We have obtained spectra of 20 objects classified sdO(D) by Green et al. (1986) or He-sdB by Beers et al. (1992) and Heber et al. (1996), and spectra of a few objects classified sdO(A), sdO(B) and sdO(C) by Green et al. (1986). The latter were obtained to provide overlap between our sample and samples analyzed in other studies (e.g. Thejll et al. 1994; Saffer et al. 1994). For objects with mV<14 spectra were obtained with the William Herschel telescope (WHT) and Utrecht échelle spectrograph (UES) in May 1995 covering the wavelength interval 3860-5025 Å with a resolution . Fainter objects were observed with the WHT and intermediate dispersion spectrograph (ISIS) in the wavelength intervals 4240-4974 Å and with a resolution of . Additional spectra were obtained with the 3.5 m telescope at Calar Alto (CA) in the wavelength interval 4181-5070 Å with a resolution . Owing to a problem in the image-derotator, the S/N of the UES observations in 1995 is poor, and to extract useful information the spectra have been binned to a resolution . Further details are given in Table 1 (click here). Moreover, with low S/N in the UES spectra, and the substantial widths of important absorption lines relative to individual échelle orders in the UES spectra, there were rectification difficulties which could only be resolved where a low-resolution CA or ISIS spectrum was available. Thus, for example, the blue wing of the H profile in our UES spectrum of PG0909+276 is incorrect (Fig. 5 (click here)).
It was noted that spectra of three objects did not resemble subdwarf spectra at all.
For the remainder it became evident that several objects show spectra substantially different from that expected from previous observations of the prototype PG1544+488. Although the majority show essentially no Balmer lines, weak HeII and possibly a number of weak metal lines, in most cases HeII4686Å is much stronger than in PG1544+488. Whilst it had been supposed that several objects would indicate effective temperatures around and surface gravities between those of the extreme helium giants and the classical hydrogen-rich subdwarf B stars, the observed strength of HeII4686Å is more often similar to that seen in sdOB and sdO stars. It is therefore necessary to review the classification of these stars.
To be properly useful, this review had to draw on a larger sample of spectral types than was provided by the observations described above. A number of 2.5 Å spectra were obtained in a series of studies of hot subluminous stars at high galactic latitudes (Moehler et al. 1990) at the Calar Alto and the European Southern Observatories. These have been used to provide both comparison spectra and examples of hot subdwarf spectral types not already represented. These spectra include the well-known subdwarfs Feige110 and BD+284211. They also include PG1701+359 and PG2337+070 (Theissen et al. 1993), and PG1047+003, PG1559+048, PG1559+076, PHL4, and PG1537-046, which have not previously been published. Since B-type subdwarf spectra are also similar to those of horizontal-branch B (HBB) stars, we have also examined a spectrum of PG1705+537 from the Moehler et al. (1990) sample. Echelle spectra of three helium stars obtained at ESO (LSS5121: Heber et al. 1986), the WHT (LSIV+62) and the AAT (HD 144941: Harrison & Jeffery 1996) have also been used for comparison purposes. The resolution of the échelle spectra was degraded to match the remainder of the sample.
Figure 1: Schematic of classifications for 10 Å resolution spectra of hot subdwarfs. The boxes are labelled by Drilling (1996) nomenclature in bold type and their equivalents as described in the text