Non-LTE Abundances of Magnesium, Aluminum and Sulfur in OB Stars Near the Solar Circle

Non-LTE abundances of magnesium, aluminum and sulfur are derived for a sample of 23 low-v \sin i stars belonging to six northern OB associations of the Galactic disk within 1 kpc of the Sun. The abundances are obtained from the fitting of synthetic line profiles to high resolution spectra. A comparison of our results with HII region abundances indicates good agreement for sulfur while the cepheid abundances are higher. The derived abundances of Mg show good overlap with the cepheid results. The aluminum abundances for OB stars are significantly below the cepheid values. But, the OB star results show a dependence with effective temperature and need further investigation. The high Al abundances in the cepheids could be the result of mixing. A discussion of the oxygen abundance in objects near the solar circle suggests that the current mean galactic oxygen abundance in this region is 8.6-8.7 and in agreement with the recently revised oxygen abundance in the solar photosphere. Meaningful comparisons of the absolute S, Al and Mg abundances in OB stars with the Sun must await a reinvestigation of these elements, as well as the meteoritic reference element Si, with 3D hydrodynamical model atmospheres for the Sun. No abundance gradients are found within the limited range in galactocentric distances in the present study. Such variations would be expected only if there were large metallicity gradients in the disk.Comment: 3 figures, accepted for publication in A&A, needs aa.st

Mixing in two magnetic OB stars discovered by the MiMeS collaboration

Recent observational and theoretical arguments suggest that magnetic OB stars may suffer more mixing than their non magnetic analogs. We present the results of an NLTE abundance study revealing a lack of CN-cycled material at the surface of two magnetic stars discovered by the MiMeS project (NGC2244 #201 and HD 57682). The existence of a strong magnetic field is therefore not a sufficient condition for deep mixing in main-sequence OB stars.Comment: 2 pages, no figures. To appear in proceedings of IAUS272 'Active OB Stars: Structure, Evolution, Mass Loss and Critical Limits