I

Figure 18: Comparison of abundances from B-type dwarfs and giants (in the solar neighbourhood) and their evolved progeny, BA-type supergiants (out to ~2.5 kpc distance) as obtained in our work and from the literature. In contrast to all previous studies we find highly homogeneous abundances in the unevolved B stars (except for N, which is sensitive to mixing with nuclear-processed matter even on the main sequence). Results from the other studies span a range of ~1 order of magnitude in abundance (as in Fig. 1). While our B star sample is only small at present, the results are significant as the comparison with one of the most careful analyses so far (Kilian 1992) for the same stars shows. The chemical homogeneity of the ISM in the solar neighbourhood (Sofia & Meyer 2001) is hereby confirmed on the basis of B stars for the first time. Moreover, the B star abundances for elements unaffected by stellar evolution (O, Mg) are confirmed by the study of BA-type supergiants. These show a slightly wider spread because a large volume was sampled such that an influence of the Galactic abundance gradient/intermediate-scale inhomogeneities can become apparent. Solar abundances (©) are adopted from Grevesse & Sauval (1998) and Asplund et al. (2005). Bin sizes are related to the 1-<7 scatter in the individual studies. From Nieva & Przybilla (2008a,b: C & N in OB stars). Preliminary results from Firnstein & Przybilla (in prep.) and Przybilla, Nieva & Heber (in prep.).

stars of average rotation similar to our sample objects. Therefore, a pristine value of log C/H + 12 = 8.35±0.05 is implied from the sample (Nieva & Przybilla 2008a). The pristine N abundance may be deduced from the stars with the lowest values. Both, C and N abundances, come close to the 'new' solar value of Asplund et al. (2005).

The findings from the B-star sample are supported by the results from the BA-SGs. Despite lacking the high degree of homogeneity (a result of the larger volume sampled), they show a much smaller range in abundance than previous studies. The average abundances for O and Mg are in excellent agreement with the values from the B-star sample. Carbon is depleted and nitrogen enriched as a consequence of the evolved state of the supergiants. The upper limit of C abundances in the BA-SGs is consistent with the pristine value derived from the B-stars, meeting a boundary condition imposed by mixing.

Consistency is achieved from two classes of indicators, which show completely different spectra (BA-SG spectra are dominated by neutral and single-ionized species, B-type stars by higher ionization stages). These first results imply that a re-interpretation of studies of massive, early-type stars in the context of Galacto-chemical evolution is required. It appears that previous findings of chemical inho-mogeneity (Figs. 1 & 18) were a result of the limited accuracy of the models and the analysis methodology used. A much higher degree of homogeneity is indicated from our work, which needs verification for the solar neighbourhood from a larger sample of objects. A wider range of elements should be considered. The work also requires an extension to other regions of the Milky Way to study star clusters and OB associations.

Now we have the tools at hand to determine abundances to the precision necessary for deriving unbiased elemental abundance gradients5, see Fig. 19 for an example of the dramatic improvements that can be expected. These will provide tight observational constraints on Galactochemical evolution models and may guide future improvements of the modelling.

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