Chemical abundances of M giants in the Galactic centre: A single metal-rich population with low [alpha/Fe]

Context. The formation and evolution of the Milky Way bulge is still largely an unanswered question. Some of the most essential observations needed for its modelling are the metallicity distribution and the trends of the alpha elements, as measured in stars. While bulge regions beyond R greater than or similar to 50 pc of the centre have been targeted in several surveys, the central part has escaped a detailed study due to the extreme extinction and crowding. The abundance gradients from the centre are, however, of large diagnostic value. Aims. We aim at investigating the Galactic centre environment by probing M giants in the field by avoiding supergiants and cluster members. Methods. For nine field M-giants in the Galactic centre region, we have obtained high- and low-resolution spectra observed simultaneously with CRIRES and ISAAC on UT1 and UT3 of the VLT. The low-resolution spectra provide a means of determining the effective temperatures, and the high-resolution spectra provide detailed abundances of Fe, Mg, Si, and Ca. Results. We find a metal-rich population at [Fe/H] = +0.11 +/- 0.15 and a lack of the metal-poor population, which is found further out in the bulge, corroborating earlier studies. Our [alpha/Fe] element trends, however, show low values, by following the outer bulge trends. A possible exception of the [Ca/Fe] trend is found and needs further investigation. Conclusions. The results of the analysed field M-giants in the Galactic centre region exclude a scenario with rapid formation, in which SNIIe played a dominated role in the chemical enrichment of the gas. The high metallicities with low alpha-enhancement seems to indicate a bar-like population that is, perhaps, related to the nuclear bar