Alpha element abundances and gradients in the Milky Way bulge from FLAMES-GIRAFFE spectra of 650 K giants

Abstract

Aims. We present the analysis of the [α/Fe] abundance ratios for a large number of stars at several locations in the Milky Way bulge with the aim of constraining its formation scenario. Methods. We obtained FLAMES-GIRAFFE spectra (R = 22 500) at the ESO Very Large Telescope for 650 bulge red giant branch (RGB) stars and performed spectral synthesis to measure Mg, Ca, Ti, and Si abundances. This sample is composed of 474 giant stars observed in 3 fields along the minor axis of the Galactic bulge and at latitudes b = −4•• . Another 176 stars belong to a field containing the globular cluster NGC 6553, located at b = −3• and 5• away from the other three fields along the major axis. Stellar parameters and metallicities for these stars were presented in Zoccali et al. (2008, A&A, 486, 177). We have also re-derived stellar parameters and abundances for the sample of thick and thin disk red giants analyzed in Alves-Brito et al. (2010, A&A, 513, A35). Therefore using a homogeneous abundance database for the bulge, thick and thin disk, we have performed a differential analysis minimizing systematic errors, to compare the formation scenarios of these Galactic components. Results. Our results confirm, with large number statistics, the chemical similarity between the Galactic bulge and thick disk, which are both enhanced in alpha elements when compared to the thin disk.

Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars

“…The similarity between the bulge and the local thick disk abundance trends was actually first recognised from studies of K giants (Meléndez et al 2008) and later by Alves-Brito et al (2010) and Gonzalez et al (2011). Recently it has been shown that this similarity also holds when comparing bulge giant stars with K giant stars from the thick disk in the inner parts of the Galaxy (Bensby et al 2010a).…”

“…Alternatively, the boxy/peanut-like shape of the bulge suggests an origin through dynamical instabilities in an already established inner disk (e.g., Maihara et al 1978;Combes et al 1990;Shen et al 2010). Such secular evolution could possibly explain the recent discovery of chemical similarities between the bulge and the Galactic thick disk as observed in the solar neighbourhood (Meléndez et al 2008;Alves-Brito et al 2010;Bensby et al 2010c;Gonzalez et al 2011), through the action of radial migration of stars (Sellwood & Binney 2002;Schönrich & Binney 2009;Loebman et al 2011). …”

Separation of stellar populations by an evolving bar: implications for the bulge of the Milky Way

“…• Zoccali et al 2008;Gonzalez et al 2011;Johnson et al 2013). Separating populations into broad metallicity bins, as used in ARGOS (Ness et al 2013a), we showed that at low latitudes the metal-rich population dominates with the low-metallicity and intermediate-metallicity bins providing a smaller contribution.…”

“…For instance, Clarkson et al (2011) estimate that any component younger than 5 Gyr must account for less than 3.4% of the bulge. The Milky Way bulge also exhibits a metallicity gradient in the vertical direction (Zoccali et al 2008;Gonzalez et al 2011;Johnson et al 2011Johnson et al , 2013. This vertical gradient was thought to be impossible if the bulge formed via the buckling instability.…”

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

“…6 and 7 show published magnesium abundances from Gonzalez et al (2011), Johnson et al (2014 and Alves-Brito et al (2010) that revise the same stars as in Meléndez et al (2008) and Ryde et al (2016) for red giants, from Hill et al (2011) and Gonzalez et al (2015) for Red Clump (RC) stars and from Bensby et al (2013) for microlensed dwarfs. The data are compared with models with ν SF = 3, 1 and 0.5 Gyr −1 .…”