A far-ultraviolet variability survey of the globular cluster M 80

Abstract

We have searched for variable sources in the core region of M80, using far-ultraviolet (FUV) data taken with the Advanced Camera for Surveys on board the Hubble Space Telescope. We found three sources that exhibit strong signs of variability in our data. Among these is source TDK1, which we believe to be an RRLyrae star that reached maximum brightness during our observations. The light curve shows a >~3mag FUV brightening over the course of ~5 h, with an estimated peak brightness of ~16.7mag, followed by a decrease to ~20mag. Archival optical data obtained with Wide-Field Planetary Camera 2 confirm that TDK1 is variable in all wavebands. TDK1's spectral energy distribution is reasonably fitted by a star with temperature K and radius , consistent with the suggestion that it is an RRLyrae. Based on the photometric and variability characteristics of the other two variables, we suggest that TDK2 is likely to be an SXPhoenicis star with min period, and TDK3 is likely another RRLyrae. Finally, we briefly discuss the FUV counterparts to two previously known variables in M80, the classical nova TSco and the dwarf nova DN1.