Chemical abundances along the 1G sequence of the chromosome maps: The Globular Cluster NGC 3201

TL;DR

This study analyzes chemical abundances in first-generation stars of globular cluster NGC 3201, revealing that metallicity variations and binarity influence the chromosome map's color spread, with blue stragglers contributing to the bluest stars.

Contribution

It provides the first detailed chemical abundance analysis of 24 elements in 1G stars across a wide range of the chromosome map in NGC 3201, highlighting the roles of metallicity variation and binarity.

Findings

Small metallicity variation (~0.10 dex) correlates with color spread.

Binarity is identified in stars with lowest Delta(F275W,F814W).

Blue stragglers contribute to the bluest part of the 1G sequence.

Abstract

The Hubble Space Telescope (HST) UV Legacy Survey of Galactic Globular Clusters (GCs) has investigated multiple stellar populations by means of the "chromosome map" (ChM) diagnostic tool that maximises the separation between stars with different chemical composition. One of the most challenging features revealed by ChMs analysis is the apparent inhomogeneity among stars belonging to the first population, a phenomenon largely attributed to He variations. However, this explanation is not supported by the uniformity in p-capture elements of these stars. The HST survey has revealed that the GC NGC 3201 shows an exceptionally wide coverage in the Delta(F275W,F814W) parameter of the ChM. We present a chemical abundance analysis of 24 elements in 18 giants belonging to the first population of this GC, and having a wide range in Delta(F275W,F814W). As far as the p-capture elements are concerned, the chemical abundances are typical of 1G stars, as expected from the location of our targets in the ChM. Based on radial velocities and chemical abundances arguments, we find that the three stars with the lowest Delta(F275W,F814W) values are binary candidates. This suggests that, at least those stars could be explained with binarity. These results are consistent with evidence inferred from multi-band photometry that evolved blue stragglers populate the bluest part of the 1G sequence in the ChM. The remaining 15 spectroscopic targets show a small range in the overall metallicity by ~0.10 dex, with stars at higher Delta(F275W,F814W) values having higher absolute abundances. We suggest that a small variation in metals and binarity govern the color spread of the 1G in the ChM, and that evolved blue stragglers contribute to the bluest tail of the 1G sequence.