Excess of Ca (and Sc) produced in globular cluster multiple populations: a first census in 77 Galactic globular clusters

Abstract

Multiple stellar populations in globular clusters (GCs) are distinct by their different abundances of light elements. The abundance anti-correlations point towards a nucleosynthesis origin due to high-temperature H burning, but it remains to be assessed which type of stars altered primordial abundances in GCs. In particular, the regime at very high temperature that shapes the variations in potassium as well as calcium and scandium, which has been detected in a few notable cases such as NGC 2419 and NGC 2808, is still poorly explored. We started a systematic search for excess of Ca (and Sc) in GC stars with respect to the level of unmodified field stars. This method has recently been proven to be highly efficient in revealing the outcome of the proton-capture reactions at very high temperatures. Statistically robust evidence of such excess was found in a small number of GCs (NGC 4833, NGC 6715, NGC 6402, NGC 5296, NGC 5824, and NGC 5139/ω Centauri) that join the previously known two clusters. For the first time we show that NGC 4833 is likely to host anti-correlated K and Mg abundances. All these GCs are among the most massive ones in the Galaxy. We found that the fraction of stars with Ca enhancement at 3σ above the field star distribution is a multivariate function of the GC mass and metallicity, as in other manifestations of the multiple population phenomenon in GCs. We argue that these alterations in only a few GCs can be reproduced by two different channels: either a class of ordinary stars, that is common to all GCs, acts only in particular environments, or an on-off mechanism is generated by the occurrence of a peculiar type of stars (or lack of such stars). Hot bottom-burning in asymptotic giant branch stars in the low-metallicity regime is a good candidate for the first class. Alternatively, a metallicity dependence is also expected for supermassive stars, which are predicted to preferentially form in massive GCs.

A New Set of Chisels for Galactic Archeology: Sc, V, and Zn as Taggers of Accreted Globular Clusters*

“…The elements we focus on in our investigation are generally not affected by the chemical pecularities associated with the so-called phenomenon of multi-populations in GCs (Bastian & Lardo 2018). The only possible exception is Sc, which shows possible variations in massive GCs (Carretta & Bragaglia 2021). NGC6441 and NGC6388 are indeed massive, but they do not show Sc variations according to the quoted analysis.…”

Testing for Dark Matter in the Outskirts of Globular Clusters

“…Nevertheless, the complications of orbital phase effects and the possibility that background stars contaminate the outer part of the velocity dispersion measurement means it is not yet possible to say whether the rising velocity dispersion profiles A local dark matter subhalo would deepen the gravitational potential of a cluster and could have some bearing on the multiple stellar populations of globular clusters (Gratton et al 2012). Detailed abundance studies do show some differences from field stars in NGC 6752 (Norris et al 1981) but no clear differences for NGC 6205 (Carretta & Bragaglia 2021). NGC 6752 stands out in a comparison of widths of the asymptotic giant branch and the red giant branch, but NGC 6205 does not (Lagioia et al 2021).…”

The in situ origin of the globular cluster NGC 6388 from abundances of Sc, V, and Zn of a large sample of stars

“…4) do not seem to have lower abundances than MW stars. Moreover, when examining mean values of [Sc/Fe] (the most commonly studied element among Sc, V, and Zn in GCs) collected for a large sample of GCs of any metal abundance in Carretta and Bragaglia (2021b), we again found that the average abundances of accreted GCs -apart from a couple of exceptionsare a perfect match to the MW pattern down to [Fe/H]∼ −2.5 dex.…”

“…Concerning the Zn abundance, all the mean values for the GCs are in good agreement with the pattern defined by field disc and bulge stars of the Galaxy, except for NGC 6441 from M21b. We would like to caution readers that the results for NGC 6441 -in particular those for Zn, resting on a single line lying very close to the blue spectrum border of the lower signal-to-noise ratio (S/N) -could be affected by a S/N lower than optimal, as only about a third of the requested observations were actually obtained (see Carretta and Bragaglia 2021b). In summary, our [Sc/Fe] ratio lies at the lower end of the distribution of field stars with a metallicity similar to that of NGC 6388, while the V abundance is close to the upper end.…”