On the Origin of Abundance Variations in the Milky Way’s High- α Plateau

Using multielement abundances from the Sloan Digital Sky Survey APOGEE survey, we investigate the origin of abundance variations in Milky Way (MW) disk stars on the “high- α plateau,” with −0.5 ≤ [Mg/H] ≤ −0.1 and 0.25 ≤ [Mg/Fe] ≤ 0.35. The elevated [ α /Fe] ratios of these stars imply low enrichment contributions from Type Ia supernovae (SN Ia), but it is unclear whether their abundance patterns reflect pure core-collapse supernova (CCSN) enrichment. We find that plateau stars with higher [Fe/Mg] ratios also have higher [X/Mg] ratios for other iron-peak elements, suggesting that the [Fe/Mg] variations in the plateau population do reflect variations in the SN Ia/CCSN ratio. To quantify this finding, we fit the observed abundance patterns with a two-process model, calibrated on the full MW disk, which represents each star’s abundances as the sum of a prompt CCSN process with amplitude A cc and a delayed SN Ia process with amplitude A Ia . This model is generally successful at explaining the observed trends of [X/Mg] with A Ia / A cc , which are steeper for elements with a large SN Ia contribution (e.g., Cr, Ni, Mn) and flatter for elements with low SN Ia contribution (e.g., O, Si, Ca). Our analysis does not determine the value of [Mg/Fe] corresponding to pure CCSN enrichment, but it should be at least as high as the upper edge of the plateau at [Mg/Fe] ≈ 0.35, and could be significantly higher. Compared to the two-process predictions, the observed trends of [X/Mg] with A Ia / A cc are steeper for (C+N) but shallower for Ce, providing intriguing but contradictory clues about asymptotic giant branch enrichment in the early disk.