Spectroscopic observations of HD 93521: an O-type star of population II.

The spectral type of HD 93521 is revised and is found to be O9.5 Ib-II. Its bright luminosity class stresses the contradiction between the star's distance to the galactic plane and its Population I apparent membership. A comparative analysis of the equivalent widths of He I lines in the star and 62 early-type stars yielded a detectable He overabundance in HD 93521. If the star is assumed to belong to the Population I group, its maximum nuclear lifetime is incompatible with its present position and kinematics. Therefore, HD 93521 is proposed to be in the evolutionary phase in which the Population II UV-bright stars are.