The Effect of the Electron Donor H3 on the Pre–main-sequence and Main-sequence Evolution of Low-mass, Zero-metallicity Stars

H3 has been shown (1991 work of Lenzuni and coworkers) to be the dominant positive ion in a zerometallicity gas at low temperature and intermediate to high density. It therefore affects both the number of free electrons and the opacity of the gas. The most recent H3 partition function (1995 work of Neale & Tennyson) is an order of magnitude larger at 4000 K than all previous partition functions, implying that H3 is a more important electron donor than previously thought. Here we present new Rosseland mean opacities for a hydrogen-helium gas of 1000 K T 9000 K and 14 log101⁄2 ðg cm Þ 2. In the calculation of these opacities, we have made use of the latest collision-induced absorption data as well as the most recent H3 partition function and line opacity data. It is shown that these updated and new sources of opacity give rise to a Rosseland mean opacity for a hydrogen-helium gas that is, in general, greater than that calculated in earlier works. The new opacity data are then used to model the evolution of low-mass (0.15–0.8 M ), zero-metallicity stars, from pre– main-sequence collapse to main-sequence turnoff. To investigate the effect of H3 on the evolution of low-mass, zero-metallicity stars, we repeat our calculations neglecting H3 as a source of electrons and line opacity. We find that H3 can have an effect on the structure and evolution of stars of mass 0.5 M or less. A gray atmosphere is used for the calculation, which is sufficient to demonstrate that H3 affects the evolution of very low mass stars to a greater degree than previously believed. Subject headings: atomic data — stars: evolution — stars: low-mass, brown dwarfs On-line material: machine-readable table