CCD time-series photometry of the globular cluster NGC 6981: Variable star census and physical parameter estimates

TL;DR

This study uses difference image analysis to identify and characterize variable stars in NGC 6981, refining their properties, estimating cluster parameters, and confirming its Oosterhoff type and age.

Contribution

First comprehensive variable star census in NGC 6981 using difference image analysis, including discovery of new variables and detailed physical parameter estimation.

Findings

Confirmed 29 known variables and discovered 14 new variables.

Estimated cluster metallicity as [Fe/H] ≈ -1.48 and distance ~16.7 kpc.

Confirmed Oosterhoff type I and age ~12.75 Gyr.

Abstract

We present the results from 10 nights of observations of the globular cluster NGC 6981 (M72) in the V, R and I Johnson wavebands. We employed the technique of difference image analysis to perform precision differential photometry on the time-series images, which enabled us to carry out a census of the under-studied variable star population of the cluster. We show that 20 suspected variables in the literature are actually non-variable, and we confirm the variable nature of another 29 variables while refining their ephemerides. We also detect 11 new RR Lyrae variables and 3 new SX Phe variables, bringing the total confirmed variable star count in NGC 6981 to 43. We performed Fourier decomposition of the light curves for a subset of RR Lyrae stars and used the Fourier parameters to estimate the fundamental physical parameters of the stars using relations available in the literature. Mean values of these physical parameters have allowed us to estimate the physical parameters of the parent cluster. We derive a metallicity of [Fe/H]_ZW = -1.48+-0.03 on the Zinn & West (1984) scale (or [Fe/H]_UVES = -1.38+-0.03 on the new Carretta et al. (2009) scale) for NGC 6981, and distances of ~16.73+-0.36 kpc and ~16.68+-0.36 kpc from analysis of the RR0 and RR1 stars separately. We also confirm the Oosterhoff type I classification for the cluster, and show that our colour-magnitude data is consistent with the age of ~12.75+-0.75 Gyr derived by Dotter et al (2010).