A FAR-ULTRAVIOLET SURVEY OF M80: X-RAY SOURCE COUNTERPARTS, STRANGE BLUE STRAGGLERS, AND THE RECOVERY OF NOVA T SCO

Using the Advanced Camera for Surveys on Hubble Space Telescope, we have surveyed the far-ultraviolet (FUV) and near-ultraviolet (NUV) populations in the core region of M80. The color-magnitude diagram (CMD) reveals large numbers of blue and extreme horizontal branch stars and blue stragglers, as well as approximate to 60 objects lying in the region of the CMD where accreting and detached white dwarf binaries are expected. Overall, the blue straggler stars are the most centrally concentrated population, with their radial distribution suggesting a typical blue straggler mass of about 1.2 M(circle dot). However, counterintuitively, the faint blue stragglers are significantly more centrally concentrated than the bright ones and a Kolmogorov-Smirnov test suggest only a 3.5% probability that both faint and bright blue stragglers are drawn from the same distribution. This may suggest that (some) blue stragglers get a kick during their formation. We have also been able to identify the majority of the known X-ray sources in the core with FUV bright stars. One of these FUV sources is a likely dwarf nova that was in eruption at the time of the FUV observations. This object is located at a position consistent with Nova 1860 AD, or T Scorpii. Based on its position, X-ray and UV characteristics, this system is almost certainly the source of the nova explosion. The radial distribution of the X-ray sources and of the cataclysmic variable candidates in our sample suggest masses > 1 M(circle dot).