Abundances in 78 metal-rich bulge spheroid stars from APOGEE

TL;DR

This study targets the metal-rich spheroidal bulge population by selecting 78 stars with $[Fe/H] > -0.8$ through kinematic criteria, complementing a metal-poor sample. Using APOGEE DR17 parameters, the authors derive C, N, O, Al, P, S, K, Mn, and Ce (revising Al and accounting for non-LTE effects where possible) via H-band spectral synthesis to compare with literature and chemical-evolution models. The results show alpha and iron-peak trends in line with fast early bulge enrichment, with notable excesses in P and Ce around $[Fe/H] \sim -0.7$, and an overall in situ origin for most stars, while a subset at high metallicity hints at enrichment tied to the nuclear disc and bar. These findings constrain the chemical evolution of the innermost Galaxy and suggest a chemically distinct inner population shaped by the bar dynamics.

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.

Figures (18)

• Figure 1: Kiel diagram, based on TEFF_SPEC and LOGG_SPEC from APOGEE DR17 of the present sample, compared to razera22. The RPM sample from queiroz21 is shown in the background as a hexagonally binned density distribution.
• Figure 2: [Mg/Fe] versus [Fe/H] with the histogram of metallicities for the present sample and the razera22 metal-poor sample.
• Figure 3: Fit to the 15525--15590 Å region in star a70. The main lines are indicated.
• Figure 4: Fit to Al i 16718.957, 16750.539, and 16763.359 Å lines. The synthetic spectra were computed with [Al/Fe] = 0.29 and are shown in red; we compare them with the observed spectrum, which is shown in black.
• Figure 5: Fit to P i 15711.522 Å and P i 16482.932 Å lines and the nearby CO 15717.2 Å line for star a2: 2M17031887-3421087; this confirms that the CO strength is reliable. Synthetic spectra were computed with [P/Fe] = 0.0 (green), +1.0 (blue), and the final value +0.8 (red), compared with the observed spectrum (black). The dotted line corresponds to molecular lines only.