Period changes in the RR Lyrae stars of NGC 6171 (M107)

Abstract

Based on photometric data obtained between 1935 and 2017, O −C diagrams were built for 22 RR Lyrae stars in the globular cluster NGC 6171, leading to the discovery of secular period changes in four variables for which we have calculated period change rates 𝛽. In contrast, we find that 82% of the sample stars have stable periods over the last 82 years. For the stable-period stars, the whole database has been employed to refine their periods. Among the period-changing stars, three (V10, V12, and V16) have decreasing periods that are larger than expected from stellar evolution. Despite these individual cases of significant period change rate, the global average of the measured period changes in the cluster is basically zero, in consonance with theoretical predictions for clusters with redder horizontal branches. The hitherto unpublished observations, now brought into public domain, are employed to calculate a set of times of maximum light, which is used in the present analysis.