METALLICITIES OF 4 BLUE SUPERGIANTS NEAR THE GALACTIC-CENTER

High resolution optical spectra of four blue supergiants (HD148422 B0. 5 Ib; HD178487 B0.5 Ib; HD179407 B1 Ib, HD163522 B1 Ib) which lie with in 4.5 kpc 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 stan dards in the solar neighborhood. A detailed non-LTE model atmosphere a nalysis of one star (HD163522) confirms that the LTE differential abun dances should be reliable, provided we use a comparison star with simi lar atmospheric parameters. One star (HD148422; R-g = 4.5 kpc) has a c hemical composition similar to normal nearby B-type stars while HD1784 87 (R-g = 4.1 kpc) shows a mild but consistent pattern of a metal enha nced atmosphere (with elements up by 0.1-0.3 dex). The other two stars HD163522 (R-g = 4.1 kpc) and HD179407 (R-g = 3.5 kpc) have significan tly higher metal compositions than their standard comparison stars wit h abundances enhanced by 0.3-0.40 dex and 0.20-0.5 dex respectively. A ll of the stars lie outside the Galactic plane (with distances in the range 0.5 < z < 1.4 kpc) and its is likely that they all have been eje cted 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 compa re our results with the sulphur and oxygen abundances expected from st udies of H II regions (Shaver et al. 1983, Simpson et al 1995) and fin d that for the metal rich stars, the studies are reasonably consistent , given the uncertainties in the stellar formation sites. However meta llicities of the other two stars are lower than those predicted from t he H II regions. We find that other elements (Mg, Al, Si, S, Fe) follo w enhancement patterns similar to oxygen, in contrast to both H II stu dies which produce different abundance gradients for other metals. Thi s paper, the first in a series, shows the potential of using early-typ e stars to determine metal abundances in regions of low extinction tow ard the Galactic centre, allowing an extension of the baseline for ste llar abundance gradient studies.