Helium and Multiple Populations in the Massive Globular Cluster NGC6266 (M62)

TL;DR

This study uses Hubble Space Telescope data to analyze multiple stellar populations in the massive globular cluster NGC6266, revealing a helium-enriched second population and supporting a correlation between helium variation, cluster mass, and horizontal branch morphology.

Contribution

It provides detailed evidence of helium variation in NGC6266 and its relation to cluster mass and HB extension, expanding understanding of multiple populations in massive GCs.

Findings

NGC6266 hosts a double main sequence with distinct helium contents.

The blue main sequence is highly helium-enhanced with Delta Y=0.08.

A helium spread of ~0.025 dex is present in the red main sequence.

Abstract

Recent studies suggest that the helium content of multiple stellar populations in globular clusters (GCs) is not uniform. The range of helium varies from cluster to cluster with more massive GCs having, preferentially, large helium spread. GCs with large helium variations also show extended-blue horizontal branch (HB). I exploit Hubble Space Telescope photometry to investigate multiple stellar populations in NGC6266 and infer their relative helium abundance. This cluster is an ideal target to investigate the possible connection between helium, cluster mass, and HB morphology, as it exhibits an extended HB and is among the ten more luminous GCs in the Milky Way. The analysis of color-magnitude diagrams from multi-wavelength photometry reveals that also NGC6266, similarly to other massive GCs, hosts a double main sequence (MS), with the red and the blue component made up of the 79+-1% and the 21+-1% of stars, respectively. The red MS is consistent with a stellar population with primordial helium while the blue MS is highly helium-enhanced by Delta Y=0.08+-0.01. Furthermore, the red MS exhibits an intrinsic broadening that can not be attributed to photometric errors only and is consistent with a spread in helium of ~0.025 dex. The comparison between NGC6266 and other GCs hosting helium-enriched stellar populations supports the presence of a correlation among helium variations, cluster mass, and HB extension.