The Swift/UVOT Stars Survey. II. RR Lyrae Stars in M 3 and M 15
  We present the first results of an near-ultraviolet (NUV) survey of RR Lyrae
stars from the Ultraviolet Optical Telescope (UVOT) aboard the Swift Gamma-Ray
Burst Mission. It is well-established that RR Lyrae have large amplitudes in
the far- and near-ultraviolet. We have used UVOT's unique wide-field NUV
imaging capability to perform the first systematic NUV survey of variable stars
in the Galactic globular clusters M 3 and M 15. We identify 280 variable stars,
comprising 275 RR Lyrae, two anomalous Cepheids, one classical Cepheid, one SX
Phoenicis star and one possible long-period or irregular variable. Only two of
these are new discoveries. We compare our results to previous investigations
and find excellent agreement in the periods with significantly larger
amplitudes in the NUV. We map out, for the first time, an NUV Bailey diagram
from globular clusters, showing the usual loci for fundamental mode RRab and
first overtone RRc pulsators. We show the unique sensitivity of NUV photometry
to both the temperatures and the surface gravities of RR Lyrae stars. Finally,
we show evidence of an NUV period-metallicity-luminosity relationship. Future
investigations will further examine the dependence of NUV pulsation parameters
on metallicity and Oosterhoff classification.
