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

Abstract

Chemical tagging of globular clusters (GCs) is often done using abundances of α-elements. The iron-peak elements Sc, V, and in particular Zn were proposed as an alternative to α-elements to tag accreted GCs in the metal-rich regime, where the dwarf galaxy Sagittarius and its GCs show peculiarly marked under-abundances of these heavier species with respect to Milky Way stars. A handful of stars in NGC 6388 was used to suggest an accreted origin for this GC, contradicting the results from dynamics. We tested the efficiency of the iron-peak method by using large samples of stars in NGC 6388, compared to thousands of field stars in the disc and the bulge of the Milky Way. Our abundance ratios of Sc (185 stars) and V (35 stars) for NGC 6388 are within about 1.5σ from the average for the field stars with a similar metallicity, and they are in perfect agreement for Zn (31 stars), claimed to be the most sensitive element concerning the accretion pattern. Moreover, the chemo-dynamical plots, coupled to the bifurcated age-metallicity relation of GCs in the Galaxy, clearly rule out any association of NGC 6388 to the groups of accreted GCs. Using a large set of GC abundances from the literature, we also show that the new method with Sc, V, and Zn seems to be efficient in picking up GCs related to the Sagittarius dwarf galaxy. Whether this is also generally true for accreted GCs seems to be less evident, and it should be verified with larger and homogeneous samples of stars both in the field and in GCs.