THE GLOBULAR CLUSTER NGC 5286. II. VARIABLE STARS

摘要

We present the results of a search for variable stars in the globular cluster (GC) NGC 5286, which has recentlybeen suggested to be associated with the Canis Major dwarf spheroidal galaxy. Fifty-seven variable stars weredetected, only 19 of which had previously been known. Among our detections one finds 52 RR Lyrae (22 RRcand 30 RRab), 4 long-period variables, and 1 type II Cepheid of the BL Herculis type. Periods are derived forall of the RR Lyrae as well as the Cepheid, and BV light curves are provided for all the variables. The meanperiod of the RRab variables is (Par)) = 0.656 days, and the number fraction of RRc stars is IV, INRR = 0.42, bothconsistent with an Oosterhoff II (OoII) type—thus making NGC 5286 one of the most metal-rich (Fe/H = —1.67)OoII globulars known to date. The minimum period of the RRabs, namely= 0.513 d, while stillconsistent with an OoII classification, falls toward the short end of the observed P_(ab,min)distribution for OoIIGCs. As was recently found in the case of the prototypical OoII GC M15 (NGC 7078), the distribution of starsin the Bailey diagram does not strictly conform to the previously reported locus for OoII stars. We provideFourier decomposition parameters for all of the RR Lyrae stars detected in our survey, and discuss the physicalparameters derived therefrom. The values derived for the RRcs are not consistent with those typically found forOoII clusters, which may be due to the cluster's relatively high metallicity—the latter being confirmed by ourFourier analysis of the ab-type RR Lyrae light curves. Using the recent recalibration of the RR Lyrae luminosityscale by Catelan Cortes, we derive for the cluster a revised distance modulus of (m — M)v = 16.04 mag.