Stellar Models with Enhanced Abundances of C, N, O, Ne, Na, Mg, Si, S, Ca, and Ti, in Turn, at Constant Helium and Iron Abundances

TL;DR

This study investigates how variations in elemental abundances of key metals affect stellar evolution models, isochrones, and observable properties of globular clusters, emphasizing the roles of CNO elements, Mg, and Si.

Contribution

It provides new stellar evolutionary tracks and isochrones accounting for enhanced abundances of multiple metals at fixed [Fe/H], improving modeling of chemically diverse globular clusters.

Findings

CNO-cycle dominates turnoff luminosity-age relation

Mg and Si significantly influence stellar temperatures

Na, Ca, and Ti have minimal impact on models

Abstract

Recent work has shown that most globular clusters have at least two chemically distinct components, as well as cluster-to-cluster differences in the mean [O/Fe], [Mg/Fe], and [Si/Fe] ratios at similar [Fe/H] values. In order to investigate the implications of variations in the abundances of these and other metals for H-R diagrams and predicted ages, grids of evolutionary sequences have been computed for scaled solar and enhanced alpha-element mixtures, and for mixtures in which the assumed [m/Fe] value for each of the metals C, N, O, Ne, Na, Mg, Si, S, Ca, and Ti has been increased, in turn, by 0.4 dex at constant [Fe/H]. These tracks, together with isochrones for ages from 6 to 14 Gyr, have been computed for -3.0 < [Fe/H] < -0.6, with helium abundances Y = 0.25, 0.29, and 0.33 at each [Fe/H] value, using upgraded versions of the Victoria stellar structure program and the Regina interpolation code, respectively. Turnoff luminosity versus age relations from isochrones are found to depend almost entirely on the importance of the CNO-cycle, and thereby mainly on the abundance of oxygen. Since C, N, and O, as well as Ne and S, do not contribute significantly to the opacities at low temperatures and densities, variations in their abundances do not impact the Teff scale of red giants. The latter is a strong function of the abundances of only Mg and Si (and Fe, possibly to a lesser extent), because they are so abundant and because they are strong sources of opacity at low temperatures. For these reasons, Mg and Si also have important effects on the temperatures of main-sequence stars. Due to their low abundances, Na, Ca, and Ti are of little consequence for stellar models. The effects of varying the adopted solar metals mix and the helium abundance at a fixed [Fe/H] are also briefly discussed.