A search for variable stars in the globular clusters

Abstract

A simple method for detecting possible variable RR Lyr stars and checking the variability of stars with unknown periods has been applied. It is based on the analysis of a colourcmagnitude diagram obtained from measurements of two plates (or CCD) taken simultaneously. A variable is thus at identical phase and the RR Lyrae stars are located in a definite strip. Using this method the globular clusters NGC 5286, NGC 6681, NGC 6934, NGC 6981 were investigated and about 50 new suspected variable stars were found. A knowledge of the total number of variable stars belonging to a globular cluster is an important factor connected with the evolution of these objects. This applies in particular to the RR Lyr variables, the periods and total number of which play a significant role in the yet unsolved second parameter problem. Our detailed investigation revealed that at present the information on the structure of the HB of individual globular clusters is far from complete. The search for variable stars in these objects has a long rich history. During the early stages of these studies it became clear that along with clusters with a sinall number of RR Lyr variables there exist clusters with a large number of these variables. The statistics for estimating the relative richness of these stars in globular clusters were first introduced by Kukarkin (1974). Our investigation (Kadla and Gerashchenko, 1982) showed that in both the Oosterhoff I1 and I groups there are variable-rich and variable-poor clusters. In the intermediate metallicity interval dividing these groups there coexist 0011 varia.ble-poor (IIVP) and 001 variable-rich (IVR) clusters. It was also evident that the central parts of most clusters had not been studied in detail and could contain a relatively large number of variable stars.