The metallicity distribution of bulge clump giants in Baade’s window

Abstract

Aims. We seek to constrain the formation of the Galactic bulge by analysing the detailed chemical composition of a large sample of red clump stars in Baade's window. These stars were selected to minimise the contamination by other Galactic components, so they are good tracers of the bulge metallicity distribution in Baade's window, at least for stars more metal-rich than ∼−1.5. Methods. We used an automatic procedure to measure [Fe/H] differentially with respect to the metal-rich star μLeo in a sample of 219 bulge red clump stars from R = 20 000 resolution spectra obtained with FLAMES/GIRAFFE at the VLT. 3), while stars in the metal-rich component are found to have nearly solar ratios. Kinematical differences between the two components have also been found: the metal-poor component shows kinematics compatible with an old spheroid, while the metal-rich component is consistent with a population supporting a bar. In view of their chemical and kinematical properties, we suggest different formation scenarii for the two populations: a rapid formation time scale as an old spheroid for the metal-poor component (old bulge) and for the metal-rich component, a formation on a longer time scale driven by the evolution of the bar (pseudo-bulge). The observations are described well by a simple model consisting of two components: a simple closed box model to predict the metal-poor population contribution and a local thin disc metallicity distribution, shifted in metallicity, to represent the metal-rich population. The pseudo-bulge is compatible with its being formed from the inner thin disc, assuming high (but plausible) values of the gradients in the early Galactic disc.

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

“…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.…”

“…The stellar parameters reported in Table A.1 were adopted from the detailed analyses by Zoccali et al (2006) and Lecureur et al (2007) for 43 bulge field red giants. An additional 13 field red clump stars were analysed by Hill et al (2011) based on both the UVES and the GIRAFFE spectra. We adopted the stellar parameters derived from the UVES spectra, which were not given in Hill et al (2011) but already reported in Barbuy et al (2013).…”

“…An additional 13 field red clump stars were analysed by Hill et al (2011) based on both the UVES and the GIRAFFE spectra. We adopted the stellar parameters derived from the UVES spectra, which were not given in Hill et al (2011) but already reported in Barbuy et al (2013).…”

“…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.…”