Showing papers by "Isabelle Baraffe published in 2003"

Evolutionary models for cool brown dwarfs and extrasolar giant planets. The case of HD 209458

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.

Evolutionary models for cool brown dwarfs and extrasolar giant planets. The case of HD 20945

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.

New low-mass members of the Lupus 3 dark cloud: Further indications of pre-main-sequence evolution strongly affected by accretion ?

Abstract: A spectroscopic survey of a small area at the center of the Lupus 3 star-forming core has revealed four new mid-to- late M-type members, including a M7.5 brown dwarf. One of the new members, classified as M5, displays prominent forbidden lines and strong H emission (EW(H)= 410 A), in addition to other permitted lines, and its luminosity is far below that of other members of the region with similar or later spectral types. We estimate a mass accretion rate rate of1:4 10 9 M yr 1 for this object, although with uncertainties that do not exclude values as low as 10 10 M yr 1 . Based on the H=(SII) ratio, the detection of HeI, and the CaII infrared triplet, we argue that most of the H emission is produced near the surface of the object, probably in accretion columns or at the base of jets, rather than in a low-density extended region. The strong emission-line spectrum superimposed on an unusually faint photospheric continuum thus seems to be a real, intrinsic feature rather than a result of the viewing geometry caused by an edge-on disk blocking the light from the central object. Other Lupus 3 late-type members also display noticeable underluminosity, all of them having EW(H) > 100 A as a result of the faint underlying continuum. We tentatively interpret these findings as evidence for the pre-main sequence evolution of objects with very low (possibly substellar) initial masses being significantly modified by accretion.

A Hubble Space Telescope Wide Field Planetary Camera 2 Survey for Brown Dwarf Binaries in the α Persei and Pleiades Open Clusters

Abstract: We present the results of a high-resolution imaging survey for brown dwarf binaries in two open clusters. The observations were carried out with the Wide Field Planetary Camera 2 on board the Hubble Space Telescope. Our sample consists of eight brown dwarf candidates in α Persei and 25 brown dwarf candidates in the Pleiades. We have resolved four binaries in the Pleiades with separations in the range 0094-0058, corresponding to projected separations between 11.7 and 7.2 AU. No binaries were found among the α Per targets. Three of the binaries have proper motions consistent with cluster membership in the Pleiades cluster, and for one of them we report the detection of Hα in emission and Li I absorption obtained from Keck II/ESI spectroscopy. One of the binaries does not have a proper motion consistent with Pleiades membership. We estimate that brown dwarf binaries wider than 12 AU are less frequent than 9% in the α Per and Pleiades clusters. This is consistent with an extension to substellar masses of a trend observed among stellar binaries: the maximum semimajor axis of binary systems decreases with decreasing primary mass. We find a binary frequency of two binaries over 13 brown dwarfs with confirmed proper-motion membership in the Pleiades, corresponding to a binary fraction of 15%. These binaries are limited to the separation range 7-12 AU, and their mass ratios are larger than 0.7. The observed properties of Pleiades brown dwarf binaries appear to be similar to their older counterparts in the solar neighborhood. The relatively high binary frequency (≥10%), the bias to separations smaller than about 15 AU, and the trend to high mass ratios (q ≥ 0.7) are fundamental properties of brown dwarfs. Current theories of brown dwarf formation do not appear to provide a good description of all these properties.

An HST/WFPC2 Survey for Brown Dwarf Binaries in the alpha Per and the Pleiades Open Clusters

Abstract: We present the results of a high-resolution imaging survey for brown dwarf (BD) binaries in two open clusters. The observations were carried out with WFPC2 onboard HST. Our sample consists of 8 BD candidates in the alpha Per cluster and 25 BD candidates in the Pleiades. We have resolved 4 binaries in the Pleiades with separations in the range 0".094--0".058, corresponding to projected separations between 11.7~AU and 7.2~AU. No binaries were found among the alpha Per targets. Three of the binaries have proper motions consistent with cluster membership in the Pleiades cluster, and for one of them we report the detection of Halpha in emission and LiI absorption obtained from Keck~II/ESI spectroscopy. One of the binaries does not have a proper motion consistent with Pleiades membership. We estimate that BD binaries wider than 12~AU are less frequent than 9% in the alphaPer and Pleiades clusters. This is consistent with an extension to substellar masses of a trend observed among stellar binaries: the maximum semimajor axis of binary systems decreases with decreasing primary mass. We find a binary frequency of 2 binaries over 13 BDs with confirmed proper motion membership in the Pleiades, corresponding to a binary fraction of 15%(1 sigma error bar +15%/-5%). These binaries are limited to the separation range 7-12~AU and their mass ratios are larger than 0.7. The relatively high binary frequency (>10%), the bias to separations smaller than about 15 AU and the trend to high mass ratios (q>0.7) are fundamental properties of BDs. Current theories of BD formation do not appear to provide a good description of all these properties.

Brown Dwarfs and Extrasolar Planets

Abstract: Brown dwarfs bridge the gap between the stellar and planetary mass regimes and, as they cool, their atmospheres evolve towards conditions most commonly associated with planets. Despite differences (e.g., formation and proximity to a stellar companion), it is interesting to know just how much of the knowledge gained by studying brown dwarfs can be directly applied when predicting the thermal and spectral properties of extrasolar planets. This paper briefly summarizes the characteristics of brown dwarfs and extrasolar giant planet atmospheres, and discusses the significance of irradiation on the evolution of extrasolar planets.

Evolutionary Models for Low Mass Stars and Brown Dwarfs at Young Ages

Abstract: We analyse evolutionary tracks at young ages for low mass stars with masses m :S 1.4 M0 and brown dwarfs down to one mass of Jupiter. We analyse current theoretical uncertainties due to initial conditions. Simple tests on initial conditions show the high uncertainties of models at ages ,;S 1 Myr.

Atmospheres of extrasolar giant planets and brown dwarfs

Abstract: Dept. of Physics & Astronomy, Wichita State University, Wichita, KS 67226-0032,USAABSTRACTBrown dwarfshavemasses intermediateto stars andplan-ets, but their atmospheres can span planetary plasma con-ditions as they cool over time. They are more directlyobservable than the currently known extrasolar planets.In this paper, we provide a brief summary of the atmo-spheric propertiesof browndwarfs, and applythis knowl-edge to predict the thermal and spectral properties of ex-trasolar planets.1.INTRODUCTIONSince their first discovery in the Pleiades (Teide 1 byRebolo et al. 1995) and in the solar neighborhood(Gliese 229b by Nakajima et al. 1995), brown dwarfsare found by the dozen mostly by photometric sky sur-veys (e.g. DENIS, 2MASS, and SLOAN). Most browndwarfs have been found within 50 parsecs. For some, ac-curate distances can be established by parallax, and goodresolution (R ) optical and near-infraredspectra areoften obtained. Over the years, models of atmospheresand interiors have continued to improve in their ability todescribe the global properties of brown dwarfs (Lunineet al. 1989, Burrows et al. 1993, Allard & Hauschildt1995, Tsuji et al. 1996, Chabrier et al. 2000, Allard et al.2001, to name a few). Consequently, brown dwarfs arefairly well understood and can serve as a stepping stoneto understand extrasolar giant planets.2.BROWNDWARFSFigures 1 and 2 summarize what we have learned of theoverall spectral properties of brown dwarfs from the stel-lar to the planetary temperaturerange. The topmost spec-trum is that of a low mass M dwarf star (M/M