Multiple stellar populations in the high-temperature regime: Potassium abundances in the globular cluster M 54 (NGC 6715)

TL;DR

This study investigates potassium abundances in 42 giants of the globular cluster M 54, revealing a significant spread and correlations with other elements, indicating high-temperature H-burning processes in multiple stellar populations.

Contribution

First homogeneous analysis of K in M 54 showing its relation to multiple stellar populations and high-temperature nucleosynthesis in globular clusters.

Findings

Detected a 1 dex spread in [K/Fe] among giants.

Found a significant anti-correlation between [K/Fe] and [O/Fe].

Observed a strong correlation between K and Ca.

Abstract

Among the multiple stellar populations in globular clusters (GCs) the very high-temperature H-burning regime, able to produce elements up to potassium, is still poorly explored. Here we present the first abundance analysis of K in 42 giants of NGC 6715 (M 54) with homogeneous abundances of light elements previously derived in our FLAMES survey. Owing to the large mass and low metallicity, a large excess of K could be expected in this GC, which is located in the nucleus of the Sagittarius dwarf galaxy. We actually found a spread in [K/Fe] spanning about 1 dex, with [K/Fe] presenting a significant anti-correlation with [O/Fe] ratios, regardless of the metallicity component in M 54. Evidence for a K-Mg anti-correlation also exists, but this is statistically marginal because of the lack of very Mg-poor stars in this GC. We found, however, a strong correlation between K and Ca. These observations clearl y show that the K enhancement in M 54 is probably due to the same network of nuclear reactions generating the phenomenon of multiple stellar populations, at work in a regime of very high temperature. The comparison with recent results in omega Cen is hampered by an unexplained trend with the temperatures for K abundances from optical spectroscopy, and somewhat by a limited sample size for infrared APOGEE data. There are few doubts, however, that the two most massive GCs in the Milky Way host a K-Mg anti-correlation.