The classification of helium-rich hot subdwarfs

Jeffery, C.S., Drilling, J.S., Harrison, P.M. , Heber, U., Moehler, S.

1997,A&AS 125, 501

Abstract. Spectroscopy has been obtained of 23 subluminous stars, twenty of which had been classified as 'helium-rich subdwarf B stars' from 10A resolution spectra. From inspection of the HeII, HeI, and Balmer lines, the sample clearly shows a wider range in HeII/HeI and He/H ratios than hitherto supposed for the class. We introduce a classification scheme for helium-rich hot subdwarfs with spectra dominated by H, HeI or HeII. The aim of this classification scheme is to establish reference standards which would be suitable for subsequent fine analysis.

Hot subdwarf spectra. The figure shows a composite of all hot subdwarf subtypes recognised from 2A spectra. Additional figures showing sequences of spectra of types sdO?:He4 and sdo&:He? are given in the published paper.

The classification criteria and class labels are defined in the following table. These classifications are designed to eliminate, for 2A resolution spectra, the confusion which arises from the use of acronyms such as sdO, sdOB, sdO(B), sdB. Note that the current two-dimensional classification depends primarily on the relative strengths of H-gamma, He I 4713, He I 4471, He II 4686 and He II 4543.

Future work will provide a refinement in metallicityand luminosity classifications, and identify classification standards for each subtype identified. Prototype classifications will shortly be available as either plots or FITS files by selecting the spectral subtype in the table below.