Galactic Metallicity Gradients Derived from a Sample of OB Stars
  The distribution of stellar abundances along the Galactic disk is an important constraint for models of chemical evolution and Galaxy formation. In this study we derive radial gradients of C, N, O, Mg, Al, Si, as well as S, from abundance determinations in young OB stars. Our database is composed of a sample of 69 members of 25 open clusters, OB associations and H II regions with Galactocentric distances between 4.7 and 13.2 kpc. An important feature of this abundance database is the fact that the abundances were derived self-consistently in non-LTE using a homogeneous set of stellar parameters. Such an uniform analysis is expected to reduce the magnitude of random errors, as well as the influence of systematics in the gradients defined by the abundance and Galactocentric distance. The metallicity gradients obtained in this study are, in general, flatter than the results from previous recent abundance studies of early-type stars. The slopes are found to be between -0.031 (for oxygen) and $-0.052 dex kpc^{-1}$ (for magnesium). The gradients obtained for the studied elements are quite similar and if averaged, they can be represented by a single slope of $-0.042 \pm 0.007 dex kpc^{-1}$. This value is generally consistent with an overall flattening of the radial gradients with time. 
Simone Daflon and Katia Cunha