Homogeneous abundance ratios of hydrostatic and explosive alpha-elements in globular clusters from high resolution optical spectroscopy

Abstract

Galactic globular clusters (GCs) were born shortly after the Big Bang. For such old stellar systems the initial mass function (IMF) at the high mass regime can never be observed directly, because stars more massive than about 1 Mo have evolved since longtime. However, the hydrostatic to explosive alpha-element ratio (HEx ratio) offers a way to bypass the lack of observable high mass stars through the yields that massive stars released when exploding as supernovae, incorporated in the stars we presently observe in GCs. The HEx ratio measures the percentage of high mass stars over the total number of stars exploding as supernovae and it is an efficient probe of the ephemeral first phases of the GC evolution. We exploited a recently completed survey to assemble a dataset of very homogeneous abundances of alpha-elements in 27 GCs from [Fe/H]~ -2.4 to ~ -0.3 dex. In agreement with previous results from APOGEE, we confirm that the HEx ratio is indistinguishable for GCs formed in situ and accreted in the Galaxy, and that this ratio decreases with increasing metallicity. However, we posit that this trend is better explained by a metallicity-dependent IMF deficient in the highest mass stars at high metallicity, as corroborated by the declining [O/Mg] ratio as a function of the [Mg/H] ratio. At odds with the previous analysis based on APOGEE data, we detect an anti-correlation of HEx ratio with both present day and initial GC masses. Finally, we hypothesise that in that analysis, the stars of the GC M 54 were probably confused with stars in the core of the Sagittarius dwarf galaxy, where the cluster is presently immersed.