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 of the metal-rich, obscured globular cluster Terzan 5, using the near-infrared camera and multiobject spectrometer (NICMOS) on board the Hubble Space Telescope. This extreme cluster has approximately solar metallicity and a central density that places it in the upper few percent of all clusters. It is estimated to have the highest interaction rate of any galactic globular cluster. The large extinction toward Terzan 5 and the severe stellar crowding near the cluster center present a substantial observational challenge. Using time-series analysis, we discovered two variable stars in this cluster. The first is an RRab Lyrae variable with a period of similar to0.61 days, a longer period than that of field stars with similar high metallicities. This period is shorter, however, than the average periods of RR Lyraes found in the metal-rich globular clusters NGC 6441, NGC 6388, and 47 Tuc. The second variable is a blue star with a 7 hr period sinusoidal variation and a likely orbital period of 14 hr. This star is probably an eclipsing blue straggler or (less likely) the infrared counterpart to the low-mass X-ray binary known in Terzan 5. Because of the extreme crowding and overlapping Airy profile of the IR point-spread function, we fall short of our original goal of detecting cataclysmic variables via Pa alpha emission and detecting variable infrared emission from the location of the binary millisecond pulsar in Terzan 5.