507 research outputs found
The VLT/NaCo large program to probe the occurrence of exoplanets and brown dwarfs at wide orbits. IV. Gravitational instability rarely forms wide, giant planets
Understanding the formation and evolution of giant planets (≥1 M_(Jup)) at wide orbital separation (≥5 AU) is one of the goals of direct imaging. Over the past 15 yr, many surveys have placed strong constraints on the occurrence rate of wide-orbit giants, mostly based on non-detections, but very few have tried to make a direct link with planet formation theories. In the present work, we combine the results of our previously published VLT/NaCo large program with the results of 12 past imaging surveys to constitute a statistical sample of 199 FGK stars within 100 pc, including three stars with sub-stellar companions. Using Monte Carlo simulations and assuming linear flat distributions for the mass and semi-major axis of planets, we estimate the sub-stellar companion frequency to be within 0.75–5.70% at the 68% confidence level (CL) within 20–300 AU and 0.5–75 M_(Jup), which is compatible with previously published results. We also compare our results with the predictions of state-of-the-art population synthesis models based on the gravitational instability (GI) formation scenario with and without scattering. We estimate that in both the scattered and non-scattered populations, we would be able to detect more than 30% of companions in the 1–75 M_(Jup) range (95% CL). With the threesub-stellar detections in our sample, we estimate the fraction of stars that host a planetary system formed by GI to be within 1.0–8.6% (95% CL). We also conclude that even though GI is not common, it predicts a mass distribution of wide-orbit massive companions that is much closer to what is observed than what the core accretion scenario predicts. Finally, we associate the present paper with the release of the Direct Imaging Virtual Archive (DIVA), a public database that aims at gathering the results of past, present, and future direct imaging surveys
Near-infrared scattered light properties of the HR 4796 A dust ring A measured scattering phase function from 13.6° to 166.6°
Context. HR 4796 A is surrounded by a debris disc, observed in scattered light as an inclined ring with a high surface brightness. Past observations have raised several questions. First, a strong brightness asymmetry detected in polarised reflected light has recently challenged our understanding of scattering by the dust particles in this system. Secondly, the morphology of the ring strongly suggests the presence of planets, although no planets have been detected to date.
Aims. We aim here at measuring with high accuracy the morphology and photometry of the ring in scattered light, in order to derive the phase function of the dust and constrain its near-infrared spectral properties. We also want to constrain the presence of planets and set improved constraints on the origin of the observed ring morphology.
Methods. We obtained high-angular resolution coronagraphic images of the circumstellar environment around HR 4796 A with VLT/SPHERE during the commissioning of the instrument in May 2014 and during guaranteed-time observations in February 2015. The observations reveal for the first time the entire ring of dust, including the semi-minor axis that was previously hidden either behind the coronagraphic spot or in the speckle noise.
Results. We determine empirically the scattering phase function of the dust in the H band from 13.6° to 166.6°. It shows a prominent peak of forward scattering, never detected before, for scattering angles below 30°. We analyse the reflectance spectra of the disc from the 0.95 μm to 1.6 μm, confirming the red colour of the dust, and derive detection limits on the presence of planetary mass objects.
