B-STARS AND THE CHEMICAL EVOLUTION OF THE GALACTIC DISK

We determine stellar parameters and He, C, N, O, Mg, Al, and Si abunda nces of a sample of 16 early B main-sequence stars in galactocentric d istances of R-GC = 5-14 kpc by reanalyzing and extending the observati ons of Kaufer et al. (1994) towards the Galactic center. The analysis uses Kurucz ATLAS 9 LTE atmospheres and Giddings-Butler DETAIL/SURFACE non-LTE line formation. It is shown that beside non-LTE effects the m etallicity of the underlying atmosphere has a non-negligible impact on the temperature structure and the subsequent line formation. The loca l field is found to have an underabundance of about -0.2dex relative t o the Sun. Contrary to part of recent B-star and H II-region determina tions we derive an oxygen abundance gradient Delta[O/H]Delta R-GC = (- 0.07 +/- 0.02) dex/kpc for the Galactic disk, typical for normal spira l galaxies of similar Hubble type. This steep gradient is in contrast to the recent evidence that (a) our Milky Way is actually a barred spi ral galaxy, and (b) barred galaxies generally have almost flat abundan ce gradients throughout their disks. Comparing our results with numeri cal models of the chemodynamical evolution of barred galaxies we deriv e an age of about 1 Gyr for the central bar structure of the Galaxy. W e argue that our bar is still too young to exert a notable flattening effect on the chemical abundance gradient of the Galactic disk.