Two-dimensional Ultraviolet Spectral Typing of O-Type Stars

Abstract

We present equivalent-width measurements of ultraviolet photospheric lines that are useful as spectral type criteria; these include 23 lines of He II, C III, N III, N IV, O IV, Si III, Si IV, S V, Fe IV, and Fe V in lUE spectra of 67 O3 to BO stars of all luminosity classes. Seven lines and two line ratios are particularly sensitive to spectral type, and we show how the ultraviolet spectral typing diagnostics lead to types that are consistent with optical types. There are few luminosity-sensitive photospheric lines in the ultraviolet but we find that the N IV λ1718 line does have a significant luminosity variation among most 0 subtypes. This criterion leads to ultraviolet classes that are approximately in agreement with optical luminosity classes. This scheme was developed in order to estimate the spectral types and luminosity classes of tomographically separated component spectra of 0-type binary systems.