Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars - VI. Age and abundance structure of the stellar populations in the central sub-kpc of the Milky Way

摘要

We present a detailed elemental abundance study of 90 F and G dwarf, turn-off, and subgiant stars in the Galactic bulge. Based on high-resolution spectra acquired during gravitational microlensing events, stellar ages and abundances for 11 elements (Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Zn, Y and Ba) have been determined. Four main findings are presented: (1) a wide metallicity distribution with distinct peaks at [Fe/H] = ?1.09, ?0.63, ?0.20, + 0.12, + 0.41 ; (2) a highfraction of intermediate-age to young stars where at [Fe/H] & 0 more than 35% are younger than 8?Gyr, and for [Fe/H] ? ?0.5 most stars are 10?Gyr or older; (3) several episodes of significant star formation in the bulge has been identified: 3,?6,?8, and 11?Gyr ago; (4) tentatively the “knee” in the α -element abundance trends of the sub-solar metallicity bulge is located at a slightly higher [Fe/H] than in the local thick disk. These findings show that the Galactic bulge has complex age and abundance properties that appear to be tightly connected to the main Galactic stellar populations. In particular, the peaks in the metallicity distribution, the star formation episodes, and the abundance trends, show similarities with the properties of the Galactic thin and thick disks. At the same time, the star formation rate appears to have been slightly faster in the bulge than in the local thick disk, which most likely is an indication of the denser stellar environment closer to the Galactic centre. There are also additional components not seen outside the bulge region, and that most likely can be associated with the Galactic bar. Our results strengthen the observational evidence that support the idea of a secular origin for the Galactic bulge, formed out of the other main Galactic stellar populations present in the central regions of our Galaxy. Additionally, our analysis of this enlarged sample suggests that the ( V ? I )_(0) colour of the bulge red clump should be revised to 1.09.