Astron. Astrophys. 326, 763-779 (1997)

Metallicities of 4 blue supergiants near the Galactic centre

S.J. Smartt ^{1, 2}, P.L. Dufton ¹ and D.J. Lennon <sup>3, 4

1</sup> The Department of Pure and Applied Physics, The Queen's University of Belfast, Belfast BT7 1NN, Northern Ireland
² ING Telescopes, Apartado de Correos 368, Santa Cruz De La Palma, E-38780 Canary Islands, Spain
³ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, D-85740 Garching bei München, Germany
⁴ Institut Für Astronomie und Astrophysik der Universität München, Scheinerstrasse 1, D-81679 München, Germany

Received 1 July 1996 / Accepted 28 January 1997

Abstract

High resolution optical spectra of four blue supergiants (HD148422 B0.5 Ib; HD178487 B0.5 Ib; HD179407 B1 Ib, HD163522 B1 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 (HD163522) confirms that the LTE differential abundances should be reliable, provided we use a comparison star with similar atmospheric parameters.

One star (HD148422; R_g = 4.5 kpc) has a chemical composition similar to normal nearby B-type stars while HD178487 (R_g = 4.1 kpc) shows a mild but consistent pattern of a metal enhanced 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 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 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 metallicities 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 in regions of low extinction toward the Galactic centre, allowing an extension of the baseline for stellar abundance gradient studies.

Key words: B-stars -- supergiants -- abundances -- the Galaxy: centre

Send offprint requests to: S.J. Smartt, at La Palma address.

SIMBAD Objects

Contents

• 1. Introduction
• 2. Observational data
  • 2.1. Photometry and selection of targets
  • 2.2. Spectroscopic observations
• 3. Method of analysis
  • 3.1. Classification of the spectra
  • 3.2. LTE stellar atmospheric parameters
  • 3.3. Microturbulent velocities in LTE
• 4. Metal abundances from LTE differential analysis
  • 4.1. HD148422
  • 4.2. HD178487
  • 4.3. HD179407
  • 4.4. HD163522
• 5. Carbon, nitrogen and helium abundances
• 6. Non-LTE differential analysis of HD163522 and comparison with LTE results
  • 6.1. Non-LTE model atmospheres
  • 6.2. Atmospheric parameters
  • 6.3. Metal abundances and microturbulent velocities in non-LTE
• 7. Discussion of results
  • 7.1. Distances, Galactic positions and evolutionary status
  • 7.2. Origins, radial velocity considerations, and Galactic structure
  • 7.3. Metallicity variations towards the Galactic centre
  • 7.4. Summary
• Acknowledgements
• References

© European Southern Observatory (ESO) 1997

Online publication: October 15, 1997
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