APOGEE DR14/DR15 Abundances in the Inner Milky Way

We present an overview of the distributions of 11 elemental abundances in the Milky Way’s (MW) inner regions, as traced by APOGEE stars released as part of the Sloan Digital Sky Survey Data Release 14/15 (DR14/DR15), including O, Mg, Si, Ca, Cr, Mn, Co, Ni, Na, Al, and K. This sample spans ∼4000 stars with RGC ≤ 4.0 kpc, enabling the most comprehensive study to date of these abundances and their variations within the innermost few kiloparsecs of the MW. We describe the observed abundance patterns ([X/Fe]–[Fe/H]), compare to previous literature results and to patterns in stars at the solar Galactocentric radius (RGC), and discuss possible trends with DR14/DR15 effective temperatures. We find that the position of the [Mg/Fe]–[Fe/H] “knee” is nearly constant with RGC, indicating a well-mixed star-forming medium or high levels of radial migration in the early inner Galaxy. We quantify the linear correlation between pairs of elements in different subsamples of stars and find that these relationships vary; some abundance correlations are very similar between the α-rich and α-poor stars, but others differ significantly, suggesting variations in the metallicity dependencies of certain supernova yields. These empirical trends will form the basis for more detailed future explorations and for the refinement of model comparison metrics. That the inner MW abundances appear dominated by a single chemical evolutionary track and that they extend to such high metallicities underscore the unique importance of this part of the Galaxy for constraining the ingredients of chemical evolution modeling and for improving our understanding of the evolution of the Galaxy as a whole.