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

TL;DR

This study analyzes 82 years of photometric data of RR Lyrae stars in NGC 6171, revealing mostly stable periods with some secular changes, and refines their period measurements.

Contribution

It provides the first long-term analysis of period changes in RR Lyrae stars in NGC 6171, including new period change rates and refined periods.

Findings

82% of stars have stable periods over 82 years

Four stars show significant secular period changes

The average period change rate aligns with theoretical predictions

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 4 variables for which we have calculated their period change rates $\beta$. 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 data base has been employed to refine their periods. Among the period changing stars, three (V10, V12 and V16) have decreasing periods larger than expected from stellar evolution. Despite these individual cases of significant period change rate, the golbal 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 The hitherto unpublished observations, now brought into public domain, are employed to calculate a set of times of maximum light which are used in the present analysis.