Wide-field multi-color photometry of the Galactic globular cluster NGC 1261

TL;DR

This study provides detailed multi-color photometry of the globular cluster NGC 1261, revealing stellar population inhomogeneities, radial distribution variations, and estimates of metallicity, contributing to understanding its complex stellar makeup.

Contribution

It offers new multi-color photometric data and analysis of stellar population inhomogeneities and radial distributions in NGC 1261, a rarely studied globular cluster.

Findings

Radial distribution of SGB stars depends on U magnitude.

RGB stars' color varies with distance from cluster center.

Estimated metallicity of NGC 1261 is around -1.35 dex.

Abstract

(Abriged)This work studies in more detail the stellar population, including its photometric properties and characteristics, in the rarely studied southern Galactic globular cluster NGC 1261. We focus on the brighter sequences of the cluster's color-magnitude diagram (CMD). Like in our previous works, we rely upon photometry in several passbands to achieve more reliable results and conclusions. We carried out and analyzed new multi-color photometry of NGC 1261 in UBVI reaching below the turnoff point in all passbands in a fairly extended cluster field, about 14'x14'. We found several signs of the inhomogeneity ("multiplicity") in the stellar population. The most prominent of them are: (1) the dependence of the radial distribution of sub-giant branch (SGB) stars in the cluster on their U magnitude, with brighter stars less centrally concentrated at the 99.9 \% level than their fainter counterparts; (2) the dependence of the location of red giant branch (RGB) stars in the U-(U-B) CMD on their radial distance from the cluster center, with the portion of stars bluer in the (U-B) color increasing towards the cluster outskirts. Additionally, the radial variation of the RGB luminosity function in the bump region is suspected. We assume that both the SGB stars brighter in the U and the RGB stars bluer in the (U-B) color are probably associated with blue horizontal branch stars, because of a similarity in their radial distribution in the cluster. We estimated the metalicity of NGC 1261 from the slope of the RGB in U-based CMDs and the location of the RGB bump on the branch. These metallicity indicators give [Fe/H]zw = -1.34 +/- 0.16 dex and [Fe/H]zw = -1.41 +/- 0.10 dex, respectively. We isolated 18 probable blue straggler candidates. They are more centrally concentrated than the lower red giants of comparable brightness at the 97.9 \% level.