A SEARCH FOR RR LYRAE VARIABLES IN THE CENTRAL REGION OF M54

Abstract

We have analyzed archival Hubble Space Telescope (HST) Wide Field Planetary Camera 2 (WFPC2) observations of the central region of the globular cluster M54. The 12 irregularly timed observations (6 F555W, 6 F814W) were reduced using the HSTphot stellar photometry package. Our statistical analysis of the HSTphot photometry yielded 50 variable stars that are likely RR Lyrae candidates. These candidates were missed by previous ground-based searches for RR Lyraes due to the extreme crowding in the core of M54. We performed two checks on our sample of RR Lyrae candidates to determine how different they were from the other variable candidates. The first test was a numerical analysis comparing the ratio of variable candidates to the total number of stars within the blue horizontal branch, the RR Lyrae region, and the red giant branch. We found ratios of 0.61%, 13%, and 1.1% in these regions, respectively. Our next test was a two-sample Kolmogorov-Smirnov (K-S) test, which allowed us to find out how different our 50 RR Lyrae candidates were from the 50 brightest other variable candidates. According to the K-S test, there is only a 0.0004% probability that these two populations are similar, indicating that our RR Lyrae candidates do indeed exhibit some real fluctuation in their brightness. We sampled the light curves of confirmed RR Lyrae light curves to simulate the HST observing conditions and found that a signature of variability in 2911 out of the 4000 (72%) generated stars. We estimate that there maybe as many as 19 more RR Lyrae candidates that were undetected in the WFPC2 field of view. The presence of additional RR Lyraes has implications into the current Oosterhoff classification for the cluster.