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 comparison to far-ultraviolet (FUV) observations obtained with the Hopkins 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 than 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 assumptions have been made to reduce computer time and improve convergence, at the inevitable expense of some accuracy. Nevertheless, models in the grid reproduce the overall continuum shape and most of the absorption features seen in HUT spectra of four evolved stars at temperatures of 17,000, 29,900, 36,100, and 55,000 K. The most serious discrepancy 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 measure 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 ratio stellar spectra, they are sufficiently similar to the observed stars to provide a considerable advantage over existing models for the analysis of the FUV spectra of composite systems, such as elliptical galaxies and globular clusters, where the advantages of having a large, well-sampled grid of models tend to outweigh the known inadequacies of the individual grid points.