Hydrostatic and explosive $\alpha$-element chemical abundances of Milky Way globular clusters, halo substructures, and satellite galaxies

Stellar atmospheric element abundance ratios of stars retain information about their birth conditions, helping elucidate their origin and nature. In this letter, we analyse and contrast the hydrostatic and explosive $\alpha$-element abundance ratios, and the ratio of the two (the hex ratio), for a large sample of Galactic globular clusters (GCs), halo substructures, satellite galaxies, and the Milky Way high-/low-$\alpha$ discs using data from the $APOGEE$ survey. Our results show that: $i$) Milky Way GCs and halo substructures appear to have qualitatively similar hex ratios across a broad range of [Fe/H], that are higher than that of dwarf satellite galaxies of similar [Fe/H]; $ii$) for all stellar populations studied, there is a trend in the hex ratio with [Fe/H]; $iii$) there is a weak trend in the hex ratio with respect to age for Galactic GCs, but not with initial or final GC mass; $iv$) there are no differences in the hex ratio between GCs formed $in$ $situ$ versus those labelled as accreted.