A Far-UV Variability Survey of the Globular Cluster M80

TL;DR

This study conducted a far-UV variability survey of M80's core, discovering three variable sources, including an RR Lyrae star, and providing insights into their properties and classifications.

Contribution

First far-UV variability survey of M80's core identifying and characterizing three variable stars with multi-wavelength confirmation.

Findings

Identified three variable sources, including an RR Lyrae star.

Confirmed variability of TDK1 across multiple wavebands.

Estimated physical parameters of TDK1 consistent with RR Lyrae characteristics.

Abstract

We have searched for variable sources in the core region of M80, using far ultra-violet 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 RR Lyrae star that reached maximum brightness during our observations. The light curve shows a >3 mag FUV brightening over the course of ~5 hours, with an estimated peak brightness of ~16.7 mag, followed by a decrease to ~20 mag. Archival optical data obtained with WFPC2 confirm that TDK1 is variable in all wavebands. TDK1's SED is reasonably fit by a star with temperature T(eff)=6700K and radius R=4.2R(sun), consistent with the suggestion that it is an RR Lyrae. Based on the photometric and variability characteristics of the other two variables, we suggest that TDK2 is likely to be an SX Phoenicis star with ~55 minutes period, and TDK3 is likely another RR Lyrae. Finally, we briefly discuss the FUV counterparts to two previously known variables in M80, the classical nova T Sco and the dwarf nova DN1.