Models for Metal-Poor Stars with Different Initial Abundances of C, N, O, Mg, and Si. III. Grids of Isochrones for $-2.5\le$ [Fe/H] $\le -0.5$ and Helium Abundances $Y=0.25$ and $0.29$ at Each Metallicity

Abstract

Stellar evolutionary tracks for $0.12\le M/M_{\odot }\le 1.0$ have been computed for each of several variations in the abundances of C, N, and O, assuming mass-fraction helium abundances $Y=0.25$ and $0.29$, and 11 metallicities in the range $-1.5\le$ [Fe/H] $\le -0.5$, in 0.2 dex increments. Such computations are provided for mixtures with [O/Fe] between $+0.4$ and $+0.8$, for different C:N:O ratios at a fixed value of [CNO/Fe], and for enhanced C. Computer codes are provided to interpolate within these grids to produce isochrones for ages $>7$ Gyr and to generate magnitudes and colours for many broad-band filters using bolometric corrections based on MARCS model atmospheres and synthetic spectra. The models are compared with (i) similar computations produced by other workers, (ii) observed UV, optical, and IR colour-magnitude diagrams (CMDs), (iii) the effective temperatures, $(V-I_C{)}_0$ and $(V-K_S{)}_0$ colours of Pop. II stars in the solar neighbourhood, and (iv) empirical data for the absolute magnitude of the tip of the giant branch (TRGB). The isochrones are especially successful in reproducing the observed morphologies of observed CMDs and in satisfying the TRGB constraints. They also fare quite well in explaining the IR colours of low mass stars in globular clusters, indicating that they have [O/Fe] $\approx +0.6$, though some challenges remain.

Accepted for publication in the MNRAS; the paper consists of 20 pages, incuding 13 figures

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