Optical and UV Spectroscopy of UV Bright Stars (Identified with UIT) in the Globular Cluster M79

Previous abstract Next abstract

Session 74 -- Globular Clusters
Display presentation, Friday, January 14, 9:30-6:45, Salons I/II Room (Crystal Gateway)

[74.09] Optical and UV Spectroscopy of UV Bright Stars (Identified with UIT) in the Globular Cluster M79

K.-P. Cheng (Hughes STX/Catholic U.), R. Hill (Hughes STX), P. Hintzen, T.P. Stecher (GSFC/LASP), R.C. Bohlin (STScI), R.W. O'Connell (U Va), M.S. Roberts (NRAO), A.M. Smith (GSFC/LASP)

Observations of the globular cluster M79 with the Ultraviolet Imaging Telescope (UIT) have resulted in UV detections of $\sim$120 stellar sources, of which 91 are blue or extremely blue horizontal-branch (HB) stars. We present optical spectroscopy, obtained at CTIO, of 11 of these stars, including two hot stars brighter than the HB by at least 1.5 mag at 1520 \AA. The star designated UIT1 has the steepest continuum of any uncrowded UV source in the cluster and appears to be on the descending, pre-white dwarf portion of its evolutionary track. It is currently either a post-asymptotic giant branch (post-AGB) star or an extreme HB star that will never reach the AGB. We also present an IUE low-dispersion SWP spectrum of UIT 1. A fit of this spectrum to flux distributions computed from the low-metallicity LTE model atmospheres of Kurucz (1992) is performed with the goal of constraining the poorly known theoretical parameters that govern post-HB evolution. UIT 1 is discussed in terms of the sdO/sdOB/sdB classification scheme for hot subdwarfs.

Friday program listing