Ultraviolet imaging telescope observations of the horizontal branch in M13

摘要

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 are detected in the UIT B5 filter centered at 1620 A. Of an Uncrowded sample of 247 stars, 232 are well correlated with ground-based Johnson V photometry, which was Provided by K. Montgomery (1995, private communication). A color-magnitude diagram, constructed from the (m_1620-V) color index and the M_1620 magnitude, shows a well-populated, blue horizontal branch extending to a temperature in excess of 35,000 K. A King plot of radial density distribution is satisfied by an exponential of index α=3.0, which is consistent with Laget et al.