Long-term photometric monitoring of RR Lyr stars in M3

TL;DR

This study analyzes 134 RR Lyrae stars in M3 over 120 years, revealing diverse period-change behaviors, some aligning with stellar evolution models and others indicating pulsation instabilities and Blazhko effects.

Contribution

It provides a comprehensive long-term analysis of RR Lyrae period changes, highlighting the prevalence of irregularities and pulsation instabilities not fully explained by current models.

Findings

Mean period-change rates agree with evolutionary models for some stars.

Many stars exhibit irregular period variations and Blazhko modulation.

A significant fraction shows behaviors inconsistent with simple stellar evolution.

Abstract

The period-change behaviour of 134 RR Lyrae stars in the globular cluster Messier 3 (M3) is investigated on the ~120-year time base of the photometric observations. The mean period-change rates (\beta \approx 0.01 d Myr^-1) of the subsamples of variables exhibiting the most regular behaviour are in good agreement with theoretical expectations based on Horizontal-Branch stellar evolution models. However, a large fraction of variables show period changes that contradict the evolutionary expectations. Among the 134 stars studied, the period-change behaviour of only 54 variables is regular (constant or linearly changing), slight irregularities are superimposed on the regular variations in 23 cases and the remaining 57 stars display irregular period variations. The light curve of ~50 per cent of the RRab stars is not stable, i.e., these variables exhibit Blazhko modulation. The large fraction of variables with peculiar behaviour (showing light-curve modulation and/or irregular O-C variation) indicate that, probably, variables with regular period changes incompatible with their evolutionary stages also could display some kind of instability of the pulsation light curve and/or period, but the available observations have not disclosed it yet. The temporal appearence of the Blazhko effect in some stars, and the 70-90 years long regular changes preceded or followed by irregular, rapid changes of the pulsation period in some cases support this hypothesis. [...] Abstract truncated due to the limitations of astroph. See full abstract in the paper.