Potassium: A New Actor on the Globular Cluster Chemical Evolution Stage. The Case of NGC 2808

Abstract

We derive [K/Fe] abundance ratios for 119 stars in the globular cluster NGC 2808, all of them having O, Na, Mg and Al abundances homogeneously measured in previous works. We detect an intrinsic star-to-star spread in the Potassium abundance. Moreover [K/Fe] abundance ratios display statistically significant correlations with [Na/Fe] and [Al/Fe], and anti-correlations with [O/Fe] and [Mg/Fe]. All the four Mg deficient stars ([Mg/Fe]<0.0) discovered so far in NGC 2808 are enriched in K by ∼0.3 dex with respect to those with normal [Mg/Fe].NGC 2808 is the second globular cluster, after NGC 2419, where a clear Mg-K anti-correlation is detected, albeit of weaker amplitude. The simultaneous correlation/anti-correlation of [K/Fe] with all the light elements usually involved in the chemical anomalies observed in globular cluster stars, strongly support the idea that these abundance patterns are due to the same self-enrichment mechanism that produces Na-O and Mg-Al anti-correlations. This finding suggests that detectable spreads in K abundances may be typical in the massive globular clusters where the self-enrichment processes are observed to produce their most extreme manifestations.

The potassium abundance in the globular clusters NGC 104, NGC 6752 and NGC 6809

“…However, the magnitude of the [K/Fe] spread, as well as the correlations with other abundance ratios, varies from cluster to cluster, emerging beyond our sensitivity limit only in clusters where the process of GC self-enrichment reached the most extreme effects (Carretta 2014;Mucciarelli et al 2015). At this stage this can be considered merely as a sound working hypothesis.…”

“…Further evidence of an intrinsic spread in the K abundances has been provided by Mucciarelli et al (2015) Mucciarelli et al (2015) conclude that the [K/Fe] enhancement detected in the Mg-poor stars of these two clusters and the K-Mg anti-correlations are ascribable to the same self-enrichment process responsible of the observed chemical anomalies. Carretta et al (2013) [Mg/Fe] anti-correlation in NGC 2419 and NGC 2808.…”

“…As per Mucciarelli et al (2015), we adopt the relation by Kirby et al (2009) that provides v t as a function of log g. This …”

A Chemical Composition Survey of the Iron-complex Globular Cluster NGC 6273 (M19)*

“…Since the full Mg-Al cycle is activated at a higher temperature than the O-N and Ne-Na cycles, the presence (or not) of a MgAl anti-correlation provides important insight into the burning temperatures achieved by the pollution sources. Similarly, a few of the most massive clusters also exhibit abundance variations that extend to elements as heavy as Si, K, and Sc, which is likely a byproduct of even higher temperature protoncapture burning (Yong et al 2005;Carretta et al 2009bCarretta et al , 2013Carretta et al , 2014Johnson & Pilachowski 2010;Cohen & Kirby 2012;Mucciarelli et al 2012Mucciarelli et al , 2015aVentura et al 2012;Carretta 2015;Roederer & Thompson 2015). Notably, many of these clusters share similar properties with NGC 6273, such as extended blue HBs.…”

Multiple Stellar Populations and Their Evolution in Globular Clusters: A Nucleosynthesis Perspective

“…3 and 4). Finally, an extremely unusual Mg-K anticorrelation was discovered in NGC 2808 (Mucciarelli et al, 2015) and in the old and metal-poor cluster GC NGC 2419 (Cohen et al, 2011;Cohen and Kirby, 2012;Mucciarelli et al, 2012) which is the most massive GC orbiting in the outermost suburbs of the halo of the Milky Way.The abundance variations of C, N, O, Na, Mg, and Al observed in GCs are clearly depicting the results of hydrogen-burning through the CNO-cycle and the NeNa-and MgAl-chains, as will be discussed in details in § 4. Importantly, the C+N+O abundance sum is found to be constant (within the measurement uncertainties) among stars of individual GCs (e.g.…”