A classification system for spectra of hot subdwarfs obtained in the blue-violet at 1.5 Å resolution or better and with S/N >10 is described. This system is an extension of that discussed by Drilling (1996) and is based on the observation that amongst hot subdwarfs there is a continuum of spectral types in three dimensions, being
At the low-temperature end of the scale, HeII4686Å is undetectable, at the high-temperature end HeI4471Å is undetectable. In between, the relative strengths of HeI4713Å, HeI4471Å and HeII4686Å establish a seven-point scale. In the H/He dimension, the relative strengths of HeI4471Å and H or of HeII4543Å and H provide a four-point scale. If neither HeI nor HeII lines are seen but high-gravity Balmer lines are present, there is no spectral measure of effective temperature. Neglecting, for the present, the density dimension, these scales define a rectangular grid of 29 classification boxes.
It is clear that the present sample of "sdB4'' stars can be arranged in extensive sequences in both temperature and H/He directions, and may be found in eleven of these boxes. Some of these are illustrated in Figs. 3 (click here) and 4 (click here). A limited sequence in surface gravity is illustrated by a comparison of PG1544+488 and PG1415+492 with spectra of the extreme helium stars LSIV and LSS5121 (Fig. 5 (click here)). A spread in gravity of 1.0 dex amongst He-rich sdO stars has already been established by Husfeld et al. (1989) and Dreizler (1993). However, the current sample contains insufficient members to quantify any general spectral discriminant.
It is also apparent from our 1.5 Å resolution spectra that many hot subdwarfs show a rich spectrum of metal absorption lines. Whilst obvious examples include PG1607+173 with a metal-line spectrum typical of early-B helium giants (cf. BD, Jeffery & Heber 1992), lines from more highly-ionized species can be also identified in spectra of the hottest objects including, for example, CIV4658Å and NV4604, 4620Å in BD. However there is such diversity that the metal lines do not provide good primary classification criteria.
Figure 6: An illustration of spectral classifications in the H/He and HeI/HeII dimensions for selected hot subdwarfs. Examples of spectral types not represented in our sample of helium-rich subdwarfs are taken from Moehler et al. (1990)
The derivation of a useful nomenclature is complicated by the need to avoid conflict with established practice, but is recognizable in terms of familiar conventions.
The prefix "sd'' is a natural designation for subdwarf in current usage, which it is appropriate to preserve.
The classes "sdO'' and "sdB'' have their antecedents in the Harvard system for normal stars, whilst the hybrid "sdOB'' is unique to the subdwarfs, indicating the presence of both HeII and HeI. It is the latter which causes the greatest difficulty, since the use of other letter and number combinations (e.g. OA, OC, OD, O1, O2, OB1, OB2, etc.) have not always followed self-evident sequences. The preferred parallel in the MK system is the sequence of subclasses "B0, O9, ..., O5, O4''. In our scheme we propose to drop the classification sdOB (which refers to a HeII/HeI ratio) and replace it with a scale running from sdB (implying no HeII) through sdO9, sdO8, ...as required. Thus a true helium-rich sdB star would show no trace of HeII, whilst a helium-rich star with no trace of HeI would be classified sdO4. It is stressed that there is no one-to-one correspondence between the current sdO subtypes and the familiar MK O subtypes. It should also be remembered that, for example, an sdO4 subdwarf is much hotter than an O4V main-sequence star. Subdwarfs which show no helium lines cannot be further subclassified and would be labelled sdB:He0.
True helium-rich B stars (only HeI and weak H) with high surface gravities include the helium stars V652 Her, LSS3184 and HD144941, but with surface gravities close to and lower than main-sequence values, these are not genuinely subluminous.
Regarding labelling the H/He ratio, there is a choice between a numerical or alphabetic scale. Previous practice (e.g. Green et al.) suggests a set of alphabetic classes, "A'' (hydrogen dominated) to "D'' (helium dominated) via "B'' and "C'', whilst white dwarf classifications (Sion 1996) adopt "A'' (pure H spectrum) to "B'' (HeI) or "O'' (HeII) via combinations such as "BA, OA'' or "OB''. The latter scheme is two-dimensional and insufficiently precise for our purposes. Moreover, in white dwarf (AOB) space, the "DA'' white dwarfs occupy the same topological position as the "sdB'' stars in hot subdwarf space! It is more convenient to provide a numerical scale running from "He1'' (some He present) through "He4'' (no hydrogen lines). The class "He0'' only applies to the H-rich sdB stars referred to above. Although this numerical scale can be mapped exactly onto a concise alphabetic scale "A'' through "D'' similar to one already introduced (Green et al. 1986), the historical use of A and B to indicate H and HeI dominated spectra leads to ambiguity. This nomenclature is best understood in terms of the classification criteria by referring to Fig. 2 (click here).
We have defined a set of classification criteria for helium-lined hot subdwarfs which can be applied to 1.5 Å\ resolution spectra of high S/N. We have proposed a classification nomenclature which may be applied to the system based on these criteria.
These have been applied to our sample of helium-rich hot subdwarfs, resulting in the classifications given in Table 2 (click here). For example, PG1715+273 shows HeII4686Å > HeI4713Å, HeII4686Å < HeI4471Å and HeI4471Å > H, and so is classified sdO7:He3.
Further spectral classes have been identified from the sample of Moehler et al. (1990). All classes identified are illustrated in Fig. 6 (click here). As the classification of subdwarfs under this scheme proceeds, standards will become better established for those classes already identified, and identified for those classes not yet established.