Individual element sensitivity for stellar evolutionary isochrones
  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.
