Chemical abundances for 11 bulge stars from high - resolution , near - IR spectra . ⋆

Context. It is debated whether the Milky Way bulge has the characteristics of a classical bulge sooner than those of a pseudobulge. Detailed abundance studies of bulge stars is a key to investigate the origin, history, and classification of the bulge. These studies can give constraints on the star-formation history, initial mass-function, and trace differences in stellar populations. Not many such studies have been made due to the large distance and large variable visual extinction along the line-of-sight towards the bulge. Therefore, near-IR investigations are to be preferred. Aims. The aim is to add to the discussion on the origin of the bulge and to study detailed abundances determined from near-IR spectra for bulge giants already investigated with optical spectra, the latter also providing the stellar parameters which are very significant for the results of the present study. Especially, the important CNO elements are better determined in the near-IR. Oxygen and other α elements are important for the investigation of the star-formation history. The C and N abundances are important for determining the evolutionary stage of the giants but also the origin of C in the bulge. Methods. High-resolution, near-infrared spectra in the H band are recorded using the CRIRES spectrometer on the Very Large Telescope. The CNO abundances can all be determined from the numerous molecular lines in the wavelength range observed. Abundances of the α elements Si, S, and Ti are also determined from the near-IR spectra. Results. [O/Fe], [Si/Fe] and [S/Fe] are enhanced up to metallicities of at least [Fe/H]=−0.3, after which they decline. This suggests that the Milky Way bulge experienced a rapid and early star-formation history like that of a classical bulge. However, a similarity between the bulge trend and the trend of the local thick disk seems present. Such a similarity could suggest that the bulge has a pseudobulge origin. The C and N abundances suggest that our giants are first-ascent red-giants or clump stars, suggesting that the measured oxygen abundances are those the stars were born with. Our [C/Fe] trend does not show any increase with [Fe/H] which could have been expected if W-R stars have contributed substantially to the C abundances. No " cosmic scatter " can be traced around our observed abundance trends; the scatter found is expected, given the observational uncertainties.