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

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 A, 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.