Abstract:
I describe the serendipitous discovery of variable stars in the central
region of the bright massive globular cluster M54 (NGC 6715) in the
nearby Sagittarius dwarf spheroidal galaxy. The stellar 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
analysis of these HST observations of M54 with the quick-look stellar
photometry codes QDWFPC2 and QLWFPC2 lead to the serendipitous discovery
of more than 50 new bright variable stars in the central region of M54.
Most of the candidate variables 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. This discovery is an
example of how quick-look photometry can be used to explore the time
domain of observations in the HST Data Archive. The HSTphot photometric
reduction package of Dolphin was then used determine accurate standard
magnitudes of the resolved stars in each of the 12 short (<=350 s)
exposures. 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. The possible
impact of these newly discovered variables may have on the Oosterhoff
classification of this bright massive globular cluster will be
discussed.