Precision age indicators that exploit chemically peculiar stars
  We would like to find a way to improve the determination of galaxy star
formation history from integrated light spectroscopy. To this end, several
classes of chemically peculiar (CP) stars arise during the course of normal
evolution in single stars and noninteracting binary stars. An aging stellar
population has periods of time in which CP stars contribute to the integrated
light, and others in which the contributions fade. The HgMn stars, for example,
occupy a narrow temperature range of 10500 to 16000 K, which maps to a narrow
range of ages. Wolf-Rayet stars, He-poor stars, Bp-Ap stars, Am-Fm stars, and C
stars all become very common in a normal stellar population at various ages
between zero and several Gyr, fading in and out in a way that is analogous to
features used in stellar spectral classification. We examine population
fractions and light fractions in order to assess the feasibility of using CP
stars as age tracers. We find that, even though CP stars do not usually
dominate in number, there are enough of them so that the CP spectral features
are detectable in high-quality integrated spectra of young and intermediate age
stellar populations. The new technique should be calibratable and useful.
Furthermore, using CP signatures as age dating tools sidesteps reliance on
photometry that is susceptible to dust and Balmer features that are susceptible
to nebular fill-in.
