Individual element sensitivity for stellar evolutionary isochrones

TL;DR

This paper investigates how individual element abundances affect stellar evolutionary models and their implications for galaxy age and composition estimates, revealing significant oxygen-age degeneracy and refining galaxy evolution insights.

Contribution

It introduces a method to incorporate element-specific temperature effects into isochrones, enhancing stellar population models and improving understanding of galaxy evolution.

Findings

Oxygen abundance strongly affects age estimates in galaxies.

Enhanced isochrones reveal older ages for early-type galaxies.

Varying Fe-peak elements increases age span and impacts galaxy evolution models.

Abstract

Stellar evolution calculations with variable abundance ratios were used to gauge the effects on temperatures, luminosities, and lifetimes in various phases. Individual elements C, N, O, Mg, Si, and Fe were included. Most of the effect relevant to integrated light models is contained in the temperature variable, as opposed to the timescale or luminosity. We derive a recipe for including abundance-sensitive temperature effects that is applicable to existing isochrone grids. The resultant enhanced isochrones are incorporated into composite stellar population models and compared with galaxy data from the Sloan Digital Sky Survey. A severe oxygen-age degeneracy is apparent, 2 - 3 Gyr per 0.1 dex in [O/R], where R represents a heavy element such as Fe. Over the range of early-type galaxy velocity dispersion, the spans of all abundance ratios are reduced but the age range increases, systematically older. Allowing Fe-peak elements the freedom to vary accentuates this increase of age span. Overall, these results sharpen the age-mass correlation known as downsizing but decrease the steepness of abundance ratio gradients. Both of these observations, in turn, imply a more robust contribution from gas free mergers in the histories of typical elliptical galaxies.