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F. Allard

Bio: F. Allard is an academic researcher from École Normale Supérieure. The author has contributed to research in topics: Brown dwarf & Stellar classification. The author has an hindex of 13, co-authored 21 publications receiving 3191 citations. Previous affiliations of F. Allard include École normale supérieure de Lyon.

Papers
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Journal ArticleDOI
TL;DR: In this article, the authors present evolutionary models for cool brown dwarfs and extra-solar giant planets and show that irradiation effects can substantially affect the radius of sub-jovian mass giant planets.
Abstract: We present evolutionary models for cool brown dwarfs and extra-solar giant planets. The models reproduce the main trends of observed methane dwarfs in near-IR color-magnitude diagrams. We also present evolutionary models for irradiated planets, coupling for the first time irradiated atmosphere profiles and inner structures. We focus on HD 209458-like systems and show that irradiation effects can substantially affect the radius of sub-jovian mass giant planets. Irradiation effects, however, cannot alone explain the large observed radius of HD 209458b. Adopting assumptions which optimise irradiation effects and taking into account the extension of the outer atmospheric layers, we still find $\\sim$ 20% discrepancy between observed and theoretical radii. An extra source of energy seems to be required to explain the observed value of the first transit planet.

1,812 citations

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TL;DR: In this paper, the authors analyse pre-main sequence evolutionary tracks for low mass stars with masses 1.4m and 1.5m and show that at low surface gravity, the common picture of vertical Hayashi lines is oversimplified.
Abstract: We analyse pre-Main Sequence evolutionary tracks for low mass stars with masses $m \\le 1.4 \\msol$ based on the Baraffe et al. (1998) input physics. We also extend the recent Chabrier et al. (2000) evolutionary models based on dusty atmosphere to young brown dwarfs down to one mass of Jupiter. We analyse current theoretical uncertainties due to molecular line lists, convection and initial conditions. Simple tests on initial conditions show the high uncertainties of models at ages $\\simle$ 1 Myr. We find a significant sensitivity of atmosphere profiles to the treatment of convection at low gravity and $\\te < 4000$ K, whereas it vanishes as gravity increases. This effect adds another source of uncertainty on evolutionary tracks at very early phases. We show that at low surface gravity ($\\log g \\simle 3.5$,) the common picture of vertical Hayashi lines with constant $\\te$ is oversimplified. The effect of a variation of initial deuterium abundance is studied. We compare our models with evolutionary tracks available in the literature and discuss the main differences. We finally analyse to which extent current observations of young systems provide a good test for pre-Main Sequence tracks.

671 citations

Journal ArticleDOI
TL;DR: The first brown dwarf atmosphere models based on theoretical calculations of absorption profiles of sodium and potassium perturbed by helium and molecular hydrogen were presented in this article. But the results were limited to the T dwarf SDSS 1624.
Abstract: We present the first brown dwarf atmosphere models based on theoretical calculations of absorption profiles of sodium and potassium perturbed by helium and molecular hydrogen. The synthetic spectra have been compared to previous calculations with Lorentz profiles and the classic van der Waals approximation, and to the observed spectrum of the T dwarf SDSS 1624. The new profiles provide increased opacities in the optical spectra of methane brown dwarfs. However, the potas- sium and sodium far wings alone cannot explain the missing opacity in the 0.85 to 1.1m range.

117 citations

Journal ArticleDOI
TL;DR: In this paper, the authors presented new coronagraphic observations obtained with VLT/SPHERE (Very Large Telescope/SpectroPolarimetric High-contrast Exoplanet REsearch) that explore, for the very first time, the innermost regions of the system down to 0.2 arcsec(0.5 au) from Sirius A.
Abstract: Sirius has always attracted a lot of scientific interest, especially after the discovery of a companion white dwarf at the end of the 19th century. Very early on, the existence of a potential third bodywas put forward to explain some of the observed properties of the system. We present new coronagraphic observations obtained with VLT/SPHERE (Very Large Telescope/SpectroPolarimetric High-contrast Exoplanet REsearch) that explore, for the very first time, the innermost regions of the system down to 0.2 arcsec(0.5 au) from Sirius A. Our observations cover the near-infrared from 0.95 to 2.3 mu m and they offer the best on-sky contrast ever reached at these angular separations. After detailing the steps of our SPHERE/IRDIFS data analysis, we present a robust method to derive detection limits for multispectral data from high-contrast imagers and spectrographs. In terms of raw performance, we report contrasts of 14.3 mag at 0.2 arcsec, similar to 16.3 mag in the 0.4-1.0 arcsec range and down to 19 mag at 3.7 arcsec. In physical units, our observations are sensitive to giant planets down to 11 M-Jup at 0.5 au, 6-7 M-Jup in the 1-2 au range and similar to 4 M-Jup at 10 au. Despite the exceptional sensitivity of our observations, we do not report the detection of additional companions around Sirius A. Using a Monte Carlo orbital analysis, we show that we can reject, with about 50 per cent probability, the existence of an 8 M-Jup planet orbiting at 1 au.

