MODELS FOR METAL-POOR STARS WITH ENHANCED ABUNDANCES OF C, N, O, Ne, Na, Mg, Si, S, Ca, AND Ti, IN TURN, AT CONSTANT HELIUM AND IRON ABUNDANCES

Abstract

Recent work has shown that most globular clusters have at least two\nchemically distinct components, as well as cluster-to-cluster differences in\nthe mean [O/Fe], [Mg/Fe], and [Si/Fe] ratios at similar [Fe/H] values. In order\nto investigate the implications of variations in the abundances of these and\nother metals for H-R diagrams and predicted ages, grids of evolutionary\nsequences have been computed for scaled solar and enhanced alpha-element\nmixtures, and for mixtures in which the assumed [m/Fe] value for each of the\nmetals C, N, O, Ne, Na, Mg, Si, S, Ca, and Ti has been increased, in turn, by\n0.4 dex at constant [Fe/H]. These tracks, together with isochrones for ages\nfrom 6 to 14 Gyr, have been computed for -3.0 < [Fe/H] < -0.6, with helium\nabundances Y = 0.25, 0.29, and 0.33 at each [Fe/H] value, using upgraded\nversions of the Victoria stellar structure program and the Regina interpolation\ncode, respectively. Turnoff luminosity versus age relations from isochrones are\nfound to depend almost entirely on the importance of the CNO-cycle, and thereby\nmainly on the abundance of oxygen. Since C, N, and O, as well as Ne and S, do\nnot contribute significantly to the opacities at low temperatures and\ndensities, variations in their abundances do not impact the Teff scale of red\ngiants. The latter is a strong function of the abundances of only Mg and Si\n(and Fe, possibly to a lesser extent), because they are so abundant and because\nthey are strong sources of opacity at low temperatures. For these reasons, Mg\nand Si also have important effects on the temperatures of main-sequence stars.\nDue to their low abundances, Na, Ca, and Ti are of little consequence for\nstellar models. The effects of varying the adopted solar metals mix and the\nhelium abundance at a fixed [Fe/H] are also briefly discussed.\n