2018A&A...616A.135M

Our goal is to provide a quantification of several spectral classification criteria for O stars. We collect high-spectral resolution spectra of 105 Galactic O-type stars from various archives. We measured equivalent widths of classification lines. We defined average values of classification criteria for given spectral types and luminosity classes. We find that the ratio HeI 4471 to HeII 4542 well matches the published ratios for spectral types. We have quantified equivalent width ratios of helium and silicon lines among O8-O9.7 stars to refine spectral class typing in this spectral range. We present quantitative criteria to separate between luminosity class V,IV-III-II (grouped), andI among O3-O8.5 stars, mainly based on the strength of HeII 4686. We find that these criteria also define very well the f, (f), and ((f)) classes for O3-O7.5 stars. Among O9-O9.7 stars we quantify the ratios of HeII 4686 to HeI 4713 and SiIV 4089 to HeI 4026 for all luminosity classes. The tabulated values of the classification criteria should help classify any new O-type stars. The final step of the classification process should rely on a direct comparison to standard stars of the assigned spectral type or luminosity class.