114 citations

Journal ArticleDOI
TL;DR: In this article, the authors presented the discovery of fifteen new T2.5-T7.5 dwarfs, with estimated distances between �24-93pc, identified in the first three main data releases of the UKIRT Infrared Deep Sky Survey.
Abstract: We present the discovery of fifteen new T2.5-T7.5 dwarfs (with estimated distances between �24–93pc), identified in the first three main data releases of the UKIRT Infrared Deep Sky Survey. This brings the total number of T dwarfs discovered in the Large Area Survey (to date) to 28. These discoveries are confirmed by near infrared spectroscopy, from which we derive spectral types on the unified scheme of Burgasser et al. (2006). Seven of the new T dwarfs have spectral types of T2.5-T4.5, five have spectral types of T5-T5.5, one is a T6.5p, and two are T7-7.5. We assess spectral morphology and colours to identify T dwarfs in our sample that may have non-typical physical properties (by comparison to solar neighbourhood populations), and find that one of these new T dwarfs may be metal poor, three may have low surface gravity, and one may have high surface gravity. The colours of the full sample of LAS T dwarfs show a possible trend to bluer Y J with decreasing effective temperature, and some interesting colour changes in J H and z J (deserving further investigation) beyond T8. The LAS T dwarf sample from the first and second main data releases show good evidence for a consistent level of completion to J=19. By accounting for the main sources of incompleteness (selection, follow-up and spatial) as well as the effects of unresolved binarity and Malmquist bias, we estimate that there are 17±4 >T4 dwarfs in the J 619 volume of the LAS second data release. Comparing this to theoretical predictions is most consistent with a sub-stellar mass function exponent α between -1.0 and 0. This is consistent with the latest 2MASS/SDSS constraint (which is based on lower number statistics), and is significantly lower than the α � 1.0 suggested by L dwarf field populations, possibly a result of the lower mass range probed by the T dwarf class.

112 citations


Cited by
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TL;DR: A review of the present-day mass function and initial mass function in various components of the Galaxy (disk, spheroid, young, and globular clusters) and in conditions characteristic of early star formation is presented in this paper.
Abstract: We review recent determinations of the present-day mass function (PDMF) and initial mass function (IMF) in various components of the Galaxy—disk, spheroid, young, and globular clusters—and in conditions characteristic of early star formation. As a general feature, the IMF is found to depend weakly on the environment and to be well described by a power-law form forM , and a lognormal form below, except possibly for m!1 early star formation conditions. The disk IMF for single objects has a characteristic mass around M , m!0.08 c and a variance in logarithmic mass , whereas the IMF for multiple systems hasM , and . j!0.7 m!0.2 j!0.6 c The extension of the single MF into the brown dwarf regime is in good agreement with present estimates of L- and T-dwarf densities and yields a disk brown dwarf number density comparable to the stellar one, n!n! BD " pc !3 .T he IMF of young clusters is found to be consistent with the disk fi eld IMF, providing the same correction 0.1 for unresolved binaries, confirming the fact that young star clusters and disk field stars represent the same stellar population. Dynamical effects, yielding depletion of the lowest mass objects, are found to become consequential for ages!130 Myr. The spheroid IMF relies on much less robust grounds. The large metallicity spread in the local subdwarf photometric sample, in particular, remains puzzling. Recent observations suggest that there is a continuous kinematic shear between the thick-disk population, present in local samples, and the genuine spheroid one. This enables us to derive only an upper limit for the spheroid mass density and IMF. Within all the uncertainties, the latter is found to be similar to the one derived for globular clusters and is well represented also by a lognormal form with a characteristic mass slightly larger than for the disk, M , ,e xcluding as ignif icant population of m!0.2-0.3 c brown dwarfs in globular clusters and in the spheroid. The IMF characteristic of early star formation at large redshift remains undetermined, but different observational constraints suggest that it does not extend below!1M , .T hese results suggest a characteristic mass for star formation that decreases with time, from conditions prevailing at large redshift to conditions characteristic of the spheroid (or thick disk) to present-day conditions.Theseconclusions,however, remain speculative, given the large uncertainties in the spheroid and early star IMF determinations. These IMFs allow a reasonably robust determination of the Galactic present-day and initial stellar and brown dwarf contents. They also have important galactic implications beyond the Milky Way in yielding more accurate mass-to-light ratio determinations. The mass-to-light ratios obtained with the disk and the spheroid IMF yield values 1.8-1.4 times smaller than for a Salpeter IMF, respectively, in agreement with various recent dynamical determinations. This general IMF determination is examined in the context of star formation theory. None of the theories based on a Jeans-type mechanism, where fragmentation is due only to gravity, can fulfill all the observational constraints on star formation and predict a large number of substellar objects. On the other hand, recent numerical simulations of compressible turbulence, in particular in super-Alfvenic conditions, seem to reproduce both qualitatively and quantitatively the stellar and substellar IMF and thus provide an appealing theoretical foundation. In this picture, star formation is induced by the dissipation of large-scale turbulence to smaller scales through radiative MHD shocks, producing filamentary structures. These shocks produce local nonequilibrium structures with large density contrasts, which collapse eventually in gravitationally bound objects under the combined influence of turbulence and gravity. The concept of a single Jeans mass is replaced by a distribution of local Jeans masses, representative of the lognormal probability density function of the turbulent gas. Objects below the mean thermal Jeans mass still have a possibility to collapse, although with a decreasing probability.

