Astron. Astrophys. 363, 537-554 (2000)

The Galactic metallicity gradient

W.R.J. Rolleston ¹, S.J. Smartt ^2,3, P.L. Dufton ^{*  1} and R.S.I. Ryans <sup>1

1</sup> Department of Pure and Applied Physics, The Queen's University of Belfast, Belfast BT7 1NN, N. Ireland, UK (R.Rolleston@qub.ac.uk)
² The Isaac Newton Group of Telescopes, Apartado de Correos 368, 38780 Santa Cruz de La Palma, Canary Islands, Spain
³ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, England, UK

Received 16 February 2000 / Accepted 11 September 2000

Abstract

We have previously published intermediate to high resolution spectroscopic observations of approximately 80 early B-type main-sequence stars situated in 19 Galactic open clusters/associations with Galactocentric distances distributed over  kpc. This current study collates and re-analyses these equivalent-width datasets using LTE and non-LTE model atmosphere techniques, in order to determine the stellar atmospheric parameters and abundance estimates for C, N, O, Mg, Al and Si. The latter should be representative of the present-day Galactic interstellar medium. Our extensive observational dataset permits the identification of sub-samples of stars with similar atmospheric parameters and of homogeneous subsets of lines. As such, this investigation represents the most extensive and systematic study of its kind to date.

We conclude that the distribution of light elements (C, O, Mg & Si) in the Galactic disk can be represented by a linear, radial gradient of  dex kpc^-1. Our results for nitrogen and oxygen viz.  dex kpc^-1 and  dex kpc are in excellent agreement with that found from the study of H II regions. We have also examined our datasets for evidence of an abrupt discontinuity in the metallicity of the Galactic disk near a Galactocentric distance of 10 kpc (see Twarog et al. 1997). However, there is no evidence to suggest that our data would be better fitted with a two-zone model. Moreover, we observe a N/O gradient of  dex kpc^-1 which is consistent with that found for other spiral galaxies (Vila-Costas & Edmunds 1993).

Key words: stars: abundances stars: early-type Galaxy: abundances Galaxy: evolution

* Visiting Astronomer, ING Telescopes, La Palma

Send offprint requests to: W.R.J. Rolleston

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Contents

• 1. Introduction
• 2. B-type stars as tracers of Galactic chemical composition
• 3. Source of spectroscopic data
• 4. LTE abundance analysis
  • 4.1. The full sample
  • 4.2. Homogeneous subsets of lines
  • 4.3. Homogeneous subsets of stars selected by temperature constraints
  • 4.4. Photometric and spectroscopic temperatures
  • 4.5. Microturbulence,
  • 4.6. Galactocentric distance
  • 4.7. LTE results
    • 4.7.1. Carbon
    • 4.7.2. Nitrogen
    • 4.7.3. Oxygen
    • 4.7.4. Magnesium
    • 4.7.5. Aluminium
    • 4.7.6. Silicon
• 5. Non-LTE abundance analysis - verification of the LTE approach
  • 5.1. Method of analysis
    • 5.1.1. Non-LTE model atmospheres
    • 5.1.2. Non-LTE stellar atmospheric parameters
    • 5.1.3. Homogeneous subsets of lines/stars
  • 5.2. Non-LTE results
    • 5.2.1. Oxygen
    • 5.2.2. Magnesium
    • 5.2.3. Conclusions
• 6. The nature of the observed Galactic abundance gradient
  • 6.1. A comparison with previous B-star studies
  • 6.2. A comparison with Galactic nebular studies
  • 6.3. The peculiarity of the distant star S283-2
  • 6.4. Linear relationship vs step function decline in metallicity
• 7. Discussion: abundance ratios in the disk and chemical evolution
  • 7.1. Chemical evolution models
  • 7.2. Abundance ratios
  • 7.3. The Milky Way as a normal spiral galaxy
• 8. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 2000

Online publication: December 11, 2000
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