On the nature of HD 93521, a high galactic latitude O-star.

Classical non-LTE model atmosphere techniques are used to analyze a high signal-to-noise, high-resolution spectrum of HD 93521 in the wavelength region 4070-4510 A. Abundances of nitrogen, oxygen, magnesium, and silicon were all found to be approximately solar. Attempts to fit the UV continuum flux with model fluxes imply a UV deficiency relative to the V-band. The present results are consistent with HD 93521 being a Population I late O-type shell star. It is inferred that the overabundance of helium is the result of turbulent diffusion, induced by the star's large rotational velocity, mixing CNO-cycled material into its atmosphere. It is suggested that HD 93521 is evolving on a track intermediate between homogeneous and normal evolutionary tracks and may exhibit moderate changes in CNO surface abundances which are masked by the star's large projected rotational velocity of 400 km/s.