LIGHT, ALPHA, AND Fe-PEAK ELEMENT ABUNDANCES IN THE GALACTIC BULGE

Abstract

We present radial velocities and chemical abundances of O, Na, Mg, Al, Si, Ca, Cr, Fe, Co, Ni, and Cu for a sample of 156 red giant branch stars in two Galactic bulge fields centered near (l,b)=(+5.25,-3.02) and (0,-12). The (+5.25,-3.02) field also includes observations of the bulge globular cluster NGC 6553. The results are based on high resolution (R∼20,000), high signal-to-noise (S/N 70) FLAMES-GIRAFFE spectra obtained through the ESO archive. However, we only selected a subset of the original observations that included spectra with both high S/N and that did not show strong TiO absorption bands. The present work extends previous analyses of this data set beyond Fe and the α-elements Mg, Si, Ca, and Ti. While we find reasonable agreement with past work, the data presented here indicate that the bulge may exhibit a different chemical composition than the local thick disk, especially at [Fe/H] -0.5. In particular, the bulge [α/Fe] ratios may remain enhanced to a slightly higher [Fe/H] than the thick disk and the Fe-peak elements Co, Ni, and Cu appear enhanced compared to the disk. There is also some evidence that the [Na/Fe] (but not [Al/Fe]) trends between the bulge and local disk may be different at low and high metallicity. We also find that the velocity dispersion decreases as a function of increasing [Fe/H] for both fields, and do not detect any significant cold, high velocity population. A comparison with chemical enrichment models indicates that a significant fraction of hypernovae are required to explain the bulge abundance trends, and that initial mass functions that are steep, top-heavy (and do not include strong outflow), or truncated to avoid including contributions from stars >40 M ⊙ are ruled out, in particular because of disagreement with the Fe-peak abundance data. For most elements, the NGC 6553 stars exhibit nearly identical abundance trends to comparable metallicity bulge field stars. However, the star-to-star scatter and mean [Na/Fe] ratios appear higher in the cluster, perhaps indicating additional self-enrichment.

The GIRAFFE Inner Bulge Survey (GIBS)

“…In Bensby et al (2013) suggested that the position of the knee in the [α/Fe] plot is found at 0.1-0.2 dex higher metallicities in the bulge than in the thick disc. A similar result was proposed by Johnson et al (2014) who added the analysis of Fe-peak elements finding in particular that Co, Ni, and Cu appear enhanced compared to the disc. We note that the results presented in Johnson et al (2014) have been obtained by comparing bulge giants to dwarf stars from the disc.…”

“…A similar result was proposed by Johnson et al (2014) who added the analysis of Fe-peak elements finding in particular that Co, Ni, and Cu appear enhanced compared to the disc. We note that the results presented in Johnson et al (2014) have been obtained by comparing bulge giants to dwarf stars from the disc. This technique might suffer from systematic offsets (Meléndez et al 2008) and it thus needs to be investigated further.…”

“…The thick disc on the other hand appears to be as enhanced in α-element abundances as the metal-poor bulge giants, though covering a much smaller range in [Fe/H]. These patterns in the α-element abundances of bulge stars with respect to the disc abundances has already been observed in several studies, based on different samples and measurement techniques (Meléndez et al 2008;Alves-Brito et al 2010;Gonzalez et al 2011;Bensby et al 2010Bensby et al , 2011Ryde et al 2010;Hill et al 2011;Johnson et al 2011Johnson et al , 2013Johnson et al , 2014. In Bensby et al (2013) suggested that the position of the knee in the [α/Fe] plot is found at 0.1-0.2 dex higher metallicities in the bulge than in the thick disc.…”

Metal-poor stars towards the Galactic bulge: A population potpourri

“…As Fig. 1 indicates, however, its [Mg/Fe] ratio is elevated, at 0.17 dex, and is consistent with Johnson's et al (2014) sample that shows [Mg/Fe] = 0.14 dex around solar metallicity, providing chemical evidence for an association with the metalrich bulge (see also Fulbright et al 2007). …”

Chemical abundances in a high-velocity RR Lyrae star near the bulge

“…This was confirmed by the more recent Northern Sky Variability Survey (Kinemuchi et al 2006) might indicate that the star belongs to the thick disk or bulge, while a lower value −1.0 to −2.5 would point toward it being a (inner or outer) halo star. Towards the central regions of the Milky Way, metallicity alone becomes an insufficient parameter, and large surveys have found an overlap of the bulge component with the metal-weak thick disk and the inner halo (i.e., within 3.5 kpc, e.g., Fulbright et al 2007;Kunder et al 2012;Ness et al 2013;Johnson et al 2014). This distinction is significant because the inside-out formation of the Galaxy dictates that the oldest and most (extremely) metal-poor stars should be found in the very center (Tumlinson 2010), rendering them inner halo stars (by formation and chemistry) that happen to be located within the bulge.…”