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

Abstract

AstrophysicsChemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars , V. Evidence for a wide age distribution and a complex MDF ABSTRACTBased on high-resolution spectra obtained during gravitational microlensing events we present a detailed elemental abundance analysis of 32 dwarf and subgiant stars in the Galactic bulge. Combined with the sample of 26 stars from the previous papers in this series, we now have 58 microlensed bulge dwarfs and subgiants that have been homogeneously analysed. The main characteristics of the sample and the findings that can be drawn are: (i) the metallicity distribution (MDF) is wide and spans all metallicities between [Fe/H] = −1.9 to +0.6; (ii) the dip in the MDF around solar metallicity that was apparent in our previous analysis of a smaller sample (26 microlensed stars) is no longer evident; instead it has a complex structure and indications of multiple components are starting to emerge. A tentative interpretation is that there could be different stellar populations at interplay, each with a different scale height: the thin disk, the thick disk, and a bar population; (iii) the stars with [Fe/H] −0.1 are old with ages between 10 and 12 Gyr; (iv) the metal-rich stars with [Fe/H] −0.1 show a wide variety of ages, ranging from 2 to 12 Gyr with a distribution that has a dominant peak around 4−5 Gyr and a tail towards higher ages; (v) there are indications in the [α/Fe] − [Fe/H] abundance trends that the "knee" occurs around [Fe/H] = −0.3 to −0.2, which is a slightly higher metallicity as compared to the "knee" for the local thick disk. This suggests that the chemical enrichment of the metal-poor bulge has been somewhat faster than what is observed for the local thick disk. The results from the microlensed bulge dwarf stars in combination with other findings in the literature, in particular the evidence that the bulge has cylindrical rotation, indicate that the Milky Way could be an almost pure disk galaxy. The bulge would then just be a conglomerate of the other Galactic stellar populations (thin disk, thick disk, halo, and ...?), residing together in the central parts of the Galaxy, influenced by the Galactic bar.

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

“…We preferred to use only the sample and results from Bensby et al (2004) (2004) show excellent agreement. Nevertheless, the bulge dwarfs by Bensby et al (2013) are clearly enhanced with respect to the present bulge giants and thick disk dwarfs of Bensby et al (2004). In this case, the covariance analysis rejects the hypothesis that both subsamples are drawn from the same-population at a <0.01% significance level, but providing a downward average shift of 0.30 for the [O/Fe] values of thick disk data with respect to the bulge data.…”

“…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 .…”

“…In this case, the covariance analysis rejects the hypothesis that both subsamples are drawn from the same-population at a <0.01% significance level, but providing a downward average shift of 0.30 for the [O/Fe] values of thick disk data with respect to the bulge data. These results illustrate why this discussion is inconclusive: it is not clear if the thick disk would be more oxygen-rich than bulge stars as hinted by results from Alves-Brito et al (2010), or if the bulge stars would be more oxygen-rich than thick disk stars as can be seen from Bensby et al (2004Bensby et al ( , 2013 …”

“…However, even for the case of an archetypical α element such as magnesium, the nucleosynthesis prescription should take into account the incompleteness of the reaction network and uncertainties in the nuclear physics and the locus where the element is produced. In particular, Woosley and Weaver (1995) Mg/ 24 Mg] ratios at low metallicities (Timmes et al 1995) and in addition, most of the 24 Mg is not Bensby et al (2004Bensby et al ( , 2013. The upper magenta line is the linear regression to the bulge data and the lower blue line is the linear regression to the thick disk data.…”

“…Stellar parameters and CNO abundances from Ryde et al (2010), Zoccali et al (2006Zoccali et al ( , 2008, and Barbuy et al (2015). Alves-Brito et al (2010), and thick disk and bulge dwarfs by Bensby et al (2004Bensby et al ( , 2013. We preferred to use only the sample and results from Bensby et al (2004) (2004) show excellent agreement.…”