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

8. Conclusions

1. The abundance analyses are based on an extensive observational dataset comprising high-resolution spectroscopy of 80 B-type main-sequence stars in 19 Galactic, young open clusters/associations distributed over  kpc.

2. Using homogeneous subsets of stars and lines, we deduce significant abundance gradients for C, N, O, Mg, Al & Si. We conclude that the radial distribution of light elements in the Galactic disk can be represented by a linear gradient of  dex kpc^-1, with a steeper gradient being more appropriate for nitrogen, viz.  dex kpc^-1.

3. Abundance gradients derived for oxygen and magnesium using LTE and non-LTE model-atmosphere techniques do not differ significantly. Thus, we conclude that the LTE analysis should be reliable for estimating the magnitude of the metal abundance gradients in the present-day Galactic disk.

4. Comparison of our nitrogen and oxygen gradients for young, B-type stars are in excellent agreement with the analyses of H II regions. Thus, it would appear that the discrepancies between earlier sets of investigations have been resolved. Several factors have almost certainly contributed to the failure of detecting significant abundance gradients in the past, viz. the use of inhomogeneous datasets, and/or the use of small samples of objects with a restricted range of Galactocentric distances.

5. Twarog et al. (1997) report evidence of an abrupt discontinuity in the metallicity of the Galactic disk at  kpc. However, we do not believe that our abundance estimates of B-type stars in young, open clusters would be better fitted with a two-zone model.

6. The variation of O-Mg-Si as a function of Galactocentric distance are well correlated; further evidence that these elements have a similar nucleosynthetic origin. On the other hand, the  N/O ratio displays a linear decline of  dex kpc^-1 with . This result is in good agreement with that found for H II regions in the Milky Way (Rudolph et al. 1997) and external spiral galaxies (see Vila-Costas & Edmunds 1993). Our results suggest that secondary nitrogen production (cf. primary production) is dominant in the Galactic disk out to  kpc.

7. Garnett et al. (1997) have demonstrated that there is a remarkable correlation between the abundance gradient and the luminosity of a spiral galaxy. Comparison of our results for the Milky way with this relationship, suggests that our Galaxy has had a reasonably normal chemical history, given its mass.

© European Southern Observatory (ESO) 2000

Online publication: December 11, 2000
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