Observations of the Globular Cluster M79 (NGC 1904) with the Hopkins Ultraviolet Telescope

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Session 50 -- Globular Clusters
Display presentation, Wednesday, 9:20-6:30, Pauley Room

[50.04] Observations of the Globular Cluster M79 (NGC 1904) with the Hopkins Ultraviolet Telescope

W.V.Dixon, A.F.Davidsen (JHU), B.Dorman (UVA), H.C.Ferguson (IoA)

We present a spectrum of the globular cluster M79 obtained with the Hopkins Ultraviolet Telescope (HUT), which flew aboard the space shuttle Columbia on the Astro-1 mission in December, 1990. The spectrum extends from 1850 \AA\ to the Lyman limit at a resolution of $\sim 3$ \AA. The spectrum shows strong absorption by atomic hydrogen, but no emission features other than well-known geocoronal lines.

M79 has an extremely blue horizontal branch (HB), extending at its high-temperature end to magnitudes as faint as the main-sequence turnoff. Observations of M79 by the Ultraviolet Imaging Telescope (UIT; Hill et al. 1992, ApJ, 385, L17) indicate that most of the far-UV flux can be resolved into stars. We model the integrated spectrum by performing Monte Carlo simulations of the HB. We use the horizontal-branch evolutionary models of Dorman, Rood \& O'Connell (1993, preprint) to determine the effective temperatures and surface gravities of evolving HB and post-HB stars, and assume a mass distribution that decreases exponentially toward the end of ZAHB. The spectra of these models are interpolated from the grid of synthetic stellar fluxes computed by Kurucz (1991, CfA Preprint No. 3348) and summed to produce an integrated cluster spectrum. By allowing both the mean mass and the width of the mass distribution to vary, we are able to produce a variety of horizontal-branch morphologies.

We find the best fit to the far-UV spectrum of M79 is given by a horizontal-branch model for which the mass distribution has a mean of $\sim 0.6 M_{\sun}$ and a standard deviation of $\sim 0.06 M_{\sun}$. We discuss these results and compare them with the optical color-magnitude diagram of Ferraro et al. (1992, MNRAS, 256, 391) and the UIT photometry.

The Hopkins Ultraviolet Telescope Project is supported by NASA contract NAS 5-27000 to the Johns Hopkins University.

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