, and develop a similar regolith structure. Alternatively, the head could be predominantly
a monolithic fragment of the same material, but with higher density than the porous rubble pile
body. In this case less processing of the surface of the head would be required to disguise its nature. There are various processes that could alter an asteroid’s surface, although not all have been
confirmed by observations. They include space weathering, collisions, gravitational torques and
tidal forces, YORP torques and possibly YORP-induced ‘seismic shaking’. However, each of these
processes may affect each lobe in different ways and so both lobes would need to have experienced
precisely the right combination of evolutionary processes to end up identical, and leave no trace
that the high-density fragment was present. Of course, we cannot rule out the possibility that the
fragment is comprised of high-density metallic material, that was sufficiently small to be subsumed
by the coalescing silicate material, and thus buried somewhere towards the ‘head’ region. Michel &
Richardson (2013) examined the impact scenario using an N-body simulation, resulting in a body
with multiple attaching relatively-large lobes. But there are a number of issues that preclude conclusive comparison with Itokawa. Firstly the initial body is approximately 50× larger than Itokawa,
with the largest remaining fragment being ∼40× larger. The results for a simulation using a much
smaller body may be completely different. Furthermore, there are many large lobes produced in this
simulation, rather than the simple ‘bi-lobed’ structure observed. Nor are density inhomogeneities
between lobes considered. With just one permutation being simulated, the probability of ending
up with something that resembles Itokawa is unknown. On the other hand, the simulations do provide a means to produce the uniform surface composition and topography. It is clearly important to
develop simulations of this kind, and reproduction of a bi-lobed Itokawa with the density disparity
that we report represents a fascinating challenge for the modelling community.
d) A fourth scenario involves a single consolidated body that was spun-up by YORP, leading
to the migration of regolith particles towards the equatorial region (Ostro et al. 2006; Scheeres et
al. 2006). This material was eventually lifted off the surface, with the orbiting material coalescing
into a satellite (Walsh et al. 2008). Both the process of regolith transport and ejection, and the reaggregation and subsequent dynamics of the secondary, could alter the densities of both primary
and secondary. In the case of the (66391) 1999 KW4 binary system, which shows similar density
differences between both components of the system (Ostro et al. 2006), it has been proposed that
the rapid spin rate of the primary leads to enhanced porosity and thus a lowering of its density
(Scheeres et al. 2006). Conversely, porosity is reduced in the secondary due to dynamical instabilities leading to periodic ‘shaking’, thereby increasing the density of the secondary. This scenario,
or some variation of it, is certainly feasible for the ‘head’ of Itokawa. Although Itokawa’s current
long rotation period precludes porosity decrease from rapid spin rate, the density of the ‘body’ will
be determined by its original, very different formation environment (presumably involving the reaggregation of fragments from a high-speed collision with another body), coupled with the residual
effects of the earlier regolith-loss process. Once the binary system has stabilized, Binary YORP (or
´
‘BYORP’) (Cuk & Burns 2005) could have caused the orbital semi-major axis to steadily evolve,
slowly guiding the smaller secondary in towards the main primary body until it eventually made
gentle contact and settled (although we note that BYORP is not strictly required for such a system
to collapse - see Jacobson & Scheeres 2011). Such ‘contact binaries’ have been observed, with
perhaps the most striking example being 1996 HW1 (Magri et al. 2011). Of course, the existing
9
10. S. C. Lowry et al.: YORP Spin-up Detected on Asteroid Itokawa
distribution of fine-grain regolith on Itokawa may be an important constraint for any modelling
tests of this scenario.
Whichever scenario is correct, based on our measurement of a density inhomogeneity, we can
now infer with a high degree of confidence that Itokawa formed from the merging of two separate asteroids, either in the aftermath of a catastrophic disruption of a larger differentiated body, or
from the collapse of a binary system. We also successfully demonstrate that an observational measurement of radiative torques, when combined with a detailed spacecraft shape model, can provide
insight into the interior structure of an asteroid.
Acknowledgements. We thank the anonymous referee for their helpful comments. We thank all the staff at the observatories involved in this study for their support. This work was based on observations at the following observatories: ESO,
Chile (PID: 185.C-1033); The Liverpool Telescope, La Palma; Palomar Observatory, California; The Isaac Newton Group,
La Palma; Steward Observatory, Arizona; Table Mountain Observatory, California. SCL, SRD, BR, and SFG gratefully acknowledge support from the UK Science and Technology Facilities Council. SCL acknowledges support from the Southeast
Physics Network (SEPnet). CS received funding from the European Union Seventh Framework Programme (FP7/20072013) under grant agreement no. 268421. This work was performed in part at the Jet Propulsion Laboratory under a contract
with NASA. This work made use of the NASA/JPL HORIZONS ephemeris-generating programme. All image reduction
and processing were performed using the Image Reduction and Analysis Facility (IRAF) (Tody 1986 & 1993). 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.
