Observing multiple populations in globular clusters with the ESO archive: NGC 6388 reloaded^★

INAF - Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
e-mail: eugenio.carretta@oabo.inaf.it

Received: 18 January 2018
Accepted: 16 February 2018

Abstract

The metal-rich and old bulge globular cluster (GC) NGC 6388 is one of the most massive Galactic GCs (M ~ 10⁶ M_⊙). However, the spectroscopic properties of its multiple stellar populations rested only on 32 red giants (only 7 of which observed with UVES, the others with GIRAFFE), given the difficulties in observing a rather distant cluster, heavily contaminated by bulge and disc field stars. We bypassed the problem using the resources of the largest telescope facility ever: the European Southern Observatory (ESO) archive. By selecting member stars identified by other programmes, we derive atmospheric parameters and the full set of abundances for 15 species from high-resolution UVES spectra of another 17 red giant branch stars in NGC 6388. We confirm that no metallicity dispersion is detectable in this GC. About 30% of the stars show the primordial composition of first-generation stars, about 20% present an extremely modified second-generation composition, and 50% have an intermediate composition. The stars are clearly distributed in the Al–O and Na–O planes in three discrete groups. We find substantial hints that more than a single class of polluters is required to reproduce the composition of the intermediate component in NGC 6388. In the heavily polluted component the sum Mg+Al increases as Al increases. The sum Mg+Al+Si is constant, and is the fossil record of hot H-burning at temperatures higher than about 70 MK in the first-generation polluters that contributed to form multiple populations in this cluster.