Conclusions. We confirm which side of the disc is inclined towards the Earth. The analysis of the phase function, especially below 45°, suggests that the dust population is dominated by particles much larger than the observation wavelength, of about 20 μm. Compact Mie grains of this size are incompatible with the spectral energy distribution of the disc, however the observed rise in scattering efficiency beyond 50° points towards aggregates which could reconcile both observables. We do not detect companions orbiting the star, but our high-contrast observations provide the most stringent constraints yet on the presence of planets responsible for the morphology of the dust
New constraints on the formation and settling of dust in the atmospheres of young M and L dwarfs
We obtained medium-resolution near-infrared spectra of seven young M9.5-L3
dwarfs classified in the optical. We aim to confirm the low surface gravity of
the objects in the NIR. We also test whether atmospheric models correctly
represent the formation and the settling of dust clouds in the atmosphere of
young late-M and L dwarfs. We used ISAAC at VLT to obtain the spectra of the
targets. We compared them to those of mature and young BD, and young late-type
companions to nearby stars with known ages, in order to identify and study
gravity-sensitive features. We computed spectral indices weakly sensitive to
the surface gravity to derive near-infrared spectral types. Finally, we found
the best fit between each spectrum and synthetic spectra from the BT-Settl 2010
and 2013 models. Using the best fit, we derived the atmospheric parameters of
the objects and identify which spectral characteristics the models do not
reproduce. We confirmed that our objects are young BD and we found NIR spectral
types in agreement with the ones determined at optical wavelengths. The
spectrum of the L2-gamma dwarf 2MASSJ2322-6151 reproduces well the spectrum of
the planetary mass companion 1RXS J1609-2105b. BT-Settl models fit the spectra
and the 1-5 m SED of the L0-L3 dwarfs for temperatures between 1600-2000
K. But the models fail to reproduce the shape of the H band, and the NIR slope
of some of our targets. This fact, and the best fit solutions found with
super-solar metallicity are indicative of a lack of dust, in particular at high
altitude, in the cloud models. The modeling of the vertical mixing and of the
grain growth will be revised in the next version of the BT-Settl models. These
revisions may suppress the remaining non-reproducibilities.Comment: Accepted in A&A, February 6, 201
The VLT/NaCo large program to probe the occurrence of exoplanets and brown dwarfs at wide orbits. IV. Gravitational instability rarely forms wide, giant planets
Understanding the formation and evolution of giant planets (≥1 M_(Jup)) at wide orbital separation (≥5 AU) is one of the goals of direct imaging. Over the past 15 yr, many surveys have placed strong constraints on the occurrence rate of wide-orbit giants, mostly based on non-detections, but very few have tried to make a direct link with planet formation theories. In the present work, we combine the results of our previously published VLT/NaCo large program with the results of 12 past imaging surveys to constitute a statistical sample of 199 FGK stars within 100 pc, including three stars with sub-stellar companions. Using Monte Carlo simulations and assuming linear flat distributions for the mass and semi-major axis of planets, we estimate the sub-stellar companion frequency to be within 0.75–5.70% at the 68% confidence level (CL) within 20–300 AU and 0.5–75 M_(Jup), which is compatible with previously published results. We also compare our results with the predictions of state-of-the-art population synthesis models based on the gravitational instability (GI) formation scenario with and without scattering. We estimate that in both the scattered and non-scattered populations, we would be able to detect more than 30% of companions in the 1–75 M_(Jup) range (95% CL). With the threesub-stellar detections in our sample, we estimate the fraction of stars that host a planetary system formed by GI to be within 1.0–8.6% (95% CL). We also conclude that even though GI is not common, it predicts a mass distribution of wide-orbit massive companions that is much closer to what is observed than what the core accretion scenario predicts. Finally, we associate the present paper with the release of the Direct Imaging Virtual Archive (DIVA), a public database that aims at gathering the results of past, present, and future direct imaging surveys
The gravitational mass of Proxima Centauri measured with SPHERE from a microlensing event
Proxima Centauri, our closest stellar neighbour, is a low-mass M5 dwarf
orbiting in a triple system. An Earth-mass planet with an 11 day period has
been discovered around this star. The star's mass has been estimated only
indirectly using a mass-luminosity relation, meaning that large uncertainties
affect our knowledge of its properties. To refine the mass estimate, an
independent method has been proposed: gravitational microlensing. By taking
advantage of the close passage of Proxima Cen in front of two background stars,
it is possible to measure the astrometric shift caused by the microlensing
effect due to these close encounters and estimate the gravitational mass of the
lens (Proxima Cen). Microlensing events occurred in 2014 and 2016 with impact
parameters, the closest approach of Proxima Cen to the background star, of
1\farcs6 0\farcs1 and 0\farcs5 0\farcs1, respectively. Accurate
measurements of the positions of the background stars during the last two years
have been obtained with HST/WFC3, and with VLT/SPHERE from the ground. The
SPHERE campaign started on March 2015, and continued for more than two years,
covering 9 epochs. The parameters of Proxima Centauri's motion on the sky,
along with the pixel scale, true North, and centering of the instrument
detector were readjusted for each epoch using the background stars visible in
the IRDIS field of view. The experiment has been successful and the astrometric
shift caused by the microlensing effect has been measured for the second event
in 2016. We used this measurement to derive a mass of
0.150 (an error of 40\%) \MSun for Proxima
Centauri acting as a lens. This is the first and the only currently possible
measurement of the gravitational mass of Proxima Centauri.Comment: 10 pages, 6 figures, accepted by MNRA
Characterization of the Benchmark Binary NLTT 33370
We report the confirmation of the binary nature of the nearby, very low-mass
system NLTT 33370 with adaptive optics imaging and present resolved
near-infrared photometry and integrated light optical and near-infrared
spectroscopy to characterize the system. VLT-NaCo and LBTI-LMIRCam images show
significant orbital motion between 2013 February and 2013 April. Optical
spectra reveal weak, gravity sensitive alkali lines and strong lithium 6708
Angstrom absorption that indicate the system is younger than field age.
