A GRID OF MODEL ATMOSPHERES AND SYNTHETIC SPECTRA FOR THE FAR-ULTRAVIOLET ANALYSIS OF OLD STELLAR POPULATIONS

We present a grid of stellar synthetic spectra suitable for detailed c omparison to far-ultraviolet (FUV) observations obtained with the Hopk ins Ultraviolet Telescope (HUT), IUE, and Hubble Space Telescope (HST) . Our specific application is to study the hot stellar populations in elliptical galaxies, but we anticipate that the models will be useful for other purposes. The 1497 spectra span a range of 10,000 K less tha n or equal to T-eff less than or equal to 250,000 K and 2 less than or equal to log g less than or equal to 8.5, with three metallicities: Z = Z(.), Z = 0.1 Z(.), and Z = 0.01 Z(.). A variety of simplifying ass umptions have been made to reduce computer time and improve convergenc e, at the inevitable expense of some accuracy. Nevertheless, models in the grid reproduce the overall continuum shape and most of the absorp tion features seen in HUT spectra of four evolved stars at temperature s of 17,000, 29,900, 36,100, and 55,000 K. The most serious discrepanc y is in the cores of the Lyman series lines, where the observed lines are not as deep as those obtained from the models. Until this problem is resolved, the Lyman series lines will not provide a very accurate m easure of T-eff or log g. While the synthetic spectra in this grid may not be appropriate for detailed analysis of high signal-to-noise rati o stellar spectra, they are sufficiently similar to the observed stars to provide a considerable advantage over existing models for the anal ysis of the FUV spectra of composite systems, such as elliptical galax ies and globular clusters, where the advantages of having a large, wel l-sampled grid of models tend to outweigh the known inadequacies of th e individual grid points.