Precise abundance analysis of the outer halo globular cluster M 75

Abstract

Globular clusters (GCs) are the oldest stellar systems in the Milky Way. Long time considered as simple stellar populations, nowadays we recognize their complex star formation history through precise abundance analysis of a variety of chemical elements in individual cluster members. Although we do not necessarily see clues for multiple populations in all GC colourmagnitude diagrams, all GCs present significant spreads and certain anticorrelations between their light and α element abundances. Furthermore, the heavy element abundances in individual stars of the primordial generation and their comparison to halo field stars and dwarf galaxies could provide us with valuable information about the very first stars that could have formed in GCs. M75 is a unique outer halo (galactocentric distance of ∼ 15 kpc) GC with a peculiar Horizontal Branch morphology. Here we present the first abundance measurements of 16 individual red giants from high resolution spectroscopy. The cluster is metal rich ([Fe/H] = −1.17±0.02), α-enhanced, and shows a marginal spread in [Fe/H] of 0.07 dex, typical of most GCs of similar luminosity. The ONa anticorrelation is clearly visible, showing at least two generations of stars, formed on a short timescale. We also discuss r- and s-process element abundances in the context of the earliest cluster enrichment phases.