Stellar Variability in the Metal‐rich, Obscured Globular Cluster Terzan 5

We present the results of a search for variability in and near the core ofthe metal-rich, obscured globular cluster Terzan 5, using NICMOS on HST. Thisextreme cluster has approximately solar metallicity and a central density thatplaces it in the upper few percent of all clusters. It is estimated to have thehighest interaction rate of any galactic globular cluster. The large extinctiontowards Terzan 5 and the severe stellar crowding near the cluster centerpresent a substantial observational challenge. Using time series analysis wediscovered two variable stars in this cluster. The first is a RRab Lyraevariable with a period of ~0.61 days, a longer period than that of field starswith similar high metallicities. This period is, however, shorter than theaverage periods of RR Lyraes found in the metal-rich globular clusters NGC6441, NGC 6388 and 47 Tuc. The second variable is a blue star with a 7-hourperiod sinusoidal variation and a likely orbital period of 14 hours. This staris probably an eclipsing blue straggler, or (less likely) the infraredcounterpart to the low mass X-ray binary known in Terzan 5. Due to the extremecrowding and overlapping Airy profile of the IR PSF, we fall short of ouroriginal goal of detecting CVs via Palpha emission and detecting variableinfrared emission from the location of the binary MSP in Terzan 5.Comment: 17 pages including 12 figures. Accepted for publication in Ap