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Session 11 - Elliptical Galaxies.
Display session, Monday, June 10
Great Hall,

[11.06] A Grid of Model Atmospheres and Synthetic Spectra for the Far Ultraviolet Analysis of Old Stellar Populations

T. M. Brown, A. F. Davidsen (JHU), H. C. Ferguson (STScI)

We present a grid of stellar synthetic spectra suitable for detailed comparison to far-ultraviolet (FUV) observations obtained with the Hopkins Ultraviolet Telescope, IUE, and 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 1,497 spectra span a range of 10,000 K \leq T_eff \leq 250,000 K and 2 \leq log g \leq 8.5, with three metallicities: Z = Z_\sun, Z = 0.1 Z_\sun, and Z = 0.01 Z_\sun. 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 17000, 29900, 36100, and 55000 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 reliable measure of T_eff or log g.

While the synthetic spectra in this grid may not be appropriate for detailed analysis of high S/N 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.

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