Time-Series BVI Photometry for the Globular Cluster NGC 6981 (M72)
  We present new BVI photometry of the globular cluster NGC 6981 (M72), based
mostly on ground-based CCD archive images. We present a new color-magnitude
diagram (CMD) that reaches almost four magnitudes below the turn-off level. We
performed new derivations of metallicity and morphological parameters of the
evolved sequences, in good agreement with previous authors, obtaining a value
of [Fe/H] ~ -1.50 in the new UVES scale. We also identify the cluster's blue
straggler population. Comparing the radial distribution of these stars with the
red giant branch population, we find that the blue stragglers are more
centrally concentrated, as found in previous studies of blue stragglers in
globular clusters. Taking advantage of the large field of view covered by our
study, we analyzed the surface density profile of the cluster, finding
extratidal main sequence stars out to r ~ 14.1 arcmin or about twice the tidal
radius. We speculate that this may be due to tidal disruption in the course of
M72's orbit, in which case tidal tails associated with the cluster may exist.
We also take a fresh look at the variable stars in the cluster, recovering all
previous known variables, including three SX Phoenicis stars, and adding three
previously unknown RR Lyrae (1 c-type and 2 ab-type) to the total census.
Finally, comparing our CMD with unpublished data for M3 (NGC 5272), a cluster
with similar metallicity and horizontal branch morphology, we found that both
objects are essentially coeval.
