2004ApJ...600.1025H

H⁺₃has been shown (1991 work of Lenzuni and coworkers) to be the dominant positive ion in a zero-metallicity 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 H⁺₃partition function (1995 work of Neale & Tennyson) is an order of magnitude larger at 4000 K than all previous partition functions, implying that H⁺₃is a more important electron donor than previously thought. Here we present new Rosseland mean opacities for a hydrogen-helium gas of 1000K≤T≤9000K and -14≤log₁₀[ρ(gcm^–3)]≤-2. In the calculation of these opacities, we have made use of the latest collision-induced absorption data as well as the most recent H⁺₃partition 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 H⁺₃on the evolution of low-mass, zero-metallicity stars, we repeat our calculations neglecting H⁺₃as a source of electrons and line opacity. We find that H⁺₃can 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 H⁺₃ affects the evolution of very low mass stars to a greater degree than previously believed.