Abundances in 78 metal-rich bulge spheroid stars from APOGEE

TL;DR

This study analyzes the chemical abundances of 78 metal-rich bulge stars from APOGEE, revealing their origins, chemical signatures, and the complex formation history of the Milky Way's inner regions.

Contribution

It provides detailed abundance measurements of key elements in the oldest bulge stars, offering insights into their formation and the Galaxy's early evolution.

Findings

Alpha elements follow expected trends with models.

Phosphorus and cerium show excess at certain metallicities.

Evidence of a chemically distinct population linked to the nuclear disc.

Abstract

The inner Galaxy is the most complex region of the Milky Way, comprising the bulge, inner thin and thick discs, and inner halo; the formation of the bar transferred gas and stars from the disc inward. Accretion of dwarf galaxies also occurred over the Galaxy's lifetime, merging with the original bulge. In this work, we constrain the metal-rich stars of the earliest spheroidal bulge. To study the oldest bulge stars, distributed in a spheroid, we applied kinematical and dynamical criteria in the metal-rich range [Fe/H] > -0.8. This complements our previous analysis of a symmetric sample with [Fe/H] < -0.8. We derived individual abundances through spectral synthesis for C, N, O, Al, P, S, K, Mn, and Ce using stellar parameters from APOGEE DR17, and compared the results with literature data and chemical-evolution models. The alpha elements Mg, Si, and Ca, and iron-peak elements V, Cr, Co, and Ni follow the expected trends relative to the models. Mn shows secondary behaviour. S and K display significant star-to-star scatter but remain broadly compatible with predictions. Phosphorus and cerium show an excess around [Fe/H] $\sim$ -0.7, more pronounced than in the metal-poor sample, suggesting a distinctive signature of the earliest bulge population. Diagrams of [Mg/Mn] versus [Al/Fe] and [Ni/Fe] versus [(C+N)/O] indicate an in situ origin for most stars. At super-solar metallicities, a subset shows enhanced K and Mn (possibly S) with low [Ce/Fe], hinting at enrichment linked to the nuclear disc and bar, and tracing a chemically distinct population shaped by the innermost Galaxy.