References
Abe, M., Takagi, Y., Kitazato, K., et al. 2006a, Science 312, 1334.
Abe, S., Mukai, T., Hirata, N., et al. 2006b, Science 312, 1344.
Benner, L. A. M., Ostro, S. J., Magri, C., et al. 2008, Icarus 198, 294.
Breiter, S., Bartczak, P., Czekaj, M., Oczujda, B., Vokrouhlick´ , D. 2009, A&A 507, 1073.
y
´
Cuk, M. & Burns, J. A. 2005, Icarus 176, 418.
Demura, H., Kobayashi, S., Nemoto, E., et al. 2006, Science 312, 1347.
ˇ
Durech, J., Vokrouhlick´ , D., Kaasalainen, M., et al. 2008a, A&A Letters 489, L25.
y
ˇ
Durech, J., Vokrouhlick´ , D., Kaasalainen, M., et al. 2008b, A&A 488, 345.
y
ˇ
Durech, J., Vokrouhlick´ , D., Baransky, A. R., et al. 2012, A&A 547, A10.
y
Gaskell, R. 2008, Gaskell Itokawa Shape Model V1.0. HAY-A-AMICA-5-ITOKAWASHAPE-V1.0, NASA Planetary Data
System.
Hapke, B. 1981, JGR 86, 3039.
Hapke, B. & Wells, E. 1981, JGR 86, 3055.
Jacobson, S. A. & Scheeres, D. J. 2011, Icarus 214, 161.
ˇ
Kaasalainen, M., Durech, J., Warner, B. D., et al. 2007, Nature 446, 420.
Kaasalainen, M., Torppa, J., & Muinonen, K. 2001, Icarus 153, 37.
Lowry, S. C., Fitzsimmons, A., Pravec, P., et al. 2007, Science 316, 272.
Magri, C., Howell, E. S., Nolan, M. C., et al. 2011, Icarus 214, 210.
Michel, P., & Richardson, D. C. 2013, A&A 554, L1.
M¨ ller, T. G., Sekiguchi, T., Kaasalainen, M., et al. 2005, A&A 443, 347.
u
Nolan, M. C., Magri, C., Howell, E. S., et al. 2013, Icarus 226, 629.
Ostro, S. J., Benner, L. A. M., Nolan, M. C., et al. 2004, M&PS 39, 407.
Ostro, S. J., Hudson, R. S., Benner, L. A. M., et al. 2002. In Asteroids III, eds. W. F. Bottke Jr., A. Cellino, P. Paolicchi, and
R. P. Binzel, University of Arizona Press, Tucson, p.151-168.
Ostro, S. J., Margot, J.-L., Benner, L. A. M., et al. 2006, Science 314, 1276.
Pravec, P., Vokrouhlick´ , D., Polishook, D., et al. 2010, Nature 466, 1085.
y
Rozitis, B. & Green, S. F. 2011, MNRAS 415, 2042.
Rozitis, B. & Green, S. F. 2012, MNRAS 423, 367.
10
11. S. C. Lowry et al.: YORP Spin-up Detected on Asteroid Itokawa
Rozitis, B. & Green, S. F. 2013, MNRAS 433, 603.
Rubincam, D. P. 2000, Icarus 148, 2.
Saito, J., Miyamoto, H., Nakamura, R., et al. 2006, Science 312, 1341.
Scheeres, D. J. & Gaskell, R. W. 2008, Icarus 198, 125.
Scheeres, D. J., Fahnestock, E. G., Ostro, S. J., et al. 2006, Science 314, 1280.
Scheeres, D. J., Abe, M., Yoshikawa, M., et al. 2007, Icarus 188, 425.
Slivan, S. M. 2002, Nature 419, 49.
Statler, T. S. 2009, Icarus 202, 502.
Taylor, P., Margot, J.-L., Vokrouhlick´ , D., et al. 2007, Science 316, 274.
y
Tody, D. 1986, “The IRAF Data Reduction and Analysis System” in Proc. SPIE Instrumentation in Astronomy VI, ed. D.L.
Crawford, 627, 733.
Tody, D. 1993, “IRAF in the Nineties” in Astronomical Data Analysis Software and Systems II, A.S.P. Conference Ser., Vol
52, eds. R.J. Hanisch, R.J.V. Brissenden, & J. Barnes, 173.
Vokrouhlick´ , D. & Nesvorn´ , D. 2008, AJ 136, 280.
y
y
Vokrouhlick´ , D., Nesvorny, D. & Bottke, W. F. 2003, Nature 425, 147.
y
Walsh, K. J., Richardson, D. C. & Michel, P. 2008, Nature 454, 188.
