Contemporary optical spectral classification of the OB stars - A digital atlas

Some recent developments in the optical classification of OB spectra are reviewed in terms of a comprehensive atlas of new blue-violet digital data from the CTIO 1-meter photon-counting system. These developments include the O3 spectral type; luminosity criteria for the O stars; OBN/OBC anomalies; and refined, interpolated late-O/early-B types. Examples of these phenomena are included among extensive spectral- and luminosity-class sequences, comprising 75 standard objects arranged into 27 montages and covering the wavelength range 3950-4750 A for types O3-B3 (-B8 at Ia). It is intended that this atlas serve a reference function analogous to that of the printed MK atlases, for morphological investigations of OB spectra based on digital data, which will supersede photographic techniques in most future applications.