Showing papers by "Isabelle Baraffe published in 1997"

Structure and evolution of low-mass stars

Abstract: We present extensive calculations of the structure and the evolution of low-massstars in the range 0.07-0.8 $\msol$, for metallicities $-2.0\le \mh \le 0.0$. These calculations are based on the most recent description of the microphysics characteristic of these dense and cool objects and on the lattest generation of grainless non-grey atmosphere models. We examine the evolution of the different mechanical and thermal properties of these objects as a function of mass and metallicity. We also demonstrate the inaccuracy of grey models and $T(\tau)$ relationships under these conditions. We provide detailed tables of the mass-radius-luminosity-effective temperature relations for various ages and metallicities, aimed at calibrating existing or future observations of low-mass stars and massive brown dwarfs. We derive new hydrogen-burning minimum masses, within the afore-mentioned metallicity range. These minimum masses are found to be smaller than previous estimates, a direct consequence of non-grey effects. At last, we examine the evolution of the abundance of light elements, $Li, Be$ and $B$, as a function of age, mass and metallicity.

Structure and evolution of low-mass stars

Abstract: We present extensive calculations of the structure and the evolution of low-mass stars in the range 0.07-0.8 M, for metallicities 2:0 (M=H) 0:0. These calculations are based on the most recent description of the microphysics char- acteristic of these dense and cool objects and on the latest gen- eration of grainless non-grey atmosphere models. We examine the evolution of the different mechanical and thermal properties of these objects as a function of mass and metallicity. We also demonstrate the inaccuracy of grey models and T() relation- ships under these conditions. We provide detailed tables of the mass-radius-luminosity-effective temperature relations for var- ious ages and metallicities, aimed at calibrating existing or fu- ture observations of low-mass stars and massive brown dwarfs. We derive new hydrogen-burning minimum masses, within the afore-mentioned metallicity range. These minimum masses are found to be smaller than previous estimates, a direct conse- quence of non-grey effects. At last, we examine the evolution of the abundance of light elements, Li;Be and B, as a function of age, mass and metal- licity.

Evolutionary models for metal-poor low-mass stars. Lower main sequence of globular clusters and halo field stars

Abstract: We have performed evolutionary calculations of very-low-mass stars from 0.08 to 0.8 M for different metal- licites from (M/H)= -2.0 to -1.0 and we have tabulated the me- chanical, thermal and photometric characteristics of these mod- els.Thecalculationsincludethemostrecentinteriorphysicsand improved non-grey atmosphere models. The models reproduce theentiremainsequencesoftheglobularclustersobservedwith the Hubble Space Telescope over the afore-mentioned range of metallicity.ComparisonsaremadeintheWFPC2Flightsystem including the F555, F606 and F814 lters, and in the standard Johnson-Cousins system. We examine the effects of different physical parameters, mixing-length, -enriched elements, he- lium fraction, as well as the accuracy of the photometric trans- formations of the HST data into standard systems. We derive mass-effective temperature and mass-magnitude relationships andwecomparetheresultswiththeonesobtainedwithdifferent grey-like approximations. These latter are shown to yield inac- curate relations, in particular near the hydrogen-burning limit. Wederivenewhydrogen-burningminimummasses,andthecor- responding absolute magnitudes, for the different metallicities. We predict color-magnitude diagrams in the infrared NIC- MOS lters, to be used for the next generation of the HST ob- servations, providing mass-magnitudes relationships in these colors down to the brown-dwarf limit. We show that the ex- pected signature of the stellar to substellar transition in color- magnitude diagrams is a severe blueshift in the infrared colors, due to the increasing collision-induced absorption of molecular hydrogen with increasing density and decreasing temperature. Atlast,weapplythesecalculationstotheobservedhaloeld stars, which yields a precise determination of their metallicity, and thus of their galactic origin. We nd no evidence for signif- icant differences between the halo eld stars and the globular cluster sequences.

