A classification system for O - B2 stars based on the Si IV and C IV resonance lines.

Low-dispersion ultraviolet spectra from Skylab Experiment S-019 are used to explore the variations of Si IV and C IV line strengths with temperature and luminosity. These considerations lead to a classification system in which the Si/C ratio is used to discriminate luminosity among the O stars and temperature among the O9-B2 stars of lower luminosity. Stars falling in these two regimes may be distinguished either by the presence of C IV emission or on the basis of C IV absorption strength. The log(Si IV/C IV) vs C IV diagram is proposed as a primary tool in such a classification system. The rapid variation in the Si IV/C IV ratio from less than 1/10 at O9 to greater than 10 at B1.5 for luminosity class III-V stars appears to be an especially useful criterion for the temperature classification of stars in this spectral range.