UBV Photometry of Bright Stars and Variables in the Globular Cluster NGC 6723

Abstract

Photographic photometry on the UBV system is presented for stars brighter than V = 17 m in NGC 6723. The height of the giant branch, |$\Delta V=2^\text m.3$| , and the colour of the subgiant branch, |$(B-V)_{0, \text g}=0.92$| , are consistent with a relatively high metal abundance in the cluster stars. This is supported by the value for the ultraviolet excess of the giants, |$\delta(U-B)=0.15$| , obtained from the two-colour diagram. From the position of the blue horizontal branch stars in the latter diagram, the reddening is found to be negligible. Ten new RR Lyrae stars and two red, possibly semi-regular, variables have been discovered in the cluster and one nearby H α emission star which is probably associated with the Corona Australis dark cloud was also found to be variable. BV photographic photometry is presented for these and for most of the already known variables in the cluster. Periods have been found for all the measurable RR Lyrae stars. The ratio by number of c-type to ab-type variables is 5/24 and the mean periods of the two groups are 0.291 days and 0.540 days, respectively, which are characteristic of an Oosterhoff type I cluster. The amplitude–period, amplitude–colour and period–colour diagrams for 12 RR Lyrae variables with the best photometry are consistent with a metal abundance in NGC 6723 between that of M3 and that of NGC 6171. The distribution of stars along the horizontal branch shows a marginal bias to the red side of the RR Lyrae gap and there appears to be a minimum in the number of stars as a function of ( B−V ) which coincides with the region occupied by the c-type RR Lyrae stars. This feature is also evident in the horizontal branches of two other relatively metal rich clusters, NGC 6712 and NGC 6981.