Ultraviolet Radiation from Old Stellar Populations

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Session 101 -- Evolved Stars, Supernova Remnants
Oral presentation, Friday, January 14, 2:15-3:45, Salons A/B Room (Crystal City Marriott)

[101.03] Ultraviolet Radiation from Old Stellar Populations

B. Dorman, R.W. O'Connell, R.T. Rood ((U.Va)

We present further results from a study of ultraviolet radiation from stellar populations in galaxies and globular clusters. Our object is to understand the UVX phenomenon, i.e. the amplitude and large variation of the ultraviolet component of the spectra of normal galaxy nuclei. We consider the observed correlation between the UVX and the strength of the absorption line index $\rm Mg_2$ (Burstein et.~al. , Ap.J. 328, 440 1988). We have computed a set of isochrones for a wide range in composition and age, which taken together with our grid of stellar models in advanced stages of evolution (Dorman, Rood, \& O'Connell 1993, Ap.J. 419, in press) are used to calculate synthetic UV color indices and the `specific UV luminosity', i.e. the UV flux per unit $V$ magnitude.

We compare the results of this study with observations of the galaxies and clusters in the colors $(1500-V)$ and $(1500-2500)$. In the globular clusters, the $(1500-V)$ color is dependent on the size of the population of the blue horizontal branch (HB) and post-HB UV-bright stars. The other color allows us to infer a characteristic temperature for the hot stellar component. Our calculations reproduce the integrated UV colors of the clusters with fair accuracy. We assume that the magnitude of the ultraviolet upturn in the galaxies [as quantified by the $(1500-V)$ color] is similarly dependent on their HB morphology. We thus determine the fraction of very blue stars that must be produced by an old metal-rich population in order to agree with the observations. Either high degrees of mass loss or large helium enrichment for $\rm [Fe/H] > 0$ can produce sufficiently large numbers of hot stars similar to the blue subdwarf population of our Galaxy. However since it takes a high value for the helium abundance ($Y > 0.40$) to enhance the number of blue HB stars, it is more plausible that mass loss plays the larger role. This interpretation will be reinforced if, as suggested by recent evidence, the strongest-lined galaxies are rich in $\alpha$-elements but have close to solar [Fe/H].

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