Pulsations in red supergiants with high L/M ratio -- Implications for the stellar and circumstellar structure of supernova progenitors

Abstract: We investigate the pulsational properties of RSG models --- which we evolve from ZAMS masses in the range 10 to $20 \Msun$ --- by means of linear and non-linear calculations. We find period and growth rate of the dominant fundamental mode to increase with increasing luminosity-to-mass ratio $L/M$. Our models obtain relatively large $L/M$ values due to the inclusion of rotation in the evolutionary calculations; however, the largest values are obtained at and beyond central He-exhaustion due to major internal rearrangements of the nuclear burning regions. Our non-linear calculations as well as the behavior of the linear period and growth rate of the pulsations for periods approaching the Kelvin-Helmholtz time scale of the H-rich stellar envelope point towards the possibility of large amplitude pulsations. Such properties are similar to that found in AGB stars and suggest the possibility of a ``superwind'' to occur before the RSGs explode as supernovae. We conclude that changes in global stellar properties during the last few $10^4 \yr$ before core collapse may lead to drastic changes in the pulsational and wind properties of pre-supernova stars, with marked consequences for the immediate pre-supernova structure of the star and the circumstellar medium. We compare our results with observations of long-period OH/IR variables and discuss observational evidence for our scenario from observed supernova light curves, spectra and remnants.

Pulsations in red supergiants with high L/M ratio. Implications for the stellar and circumstellar structure of supernova progenitors

Abstract: We investigate the pulsational properties of RSG models { which we evolve from ZAMS masses in the range 10 to 20M { by means of linear and non-linear calculations. We nd period and growth rate of the dominant fundamental mode to increase with increasing luminosity-to-mass ratio L=M. Our models obtain relatively large L=M values due to the inclusion of rotation in the evolutionary calculations; however, the largest values are obtained at and beyond central He-exhaustion due to major internal rearrangements of the nuclear burning regions. Our non-linear calculations as well as the behavior of the linear periodandgrowthrateofthepulsationsforperiodsapproaching the Kelvin-Helmholtz time scale of the H-rich stellar envelope pointtowardsthepossibilityoflargeamplitudepulsations.Such properties are similar to those found in AGB stars and suggest the possibility of a \superwind" to occur before the RSGs ex- plode as supernovae. We conclude that changes in global stellar properties during the last few 10 4 yr before core collapse may lead to drastic changes in the pulsational and wind properties of pre-supernova stars, with marked consequences for the imme- diate pre-supernova structure of the star and the circumstellar medium. We compare our results with observations of long- period OH/IR variables and discuss observational evidence for our scenario from observed supernova light curves, spectra and remnants.

Theory of low-mass stars, brown dwarfs and extra-solar giant planets

Abstract: We present a review of the most recent improvements in the theory of low-mass star-like objects, in the stellar and sub-stellar domains. Comparison is made with various recent observations.

Theory of low mass stars, brown dwarfs and extra-solar giant planets

Abstract: Accurate modeling of the mechanical and thermal properties of very-lowmass stars (VLMS), Brown Dwarfs (BD) and Extra-solar Giant Planets (EGP) is of prior importance for a wide range of physical and astrophysical problems, from the fundamental physics point of view to the astrophysical and cosmological implications. They provide natural laboratories to test the different theories, equations of state, nuclear reaction rates, model atmospheres aimed at describing the physics of dense and cool objects. They represent the largest stellar population in the Galaxy, and thus provide a substantial contribution to the Galactic (disk) mass budget. Finally they represent one of the most intriguing questions in our understanding of the formation of star-like objects: are planet and star formation processes really different? Is there, and if so what is, a minimum mass for the formation of star-like objects? This field has blossomed recently with the discovery of several brown dwarfs (Nakajima et al, 1995; Rebolo et al., 1995) and numerous exoplanets since 51 Pegasi (Mayor and Queloz 1995; Mayor, this conference), which provide important information to challenge the theory.

