ABSTRACT We present the results from 10 nights of observations of the globular cluster NGC 6981 (M72) in the V, R and I Johnson wavebands. We employed the technique of difference image analysis to perform precision differential photometry on the time-series images, which enabled us to carry out a census of the understudied variable star population of the cluster. We show that 20 suspected variables in the literature are actually non-variable, and we confirm the variable nature of another 29 variables while refining their ephemerides. We also detect 11 new RR Lyrae variables and three new SX Phe variables, bringing the total confirmed variable star count in NGC 6981 to 43. We performed Fourier decomposition of the light curves for a subset of RR Lyrae stars and used the Fourier parameters to estimate the fundamental physical parameters of the stars using relations available in the literature. Mean values of these physical parameters have allowed us to estimate the physical parameters of the parent cluster. We I N T RO D U C T I O N The study of Galactic globular clusters is important for many reasons. These stellar systems represent some of the oldest, and consequently metal poor, stellar populations in the Galaxy, and their scrutiny allows us to glean information regarding the formation and early evolution of the Galaxy. The spatial distribution of the clusters reveals a different aspect of the Galactic structure than other stars in the Galaxy, and their orbits and tidal tails provide constraints on the Galactic potential. Of course, what we learn about globular clusters in our own Galaxy is also applicable to other galaxies as well. Globular clusters are also believed to be a close approximation to a stellar laboratory since a cluster's members were formed at the same time from the same primordial material with the same composition, leading to a homogeneity of certain properties within each E-mail: [email protected], [email protected] (DMB); [email protected] (RFJ); [email protected] (SG); armando@astroscu. unam.mx (AAF) cluster, but with differences in these properties between clusters. Although this paradigm is being challenged by the recent discovery in some globular clusters of multimodal main sequences and subgiant branches (Piotto 2009, and references therein), indicating the existence of multiple stellar populations, most globular clusters do not exhibit such obvious deviations from a simple stellar population and the paradigm still holds. There are ∼150 globular clusters in our Galaxy for which their fundamental properties, such as metallicity, distance, age and kinematics, have been estimated by various methods (Harris 1993). One independent method for estimating at least some of these quantities is by studying the population of RR Lyrae variable stars present in most clusters. This method uses the fact that the light-curve morphology of RR Lyrae stars is connected with their fundamental stellar parameters, and consequently quantities such as metallicity, absolute magnitude and effective temperature may be calculated from the fit parameters of the Fourier decomposition of their light curves using empirical, semi-empirical or theoretical relations published in recent year
