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K.C. Schlaufman, K.J. Mighell (National Optical Astronomy Observatory)
We report the serendipitous discovery of variable stars in the central region of the globular cluster M54 (NGC 6715) in the nearby Sagittarius dwarf spheroidal galaxy. Our photometry is based on the analysis of archival Hubble Space Telescope (HST) Wide Field Planetary Camera 2 (WFPC2) observations of central region of M54 in the F555W (~V) and the F814W (~I) filters. Although these observations were designed to fill two 90-minute orbits of the HST, the elapsed time between the first and last observations of this program was nearly 8 hours (>5 orbits). The extended time coverage of these observations provides an unforeseen opportunity for the first space-based detection of horizontal-branch variable stars in M54. The HSTphot photometric reduction package of Dolphin was used determine standard magnitudes of the resolved stars in each of the 12 short (<=350 s) exposures. Candidate variables were determined by computing a variety of statistical measures including a modified Welch-Stetson variability index. Most of our andidate variable stars are found on the PC1 images of the cluster center --- a region where no variables have been reported by previous ground-based studies of variables in M54. Only 8 of the short-period variables found by Layden and Sarjedini (2000) lie within the WFPC2 field-of-view of these observations; HSTphot recovered 7 of these 8 variables. We discuss the impact our newly discovered variables may have on the Oosterhoff classification of this bright massive globular cluster.
Schlaufman's research was supported by the NOAO/KPNO Research Experiences for Undergraduates (REU) Program which is funded by the National Science Foundation through Scientific Program Order No. 3 (AST-0243875) of the Cooperative Agreement No. AST-0132798 between the Association of Universities for Research in Astronomy (AURA) and the NSF.
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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.