Stellar evolution through the ages: period variations in galactic RRab stars as derived from the GEOS database and TAROT telescopes

TL;DR

This study analyzes over a century of observational data on galactic RR Lyrae stars to investigate their period variations, providing new insights into stellar evolution and testing theoretical models.

Contribution

It compiles a comprehensive database of RR Lyrae period changes and identifies significant evolutionary period variations, including a notable number of stars with decreasing periods.

Findings

Evidence of both period increases and decreases, indicating evolutionary effects.

Large number of stars with decreasing periods (blueward evolution).

Observed period change rates are slightly larger than theoretical predictions.

Abstract

The theory of stellar evolution can be more closely tested if we have the opportunity to measure new quantities. Nowadays, observations of galactic RR Lyr stars are available on a time baseline exceeding 100 years. Therefore, we can exploit the possibility of investigating period changes, continuing the pioneering work started by V. P. Tsesevich in 1969. We collected the available times of maximum brightness of the galactic RR Lyr stars in the GEOS RR Lyr database. Moreover, we also started new observational projects, including surveys with automated telescopes, to characterise the O-C diagrams better. The database we built has proved to be a very powerful tool for tracing the period variations through the ages. We analyzed 123 stars showing a clear O-C pattern (constant, parabolic or erratic) by means of different least-squares methods. Clear evidence of period increases or decreases at constant rates has been found, suggesting evolutionary effects. The median values are beta=+0.14 day/Myr for the 27 stars showing a period increase and beta=-0.20 day/Myr for the 21 stars showing a period decrease. The large number of RR Lyr stars showing a period decrease (i.e., blueward evolution) is a new and intriguing result. There is an excess of RR Lyr stars showing large, positive $\beta$ values. Moreover, the observed beta values are slightly larger than those predicted by theoretical models.