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R.C. Peterson (UCO/Lick; Astrophysical Advances), B.W. Carney (UNC), B. Dorman, W.B. Landsman (GSFC), E.M. Green, J. Liebert (U. Arizona), R.W. O'Connell, R.T. Rood, R.P. Schiavon (UVa)
We describe our progress on our Hubble Treasury program, aimed at better determining the age and metallicity of old stellar systems. We are calculating mid-ultraviolet and optical spectra from first principles using individual stellar photospheric models, checking them against high-quality observational spectra of standard stars and clusters, and then combining them to match stellar clusters and elliptical galaxies between one and 20 Gyr old. Our first report (Peterson et al. 2003, ApJ, 588, 299) shows a half-dozen stellar spectra calculated at a metallicity one-third solar coadded in various combinations, and compared to the globular cluster G1 in the Andromeda galaxy (M31). The mid-UV reveals the presence of old hot horizontal branch stars. Currently we are generating composite spectra from weights derived from isochrones rather than empirically. The isochrones are created from stellar tracks we are computing for several elemental abundance ratios: scaled solar, oxygen-enhanced, and enhanced in the light elements Mg, Si, Ca, and Ti. We plot several composite spectra of about one-third solar metallicity and compare them to observed spectra, to illustrate their diagnostic potential for establishing the age, metallicity, and light-element abundance ratio.
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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.