Abundance analysis of APOGEE spectra for 58 metal-poor stars from the bulge spheroid

Abstract

The central part of the Galaxy host a multitude of stellar populations, including the spheroidal bulge stars, stars moved to the bulge through secular evolution of the bar, inner halo, inner thick disk, inner thin disk, as well as debris from past accretion events. We identified a sample of 58 candidate stars belonging to the stellar population of the spheroidal bulge, and analyse their abundances. The present calculations of Mg, Ca, and Si lines are in agreement with the APOGEE-ASPCAP abundances, whereas abundances of C, N, O, and Ce are re-examined. We find normal α-element enhancements in oxygen, similar to magnesium, Si, and Ca abundances, which are typical of other bulge stars surveyed in the optical in Baade’s Window. The enhancement of [O/Fe] in these stars suggests that they do not belong to accreted debris. No spread in N abundances is found, and none of the sample stars is N-rich, indicating that these stars are not second generation stars originated in globular clusters. Ce instead is enhanced in the sample stars, which points to an s-process origin such as due to enrichment from early generations of massive fast rotating stars, the so-called spinstars.

A Perspective on the Milky Way Bulge Bar as Seen from the Neutron-capture Elements Cerium and Neodymium with APOGEE

“…Beyond Ce and Nd, the chemical abundances of other heavy elements in bulge-bar stars also indicate a significant contribution of the r-process at low metallicity (Van der Swaelmen et al 2016;Duong et al 2019;Lucey et al 2022). Some studies also point to an important contribution of spinstars (massive rotating stars, an s-process site) in the abundance of neutron-capture elements of metalpoor bulge stars (Chiappini et al 2011;Prantzos et al 2018;Razera et al 2022).…”

“…The situation is similar for Nd, except that the rprocess contribution is substantial, and even dominant at such low metallicities. Razera et al (2022) also note that the sprocess enrichment present in very metal-poor bulge stars may have a significant contribution from massive rotating stars, based on yields from Frischknecht et al (2016).…”

“…In this context, the bulge may play an important role in resolving the tensions between the models and the measured s-process abundances. However, in general, the studies of the s-process elements in the bulge suffer from low numbers of stars (Johnson et al 2012;Bensby et al 2013;Van der Swaelmen et al 2016;Forsberg et al 2019;Lucey et al 2022;Razera et al 2022;Sestito et al 2023), preventing a clear picture of the s-process chemical pattern. As an exception, Duong et al (2019) analyzed the Zr, La, Ce, and Nd abundances for 832 stars in the direction of the bulge at latitudes −10°, −7°.5, and −5°.…”

“…They related this trend to the Ba production by the r-process. In addition, Razera et al (2022) derived high [Ce/Fe] ratios for 58 metal-poor bulge stars, indicating an increase of [Ce/Fe] ratio with decreasing [Fe/H]. Clearly, the scenario of the neutroncapture elements in the MW bulge remains inconclusive.…”

Chrono-chemodynamical analysis of the globular cluster NGC 6355: Looking for the fundamental bricks of the Bulge

“…The models for O, Mg, Si, Ca, V, Mn, Co, Cu, and Zn were computed with the code described in Friaça & Barbuy (2017; see also Barbuy et al 2015;Ernandes et al 2020, 2022, for V, Mn, Co, Cu, andZn). The star formation rate (SFR) was found to be best suited with ν = 1 Gyr −1 in order to fit the abundances of a selected sample of bulge stars in Razera et al (2022). Therefore, we adopted this SFR for all elements.…”