An AO-assisted variability study of four globular clusters
  The image subtraction technique applied to study variable stars in globular
clusters represented a leap in the number of new detections, with the drawback
that many of these new light curves could not be transformed to magnitudes due
to the severe crowding. In this paper we present observations of four Galactic
globular clusters, M 2 (NGC 7089), M 10 (NGC 6254), M 80 (NGC 6093) and NGC
1261, taken with the ground-layer adaptive optics module at the SOAR Telescope,
SAM. We show that the higher image quality provided by SAM allows the
calibration of the light curves of the great majority of the variables near the
cores of these clusters as well as the detection of new variables even in
clusters where image-subtraction searches were already conducted. We report the
discovery of 15 new variables in M 2 (12 RR Lyrae stars and 3 SX Phe stars), 12
new variables in M 10 (11 SX Phe and one long-period variable) and one new W
UMa-type variable in NGC 1261. No new detections are found in M 80, but
previous uncertain detections are confirmed and the corresponding light curves
are calibrated into magnitudes. Additionally, based on the number of detected
variables and new HST/UVIS photometry, we revisit a previous suggestion that M
80 may be the globular cluster with the richest population of blue stragglers
in our Galaxy.
