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Session 80 - Star Clusters in the Milky Way.
Display session, Friday, January 09
Exhibit Hall,
Images of the globular cluster M13 were obtained with the Ultraviolet Imaging Telescope (UIT) during the Astro-2 Spacelab mission. A total of 488 stars is detected in the UIT B5 filter centered on 1620ÅOf these, 427 are correlated with ground-based Johnson V photometry provided by Montgomery (1995). A color-magnitude diagram constructed from the (m_1620-V) color index and m_1620 magnitude show a well populated blue horizontal branch extending to a temperature in excess of 35,000 K . A King plot of radial density distribution is consistant with the findings of Laget et al (1992) and no radial color gradient is detected.
Recent evolutionary models by Sweigart(1997a,b) that include the effects of helium mixing into the hydrogen envelope of horizontal branch stars in addition to the latest canonical models are considered in discussing the extreme blue horizontal branch morphology. An offset towards higher luminosity of stars near the ZAHB in the 4.1<\log (T_eff)<4.3 region, and an unusually small population of post-horizontal branch stars are also discussed.
