Estimating the parameters of globular cluster M 30 (NGC 7099) from time-series photometry

Abstract

Aims. We present the analysis of 26 nights of V and I time-series observations from 2011 and 2012 of the globular cluster M30 (NGC 7099). Weused our data to search for variable stars in this cluster and refine the periods of known variables; we then used our variable star light curves toderive values for the cluster’s parameters.Methods. We used difference image analysis to reduce our data to obtain high-precision light curves of variable stars.We then estimated the clusterparameters by performing a Fourier decomposition of the light curves of RR Lyrae stars for which a good period estimate was possible. We alsoderived an estimate for the age of the cluster by fitting theoretical isochrones to our colour-magnitude diagram (CMD).Results. Out of 13 stars previously catalogued as variables, we find that only 4 are bona fide variables. We detect two new RR Lyrae variables,and confirm two additional RR Lyrae candidates from the literature. We also detect four other new variables, including an eclipsing blue stragglersystem, and an SX Phoenicis star. This amounts to a total number of confirmed variable stars in M30 of 12. We perform Fourier decomposition ofthe light curves of the RR Lyrae stars to derive cluster parameters using empirical relations.We find a cluster metallicity [Fe/H]ZW = -2.01±0.04,or [Fe/H]UVES = -2.11 ± 0.06, and a distance of 8.32 ± 0.20 kpc (using RR0 variables), 8.10 kpc (using one RR1 variable), and 8.35 ± 0.42 kpc(using our SX Phoenicis star detection in M30). Fitting isochrones to the CMD, we estimate an age of 13.0 ± 1.0 Gyr for M30.