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
4686Å
to HeII Pickering series members.
,
HeI4713Å, HeI4471Å, HeI4388Å,
HeII4686Å, and HeII4543Å.
The criteria are best illustrated in a
block diagram (Fig. 2 (click here)).
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.