The classification of helium-rich hot subdwarfs

Spectroscopy has been obtained of 23 subluminous stars, twenty of which had been classified as "helium-rich subdwarf B stars" from 10 Angstrom resolution spectra. From inspection of the He ii, He i and Balmer lines, the sample clearly shows a wider range in He ii/He i and He/H ratios than hitherto supposed for the class. We introduce a classification scheme for helium-rich hot subdwarfs based on 1.5 Angstrom spectra of high S/N, which can be extended to include all hot subdwarfs with spectra dominated by H, He i or He ii. The aim of this classification scheme is to establish reference standards which would be suitable for subsequent fine analysis. Based on observations obtained with the Calar Alto 3.5 m and William Herschel telescopes.