Chemical abundances for a sample of southern OB stars. II. The outer disk

Non-LTE abundances of carbon, nitrogen, oxygen, aluminum, magnesium, silicon, and sulfur are derived for a sample of nine OB stars that are members of seven clusters or H II regions located outside the solar circle, with Galactocentric distances between 9 and 13 kpc. The abundance distribution for these stars is found to have a peak roughly 0.3 dex lower than the average of the abundance distribution for OB stars from the inner disk. This result is in general agreement with the disk metallicity discontinuity claimed from observations of open clusters and Cepheids. The abundances obtained for this outer disk sample are all subsolar, with an average for N, O, Mg, and S of-0.39 ± 0.05. For Si and Al, we find even lower average abundances of -0.56 and -0.58, respectively.