11
12. S. C. Lowry et al.: YORP Spin-up Detected on Asteroid Itokawa
Table 1. Observational log and geometry for all optical photometry.
LC No.
Date
Rh
∆
α
Aspect angle
No. of data
Phase offset
[dd/mm/yy]
Telescope
[AU]
[AU]
[deg.]
[deg.]
points
φYORP ± 1-σ [degrees]
PAL60
1
22-24/08/01
1.30
0.32
19.2
97.1
48
0.0 ± < 0.5
PAL60
2
22-25/09/01
1.43
0.45
15.6
95.4
30
0.0 ± < 0.5
TMO
3
27-30/01/04
1.31
0.33
5.9
83.9
62
0.5 ± < 0.5
S60
4
24-25/01/07
1.40
0.46
20.9
86.0
24
4.0 ± < 0.5
S90
5
10/12/09
1.58
0.60
4.6
88.2
48
7.0 ± < 0.5
LT
6
10/01/10
1.51
0.58
20.0
87.4
99
9.0 ± < 0.5
LT
7
09/12/12
1.63
0.65
0.0
89.0
179
17.0 ± < 0.5
NTT
8
14-16/12/12
1.62
0.65
6.8
88.8
76
16.0 ± < 0.5
INT
9
20/12/12
1.61
0.69
11.4
88.7
321
15.5 ± < 0.5
PAL200
10
5-6/01/13
1.57
0.69
6.8
88.8
36
15.0 ± < 0.5
All images were taken in either the broadband V- or R-filter (λc(V) = 550 nm, λc(R) = 657-nm). Telescope key:
PAL60 - Palomar Observatory 60-inch Telescope (California, USA), TMO - Table Mountain Observatory
(California, USA), S60 - Steward Observatory 60-inch Telescope (Arizona, USA), S90 - Steward
Observatory 90-inch Bok Telescope (Arizona, USA), LT - 2m Liverpool Telescope (La Palma, Spain), NTT European Southern Observatory 3.5m New Technology Telescope (Chile), INT - 2.5m Isaac Newton
Telescope (La Palma, Spain), PAL200 - Palomar Observatory 5m Hale Telescope (California, USA).
12
13. S. C. Lowry et al.: YORP Spin-up Detected on Asteroid Itokawa
Phase Offset (degrees)
20
Itokawa
15
Phase Offset Change in 10 yrs ~ 12deg
10
5
0
0
2
4
6
Year
8
10
12
Fig. 1. Rotational phase changes (φ) in Itokawa’s lightcurves observed from August/September 2001 to
January 2013. The strong quadratic temporal variation of φ is perfectly consistent with YORP-induced rotational acceleration. The solid curve is the best-fit quadratic curve, and the dotted line connects the first and
last data points, to emphasize the deviation from a straight line profile.
13
15. S. C. Lowry et al.: YORP Spin-up Detected on Asteroid Itokawa
YORP strength (rad day 2)
4.1
x 10
8
4
3.9
3.8
3.7
3.6
3.5
6
4
2
0
2
4
Period Relative to Best Fit (hours)
6
x 10
6
Fig. 3. 1-σ uncertainty ellipse for the optimum initial rotational period and YORP strength of the spin-state
model to produce the best fit between artificial lightcurves and observations. YORP-spin up is required, with
the best-fit obtained using 12.1323789 hours for the sidereal rotation period at T 0 and a corresponding YORP
strength of 3.81 × 10−8 rad day−2 . From the 1-σ uncertainty ellipse we determine the uncertainty in period
to be 4.7 × 10−6 hours and the corresponding uncertainty in YORP to be 0.24 × 10−8 rad day−2 . Note that in
this case where we have a spacecraft shape model with a well-determined pole solution, the uncertainty in the
fitted initial period becomes the dominating influence on the uncertainty in the measured YORP value.
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16. S. C. Lowry et al.: YORP Spin-up Detected on Asteroid Itokawa
300
200
Minimum Δr
Minimum Δx
y-axis (m)
100
0
-100
30
20
10
-200
0
-10
-20
-30
-300
-300
-30 -20 -10 0 10 20 30
-200
-100
0
100
x-axis (m)
200
300
Fig. 4. Projection in the x − y plane of Itokawa’s shape model and possible new locations of the centre-of-mass
to reconcile the observed YORP effect with theory. Assuming a moderately rough surface with a uniform
spatial distribution, the ATPM calculates that the COM must exist somewhere along the solid line to reproduce
the observed YORP rotational acceleration of 3.54 × 10−8 rad day−2 . The location with the minimum required
offset from the center-of-figure is shown by the blue dot (∼14 m displacement), and the red dot shows the
location if the offset is just along the x-axis (∼21 m displacement). Locations further along the line are less
probable as they require greater and more unusual bulk density inhomogeneities to produce the larger offsets.