8,218 citations

Journal ArticleDOI
TL;DR: Modules for Experiments in Stellar Astrophysics (MESA) as mentioned in this paper is a suite of open source, robust, efficient, thread-safe libraries for a wide range of applications in computational stellar astrophysics.
Abstract: Stellar physics and evolution calculations enable a broad range of research in astrophysics. Modules for Experiments in Stellar Astrophysics (MESA) is a suite of open source, robust, efficient, thread-safe libraries for a wide range of applications in computational stellar astrophysics. A one-dimensional stellar evolution module, MESAstar, combines many of the numerical and physics modules for simulations of a wide range of stellar evolution scenarios ranging from very low mass to massive stars, including advanced evolutionary phases. MESAstar solves the fully coupled structure and composition equations simultaneously. It uses adaptive mesh refinement and sophisticated timestep controls, and supports shared memory parallelism based on OpenMP. State-of-the-art modules provide equation of state, opacity, nuclear reaction rates, element diffusion data, and atmosphere boundary conditions. Each module is constructed as a separate Fortran 95 library with its own explicitly defined public interface to facilitate independent development. Several detailed examples indicate the extensive verification and testing that is continuously performed and demonstrate the wide range of capabilities that MESA possesses. These examples include evolutionary tracks of very low mass stars, brown dwarfs, and gas giant planets to very old ages; the complete evolutionary track of a 1 M ☉ star from the pre-main sequence (PMS) to a cooling white dwarf; the solar sound speed profile; the evolution of intermediate-mass stars through the He-core burning phase and thermal pulses on the He-shell burning asymptotic giant branch phase; the interior structure of slowly pulsating B Stars and Beta Cepheids; the complete evolutionary tracks of massive stars from the PMS to the onset of core collapse; mass transfer from stars undergoing Roche lobe overflow; and the evolution of helium accretion onto a neutron star. MESA can be downloaded from the project Web site (http://mesa.sourceforge.net/).

3,474 citations

Journal ArticleDOI
TL;DR: The first extensive catalog of galactic embedded clusters is compiled, finding that the embedded cluster birthrate exceeds that of visible open clusters by an order of magnitude or more indicating a high infant mortality rate for protocluster systems.
Abstract: ▪ Abstract Stellar clusters are born embedded within giant molecular clouds (GMCs) and during their formation and early evolution are often only visible at infrared wavelengths, being heavily obscured by dust. Over the past 15 years advances in infrared detection capabilities have enabled the first systematic studies of embedded clusters in galactic molecular clouds. In this article we review the current state of empirical knowledge concerning these extremely young protocluster systems. From a survey of the literature we compile the first extensive catalog of galactic embedded clusters. We use the catalog to construct the mass function and estimate the birthrate for embedded clusters within ∼2 kpc of the sun. We find that the embedded cluster birthrate exceeds that of visible open clusters by an order of magnitude or more indicating a high infant mortality rate for protocluster systems. Less than 4–7% of embedded clusters survive emergence from molecular clouds to become bound clusters of Pleiades age. Th...

2,949 citations

Journal ArticleDOI
TL;DR: The final version published in MNRAS August 2007 included significant revisions including significant revisions to the original version April 2006.
Abstract: Final published version including significant revisions. Twenty four pages, fourteen figures. Original version April 2006; final version published in MNRAS August 2007

2,562 citations

Journal ArticleDOI
28 Nov 2008-Science
TL;DR: High-contrast observations with the Keck and Gemini telescopes have revealed three planets orbiting the star HR 8799, with projected separations of 24, 38, and 68 astronomical units.
Abstract: Direct imaging of exoplanetary systems is a powerful technique that can reveal Jupiter-like planets in wide orbits, can enable detailed characterization of planetary atmospheres, and is a key step toward imaging Earth-like planets. Imaging detections are challenging because of the combined effect of small angular separation and large luminosity contrast between a planet and its host star. High-contrast observations with the Keck and Gemini telescopes have revealed three planets orbiting the star HR 8799, with projected separations of 24, 38, and 68 astronomical units. Multi-epoch data show counter clockwise orbital motion for all three imaged planets. The low luminosity of the companions and the estimated age of the system imply planetary masses between 5 and 13 times that of Jupiter. This system resembles a scaled-up version of the outer portion of our solar system.

1,966 citations