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Chemical Abundances of OB Stars in Five OB Associations

We present LTE abundances of magnesium, aluminum, sulfur, and iron and non-LTE abundances of carbon, nitrogen, oxygen, and silicon for a sample of 15 slowly rotating B stars belonging to five OB associations: Cyg OB3, Cyg OB7, Lac OB1, Vul OB1, and Cep OB3. These OB associations lie on the Galactic plane and are situated within 3 kpc of the Sun. Of the eight elements sampled, non-LTE abundances for C, N, O, and Si, as well as LTE abundances for Al and Fe, generally show subsolar abundances, with typical underabundances of ~0.2-0.4 dex. The LTE abundances for Mg and S tend to fall closer to solar values in the five associations. Whether the somewhat larger abundances derived for Mg and S, relative to the other six elements studied, are significantly different will require further work, while the modest, but persistent, underabundances (relative to solar) found for the other elements confirm a number of previous studies of young disk OB stars lying relatively near to the Sun. The five associations studied here do not span a significant range of Galactocentric distances; however, their derived abundances agree with what would be expected based upon previous studies that have mapped abundance versus Galactocentric distance and measured abundance gradients in the Milky Way disk.