The ATPM calculates the YORP rotational acceleration line with shadowing and global self-heating effects
included. For comparison purposes, the zero YORP rotational acceleration lines with none of these effects
included (Scheeres & Gaskell 2008), and with only shadowing included (Breiter, et al. 2009), are plotted as
the dotted and dashed lines respectively. As demonstrated in Rozitis & Green (2013), if global self-heating
effects are neglected then YORP predictions are generally more accurate if shadowing is also not included.
This explains why the calculations by Scheeres & Gaskell (2008) are similar to ours as shown here.
16
17. S. C. Lowry et al.: YORP Spin-up Detected on Asteroid Itokawa
Normalized Frequency
0.20
ATPM
Measured
0.15
0.10
0.05
0.00
-1•10-6
-5•10-7
0
5•10-7
YORP Rotational Acceleration (rad d-2)
Fig. 5. Distribution of YORP rotational acceleration values acting on Itokawa predicted by ATPM for different
patchy surface roughness distributions (solid line) compared with the observed value (dotted line).
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18. S. C. Lowry et al.: YORP Spin-up Detected on Asteroid Itokawa
Body: 1750
200
Head: 2850
kg m-3
y (m)
100
0
-100
-200
-400
-300
-200
-100
0
100
200
300
400
x (m)
Body: 1700
Neck: 3320 kg m-3
Head: 1700
200
y (m)
100
0
-100
-200
-400
-300
-200
-100
0
100
200
300
400
x (m)
Fig. 6. Various stages of the thermophysical analysis used to attempt to reconcile the observed YORP acceleration with theoretical determinations. The pole-on shape model renderings were determined from imaging
data from the Hayabusa spacecraft (Gaskell 2008), and highlight Itokawa’s ‘bi-lobed’ appearance (Demura et
al. 2006). Upper Left Panel - Average surface roughness distribution of Itokawa clones that produce a YORP
rotational acceleration (or YORP spin-up). The roughness scale ranges from 0.45 (blue) to 0.55 (red). Lower
Left Panel - Regular ellipsoids that interface at x ∼ 150 m, with relative dimensions chosen for consistency
with Scheeres & Gaskell (2008), which are loosely based on values from Demura et al. (2006). A density
differential between the head and body can explain the COM offset. Upper Right Panel - Use of the true shape
model with the interface between ‘body’ and ’head’ regions of different densities at x = 150 m. Lower Right
Panel - A compressed ‘neck’ region of higher density located between the ‘body’ and ‘head’ to explain the
measured COM offset. See section 4 for details.
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19. S. C. Lowry et al.: YORP Spin-up Detected on Asteroid Itokawa
0.5
Normalized Frequency
∆x
Min ∆r
0.4
0.3
0.2
0.1
0.0
0
20
40
Offset (m)
60
80
Fig. 7. Distribution of possible center-of-mass offsets in terms of minimum distance from the center-of-figure,
∆r (solid line), and distance along the x-axis only, ∆x (dotted line), derived using the results of the Monte
Carlo analysis shown in Fig. 5. Itokawa clones with unrealistic surface roughness distributions (i.e. highly
assymetric as in Fig. 6 or its transpose) are excluded.
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20. S. C. Lowry et al.: YORP Spin-up Detected on Asteroid Itokawa
Bulk Density (kg m3)
3500
3000
2500
Head
Body
2000
1500
1000
-10
0
10
20
30
∆x Offset (m)
40
50
Fig. 8. Bulk density of Itokawa’s ‘body’ and ‘head’ as a function of center-of-mass offset (∆x) along the xaxis. The overall bulk density of Itokawa remains at 1950 kg m−3 , as determined from the Hayabusa spacecraft
(Abe et al. 2006b).
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21. S. C. Lowry et al.: YORP Spin-up Detected on Asteroid Itokawa
Normalized Mass Distribution
0.010
Neck Concentration
Uniform Density
0.008
Neck Centre
0.006
0.004
0.002
0.000
-400
-200
0
x-axis (m)
200
400
CoM Offset (m)
20
x1.7
15
x1.5
10
x1.3
5
x1.1
0
0
20
40
60
Neck Width (m)
80
100
Fig. 9. Analysis of a compressed ‘neck’ region between the ‘body’ and ‘head’ of Itokawa to explain the
centre-of-mass (COM) offset. Top Panel - The normalised mass distribution along Itokawa’s x-axis. The solid
line is for uniform density, and the dotted line is for the extreme case of the same overall mass but with a
neck of diameter 100m and density defined by a neck-density multiple of 1.7 times the overall bulk density.
The vertical dashed line indicates the location of the center of the neck defined by the minimum in the mass
distribution. Bottom Panel - The COM offset as a function of neck width and neck-density multiple. A multiple
of 1.7 corresponds to a density equal to that of solid meteorites associated with S-type asteroids.
21