New evolutionary models for pre-main sequence and main sequence low-mass stars down to the hydrogen-burning limit

We present new models for low-mass stars down to the hydrogen-burning limit that consistently couple atmosphere and interior structures, thereby superseding the widely used BCAH98 models. The new models include updated molecular linelists and solar abundances, as well as atmospheric convection parameters calibrated on 2D/3D radiative hydrodynamics simulations. Comparison of these models with observations in various colour-magnitude diagrams for various ages shows significant improvement over previous generations of models. The new models can solve flaws that are present in the previous ones, such as the prediction of optical colours that are too blue compared to M dwarf observations. They can also reproduce the four components of the young quadruple system LkCa 3 in a colour-magnitude diagram with one single isochrone, in contrast to any presently existing model. In this paper we also highlight the need for consistency when comparing models and observations, with the necessity of using evolutionary models and colours based on the same atmospheric structures.Comment: 7 pages, 8 figures, Astronomy & Astrophysics in pres

Evolution of low-mass stars and substellar objects. Contribution to the Galactic mass budget

We briefly summarize our present knowledge of the theory of low-mass stars and substellar objects and their contribution to the Galactic population.Comment: 9 pages, Latex file, Invited Review VLT Opening Symposium, Antofagasta (march 1999

Deuterium-burning in substellar objects

We consider the depletion of primordial deuterium in the interior of substellar objects as a function of mass, age and absolute magnitude in several photometric passbands. We characterize potential spectroscopic signatures of deuterium in the lines of deuterated water HDO. These results will serve as a useful, independent diagnostic to characterize the mass and/or the age of young substellar objects, and to provide an independent age determination of very young clusters. These results can serve to identify objects at the deuterium-burning limit and to confront the theoretical prediction that D-burning is a necessary condition to form star-like objects.Comment: 13 pages, Latex file, uses aasms4.sty, accepted for publication in ApJ Letter

Evolutionary models for solar metallicity low-mass stars: mass-magnitude relationships and color-magnitude diagrams

Copyright © European Southern Observatory (ESO) 199

Pre-Main Sequence models for low-mass stars and brown dwarfs

We present evolutionary models for low mass stars and brown dwarfs (m \le 1.2 \msol) based on recent improvement of the theory: equation of state, atmosphere models, ... We concentrate on early evolutionary phases from the initial deuterium burning phase to the zero-age Main Sequence. Evolutionary models for young brown dwarfs are also presented. We discuss the uncertainties of the present models. We analyse the difficulties arising when comparing models with observations for very young objects, in particular concerning the problem of reddening.Comment: 10 pages, Latex file, uses Cargesepasp.sty, to be published in ASP Conf. Series, "From Darkness to Light", eds. T. Montmerle and P. Andre, Cargese 200

The physics of extrasolar gaseous planets : from theory to observable signatures

We review our present understanding of the physical properties of substellar objects, brown dwarfs and irradiated or non-irradiated gaseous exoplanets. This includes a description of their internal properties, mechanical structure and heat content, their atmospheric properties, thermal profile and emergent spectrum, and their evolution, in particular as irradiated companions of a close parent star. The general theory can be used to make predictions in term of detectability for the future observational projects. Special attention is devoted to the evolution of the two presently detected transit planets, HD209458B and OGLE-TR-56B. For this latter, we present a consistent evolution for its recently revised mass and show that we reproduce the observed radius within its error bars. We briefly discuss differences between brown dwarfs and gaseous planets, both in terms of mass function and formation process. We outline several arguments to show that the minimum mass for deuterium burning, recently adopted officially as the limit to distinguish the two types of objects, is unlikely to play any specific role in star formation, so that such a limit is of purely semantic nature and is not supported by a physical justification.Comment: 21 pages, 3 figure

Evolutionary models for very-low-mass stars and brown dwarfs with dusty atmospheres

We present evolutionary calculations for very-low-mass stars and brown dwarfs based on synthetic spectra and non-grey atmosphere models which include dust formation and opacity, i.e. objects with \te\simle 2800 K. The interior of the most massive brown dwarfs is shown to develop a conductive core after $\sim 2$ Gyr which slows down their cooling. Comparison is made in optical and infrared color-magnitude diagrams with recent late-M and L-dwarf observations. The saturation in optical colors and the very red near-infrared colors of these objects are well explained by the onset of dust formation in the atmosphere. Comparison of the faintest presently observed L-dwarfs with these dusty evolutionary models suggests that dynamical processes such as turbulent diffusion and gravitational settling are taking place near the photosphere. As the effective temperature decreases below \te\approx 1300-1400 K, the colors of these objects move to very blue near-infrared colors, a consequence of the ongoing methane absorption in the infrared. We suggest the possibility ofa brown dwarf dearth in $J,H,K$ color-magnitude diagrams around this temperature.Comment: 38 pages, Latex file, uses aasms4.sty, accepted for publication in Ap

Evolutionary models for low-mass stars and brown dwarfs: uncertainties and limits at very young ages

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.Comment: 12 pages, Latex file, uses aa.cls, accepted for publication in A&