VLT-SINFONI near-IR spectra also show weak, gravity sensitive features and
spectral morphology that is consistent with other young, very low-mass dwarfs.
We combine the constraints from all age diagnostics to estimate a system age of
~30-200 Myr. The 1.2-4.7 micron spectral energy distribution of the components
point toward T_eff=3200 +/- 500 K and T_eff=3100 +/- 500 K for NLTT 33370 A and
B, respectively. The observed spectra, derived temperatures, and estimated age
combine to constrain the component spectral types to the range M6-M8.
Evolutionary models predict masses of 113 +/- 8 M_Jup and 106 +/- 7 M_Jup from
the estimated luminosities of the components. KPNO-Phoenix spectra allow us to
estimate the systemic radial velocity of the binary. The Galactic kinematics of
NLTT 33370AB are broadly consistent with other young stars in the Solar
neighborhood. However, definitive membership in a young, kinematic group cannot
be assigned at this time and further follow-up observations are necessary to
fully constrain the system's kinematics. The proximity, age, and late-spectral
type of this binary make it very novel and an ideal target for rapid, complete
orbit determination. The system is one of only a few model calibration
benchmarks at young ages and very low-masses.Comment: 25 pages, 3 tables, 13 figures, accepted for publication in The
Astrophysical Journa
There are No Causality Problems for Fermi's Two Atom System
A repeatedly discussed gedanken experiment, proposed by Fermi to check
Einstein causality, is reconsidered. It is shown that, contrary to a recent
statement made by Hegerfeldt, there appears no causality paradoxon in a proper
theoretical description of the experiment.Comment: 6 pages, latex, DESY 94-02
Planet Candidates from K2 Campaigns 5-8 and Follow-Up Optical Spectroscopy
We present 151 planet candidates orbiting 141 stars from K2 campaigns 5-8
(C5-C8), identified through a systematic search of K2 photometry. In addition,
we identify 16 targets as likely eclipsing binaries, based on their light curve
morphology. We obtained follow-up optical spectra of 105/141 candidate host
stars and 8/16 eclipsing binaries to improve stellar properties and to identify
spectroscopic binaries. Importantly, spectroscopy enables measurements of host
star radii with 10% precision, compared to 40% precision when
only broadband photometry is available. The improved stellar radii enable
improved planet radii. Our curated catalog of planet candidates provides a
starting point for future efforts to confirm and characterize K2 discoveries.Comment: Accepted for publication in the Astronomical Journal; 17 pages, 8
figures, 2 tables, download source for full table
60 Validated Planets from K2 Campaigns 5-8
We present a uniform analysis of 155 candidates from the second year of
NASA's mission (Campaigns 5-8), yielding 60 statistically validated
planets spanning a range of properties, with median values of = 2.5
, = 7.1 d, = 811 K, and = 11.3 mag. The
sample includes 24 planets in 11 multi-planetary systems, as well as 18 false
positives, and 77 remaining planet candidates. Of particular interest are 18
planets smaller than 2 , five orbiting stars brighter than = 10
mag, and a system of four small planets orbiting the solar-type star EPIC
212157262. We compute planetary transit parameters and false positive
probabilities using a robust statistical framework and present a complete
analysis incorporating the results of an intensive campaign of high resolution
imaging and spectroscopic observations. This work brings the yield to over
360 planets, and by extrapolation we expect that will have discovered
600 planets before the expected depletion of its on-board fuel in late
2018.Comment: 33 pages, 13 figures, 5 tables, accepted for publication in A
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