A variable star population in the open cluster NGC 6791 observed by the Kepler spacecraft

TL;DR

This study catalogs and analyzes 278 variable stars in the open cluster NGC 6791 using Kepler data and Gaia astrometry, identifying cluster members, their types, and orbital period variations, and deriving cluster properties.

Contribution

It provides the first comprehensive list of variable stars in NGC 6791 with classifications, membership probabilities, and orbital analyses, including 119 newly discovered variables.

Findings

Identified 278 variable stars, with 62 cluster members.

Detected only three stars with significant orbital period variations.

Derived cluster parameters: age 8.91 Gyr, distance ~4134 pc, [Fe/H] ~0.27.

Abstract

We present the list of variable stars we found in the Kepler superstamp data covering approximately 9 arcminutes from the central region of NGC 6791. We classified the variable stars based on the variability type and we established their cluster membership based on the available Gaia Early Data Release 3 astrometry, by means of the Bayesian Gaussian mixture models. In total we found 278 variable objects, among which 17 binaries, 45 pulsators, 62 rotational and five unclassified variables are cluster members. The remaining 28 binaries, 25 pulsators, 83 rotational, four unclassified and nine unidentified variables are either not members or their membership is not established. In the case of eclipsing binaries we calculated the mid-times of eclipses and derived ephemerides. We searched for eclipse timing variation by means of the observed minus calculated diagrams. Only three objects show significant orbital period variation. Independently of a report published just recently by Colman et al(2022) we found 119 new variables. We used isochrones calculated within the MIST project and derived the age (8.91 Gyr), average distance (4134 pc) and iron content [Fe/H] (0.26-0.28), of NGC 6791. Using the cluster members with membership probabilities greater than 0.9, we calculated the distance to the cluster of 4123(31) pc, which agrees with the result from our isochrone fitting.