Oxygen abundances in the Galactic bulge: evidence for fast chemical enrichment

Abstract

Aims.We spectroscopically characterize the Galactic Bulge to infer its star formation timescale, compared to the other Galactic components, through the chemical signature on its individual stars. Methods: .We derived iron and oxygen abundances for 50 K giants in four fields towards the Galactic bulge. High resolution (R=45 000) spectra for the target stars were collected with FLAMES-UVES at the VLT. Results: .Oxygen, as measured from the forbidden line at 6300 Å, shows a well-defined trend with [Fe/H], with [O/Fe] higher in bulge stars than in thick disk ones, which were known to be more oxygen enhanced than thin disk stars. Conclusions: .These results support a scenario in which the bulge formed before and more rapidly than the disk, and therefore the MW bulge can be regarded as a prototypical old spheroid, with a formation history similar to that of early-type (elliptical) galaxies

Tracing the evolution of the Galactic bulge with chemodynamical modelling of alpha-elements

“…Despite having reported C and N abundances, the resulting oxygen abundances appear too high, as we explain in the present work: in Barbuy et al (2015) the oxygen abundances from Zoccali et al (2006) and Lecureur et al (2007) were revised by reinspecting the C 2 (0, 1) 5635.5 and CN(5,1) 6332.2 Å lines, and a trend of lower oxygen abundances was found. The Barbuy et al (2015) primary goal was to derive reliable CN line intensities in order to disentangle a blend of the Zn i 6362.35 Å line from CN lines on its red wing.…”

“…Hydrodynamical three-dimensional (3D) model atmospheres show that this line is instead affected by granulation, increasing with decreasing metallicity and increasing temperatures, as shown by Nissen (2002), for example. Oxygen abundances in the Galactic bulge derived from these forbidden lines were presented by Zoccali et al (2006), Lecureur et al (2007), Fulbright et al (2006Fulbright et al ( , 2007, Alves-Brito et al (2010) and Johnson et al (2014). Bensby et al (2004) also used these lines to examine a sample of thick disk giants; (ii) the permitted OI λ 7771.96, 7774.18 and 7775.40 Å triplet lines measurable in dwarfs and subgiants (in supergiants, also the weaker triplet at λ 6156.03, 6156.80 and 6158.17 Å can be used); these lines are subject to strong NLTE effects.…”

“…The stars were observed in four fields, which are the Baade's Window (l = 1.14 Lecureur et al (2007) and Zoccali et al (2006Zoccali et al ( , 2008. Another 13 red clump stars in Baade's Window were observed and analysed by Hill et al (2011) in GTO time.…”

“…The C, N and O abundances of the 56 bulge giants were presented in Zoccali et al (2006), Lecureur et al (2007) and Hill et al (2011). Despite having reported C and N abundances, the resulting oxygen abundances appear too high, as we explain in the present work: in Barbuy et al (2015) the oxygen abundances from Zoccali et al (2006) and Lecureur et al (2007) were revised by reinspecting the C 2 (0, 1) 5635.5 and CN(5,1) 6332.2 Å lines, and a trend of lower oxygen abundances was found.…”

“…In particular, we re-derive the oxygen abundances for the sample analysed by Zoccali et al (2006), Lecureur et al (2007) and Hill et al (2011), now taking the carbon and nitrogen abundances into account, which is needed due to their influence in the molecular dissociative equilibrium. We also briefly discuss the comparison of oxygen abundances in the Galactic bulge and in thick disk stars.…”