Professor Stephen Smartt CBE FRS MRIA

Abstract:

High resolution optical spectra of four blue supergiants (HD148422 BO.5 Ib; HD178487 B0.5 Ib; HD179407 BI Ib, HD 163522 B l Ib) which lie within 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 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 (HD148422; Rg = 4.5 kpc) has a chemical composition similar to normal nearby B-type stars while HD 178487 (Rg = 4.1 kpc) 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 (Rg = 4.1 kpc) and HD 179407 (Rg = 3.5 kpc) have significantly higher metal compositions than their standard comparison stars with abundances enhanced by 0.30-0.40 dex and 0.20-0.5 dex respectively. All 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 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 H II regions (Shaver et al. 1983, Simpson et al 1995) and find that for the metal rich stars, the studies are reasonably consistent, given the uncertainties in the stellar formation sites. However metallicilies of the other two stars are lower than those predicted from the H II regions. We find that other elements (Mg, Al, Si, S, Fe) follow enhancement patterns similar to oxygen, in contrast to both H II 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 m regions of low extinction toward the Galactic centre, allowing an extension of the baseline for stellar abundance gradient studies.