Evolutionary models for metal-poor low-mass stars. Lower main sequence of globular clusters and halo field stars

Abstract: We have performed evolutionary calculations of very-low-mass stars from 0.08 to 0.8 $\msol$ for different metallicites from [M/H]= -2.0 to -1.0 and we have tabulated the mechanical, thermal and photometric characteristics of these models. The calculations include the most recent interior physics and improved non-grey atmosphere models. The models reproduce the entire main sequences of the globular clusters observed with the Hubble Space Telescope over the afore-mentioned range of metallicity. Comparisons are made in the WFPC2 Flight system including the F555, F606 and F814 filters, and in the standard Johnson-Cousins system. We examine the effects of different physical parameters, mixing-length, $\alpha$-enriched elements, helium fraction, as well as the accuracy of the photometric transformations of the HST data into standard systems. We derive mass-effective temperature and mass-magnitude relationships and we compare the results with the ones obtained with different grey-like approximations. These latter are shown to yield inaccurate relations, in particular near the hydrogen-burning limit. We derive new hydrogen-burning minimum masses, and the corresponding absolute magnitudes, for the different metallicities. We predict color-magnitude diagrams in the infrared NICMOS filters, to be used for the next generation of the HST observations, providing mass-magnitudes relationships in these colors down to the brown-dwarf limit. We show that the expected signature of the stellar to substellar transition in color-magnitude diagrams is a severe blueshift in the infrared colors, due to the increasing collision-induced absorption of molecular hydrogen with increasing density and decreasing temperature.

Theory of low mass stars, brown dwarfs and extra-solar giant planets.

Abstract: Accurate modeling of the mechanical and thermal properties of very-low-mass stars (VLMS), Brown Dwarfs (BD) and Extra-solar Giant Planets (EGP) is of prior importance for a wide range of physical and astrophysical problems, from the fundamental physics point of view to the astrophysical and cosmological implications. They provide natural laboratories to test the different theories, equations of state, nuclear reaction rates, model atmospheres aimed at describing the physics of dense and cool objects. They represent the largest stellar population in the Galaxy, and thus provide a substantial contribution to the Galactic (disk) mass budget. Finally they represent one of the most intriguing questions in our understanding of the formation of star-like objects: are planet and star formation processes really different ? Is there, and if so what is, a minimum mass for the formation of star-like objects ? This field has blossomed recently with the discovery of several brown dwarfs (Nakajima et al. 1995; Rebolo et al., 1995) and numerous exoplanets since 51 Pegasi (Mayor and Queloz 1995; Mayor, this conference), which provide important information to challenge the theory.

Atmosphere models for very low mass stars, brown dwarfs and exoplanets.

Abstract: Over the past decade considerable effort, both observational and theoretical, has been directed towards a more accurate determination of the stellar lower main sequence and of the sub-stellar domain covered by Brown Dwarfs and Planets. Astronomers have been looking for brown dwarfs for more than a decade, either with standard astronomical technics or with microlensing experiments. A breakthrough in the search for brown dwarfs was very recently achieved with the discovery of the first cool brown dwarf GL 229B (Nakajima et al. 1995). At the same epoch, the search for planets blossomed with the discovery of a Jupiter - mass companion of the star 51 Pegasi (Mayor and Queloz 1995). Now, the number of faint, cool stars and substellar objects is rising rapidly.

Non-linear models for the pulsating post-AGB star SAO 96709: metallic lines.

Abstract: Wehaveperformedasetofnonlinearpulsatingmod- els for post-AGB stars and have reconstructed the spectral lines for two chemical elements (BaII, CI). The present analysis is devoted to the interpretation of recent spectroscopic observa- tions of the pulsating post-AGB star SAO96709. We conrm our earlier result (Jeannin et al. 1996) that the dominant pul- sation mode is the rst overtone and nd an increase of the second overtone in the Fourier spectrum with decreasing mass and metallicity Z. The Doppler velocities deduced from the the- oretical line proles vary from 10 to 15 km.s 1 and are in a good agreement with observations. In order to reproduce the observed line broadening, a very large microturbulent velocity has to be introduced, probably with some variations during the pulsational cycle.