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Astron. Astrophys. 327, 1054-1069


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

I. Baraffe1, G. Chabrier1, F. Allard2, and P.H. Hauschildt3

1C.R.A.L. (UMR 5574 CNRS), Ecole Normale Supérieure, F-69364 Lyon Cedex 07, France (ibaraffe, chabrier @ens-lyon.fr)
2Dept. of Physics, Wichita State University, Wichita, KS 67260-0032, USA (allard@eureka.physics.twsu.edu)
3Dept. of Physics and Astronomy, University of Georgia Athens, GA 30602-2451, USA (yeti@hal.physast.uga.edu)

Received 12 November 1996 / Accepted 11 April 1997

Abstract

We have performed evolutionary calculations of very-low-mass stars from 0.08 to 0.8 $M_\odot$ 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.

At last, we apply these calculations to the observed halo field stars, which yields a precise determination of their metallicity, and thus of their galactic origin. We find no evidence for significant differences between the halo field stars and the globular cluster sequences.

Key words: stars: low-mass, brown dwarfs - stars: evolution - stars: globular clusters


© European Southern Observatory (ESO) 1997

Online publication: October 20, 1997
Last change: April 6, 1998
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