Time-Series BVI Photometry for the Globular Cluster NGC 6981 (M72)

TL;DR

This study provides new BVI photometry and a detailed analysis of NGC 6981 (M72), including its stellar populations, surface density profile, and variable stars, revealing potential tidal disruption and confirming its age similarity to M3.

Contribution

The paper offers the first comprehensive BVI photometry of NGC 6981, identifies its blue straggler and variable star populations, and suggests possible tidal disruption effects.

Findings

Blue stragglers are more centrally concentrated.

Extratidal main sequence stars suggest tidal disruption.

NGC 6981 and M3 are essentially coeval.

Abstract

We present new BVI photometry of the globular cluster NGC 6981 (M72), based mostly on ground-based CCD archive images. We present a new color-magnitude diagram (CMD) that reaches almost four magnitudes below the turn-off level. We performed new derivations of metallicity and morphological parameters of the evolved sequences, in good agreement with previous authors, obtaining a value of [Fe/H] ~ -1.50 in the new UVES scale. We also identify the cluster's blue straggler population. Comparing the radial distribution of these stars with the red giant branch population, we find that the blue stragglers are more centrally concentrated, as found in previous studies of blue stragglers in globular clusters. Taking advantage of the large field of view covered by our study, we analyzed the surface density profile of the cluster, finding extratidal main sequence stars out to r ~ 14.1 arcmin or about twice the tidal radius. We speculate that this may be due to tidal disruption in the course of M72's orbit, in which case tidal tails associated with the cluster may exist. We also take a fresh look at the variable stars in the cluster, recovering all previous known variables, including three SX Phoenicis stars, and adding three previously unknown RR Lyrae (1 c-type and 2 ab-type) to the total census. Finally, comparing our CMD with unpublished data for M3 (NGC 5272), a cluster with similar metallicity and horizontal branch morphology, we found that both objects are essentially coeval.