The Clusters AgeS Experiment (CASE): Variable Stars in the Globular Cluster M4

TL;DR

This study analyzes variable stars in globular cluster M4 over 14 years, discovering new variables, characterizing binary systems, and exploring stellar evolution and cluster properties with detailed light curves and extinction mapping.

Contribution

It provides the first extensive light curve analysis of M4 variables, including new variable detections, detailed binary system parameters, and insights into stellar evolution in globular clusters.

Findings

Discovered 10 new variable stars in M4.

Identified contact binaries with record-low mass ratio q=0.06.

Mapped differential extinction and determined cluster reddening E(B-V)=0.392 mag.

Abstract

Based on over 3000 BV images of M4 collected in years 1995-2009 we obtain light curves of 22 variables, 10 of which are newly detected objects. We identify four detached eclipsing binaries and eight contact binaries. Accurate periods are found for all but two variables. Nineteen variables are proper-motion members of the cluster, and the remaining three are field stars. Five variables are optical counterparts of X-ray sources. For one of the variables unassociated with X-ray sources we report a flare lasting for about 90 min and reaching an amplitude of 0.11 mag in V. One of the new contact binaries has a record-low mass ratio q=0.06. Another four such systems show season-to season luminosity variations probably related to magnetic activity cycles, whose lengths are surprisingly similar to that of the solar cycle despite a huge difference in rotational periods. The location of contact binaries on the color-magnitude diagram of M4 strongly suggests that at least in globular clusters the principal factor enabling EW systems to form from close but detached binaries is stellar evolution. We identify 46 blue and yellow stragglers in M4 and discuss their properties. We also derive a map of the differential extinction in the central part of M4, and determine the reddening of a selected reference region, E(B-V)= 0.392 mag.