1997A&A...326..763S

High resolution optical spectra of four blue supergiants (HD 148422 B0.5Ib; HD 178487 B0.5Ib; HD 179407 B1Ib, HD 163522 B1Ib) which lie within 4.5kpc of the Galactic centre are presented. Careful differential LTE model atmosphere analyses are used to quantify the differences in photospheric metal abundances between these stars and MK spectral standards in the solar neighborhood. A detailed non-LTE model atmosphere analysis of one star (HD 163522) confirms that the LTE differential abundances should be reliable, provided we use a comparison star with similar atmospheric parameters. One star (HD 148422; R_g=4.5kpc) has a chemical composition similar to normal nearby B-type stars while HD 178487 (R_g=4.1kpc) shows a mild but consistent pattern of a metal enhanced atmosphere (with elements up by 0.1-0.3dex). The other two stars HD 163522 (R_g=4.1kpc) and HD 179407 (R_g=3.5kpc) have significantly higher metal compositions than their standard comparison stars with abundances enhanced by 0.30-0.40dex and 0.20-0.5dex respectively. All of the stars lie outside the Galactic plane (with distances in the range 0.5<z<1.4kpc) and its is likely that they all have been ejected from the disk at an earlier point in their lifetime. Their radial velocities are examined and we consider possible ejection mechanisms which constrain their regions of origin in the Galactic disk. We compare our results with the sulphur and oxygen abundances expected from studies of HII regions (Shaver et al. (1983MNRAS.204...53S), Simpson et al. (1995ApJ...444..721S)) and find that for the metal rich stars, the studies are reasonably consistent, given the uncertainties in the stellar formation sites. However metallicities of the other two stars are lower than those predicted from the HII regions. We find that other elements (Mg, Al, Si, S, Fe) follow enhancement patterns similar to oxygen, in contrast to both HII studies which produce different abundance gradients for other metals. This paper, the first in a series, shows the potential of using early-type stars to determine metal abundances in regions of low extinction toward the Galactic centre, allowing an extension of the baseline for stellar abundance gradient studies.