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Drilling (1996) discussed the classification of the spectra of
hot subdwarfs of 1.5 to 10 Å resolution and has identified 8
distinct classes, all of which had been previously defined in the
literature. The spectra of ALL hot subdwarfs classified to date fit
into one of these 8 classes which can be arranged in a two-parameter
sequence of HeI/HeII versus He/H. Some duplicity of
nomenclature exists, which Drilling sought to rationalise with a
provisional scheme which is amplified here.
- (smallest HeII/HeI). "Stars with colours
corresponding to those of a B star and in which the
Balmer lines are abnormally broad for the colour as
compared with Pop. I main-sequence stars. Some, but not
all, subdwarf B stars have HeI lines that are weak for
their colour'' (Sargent & Searle 1968).
Closer examination suggests that there is little to
distinguish classes sdB2 and sdB3 quantitatively. In
terms of our classification lines, both classes fulfil
the criterion that HeI 4471 is present but weaker than
- sdB1. "Higher-gravity Balmer absorption over a large
colour range, no HeI 4471'' (Green et al. sdB).
- sdB2. "sdB stars with a suggestion of HeI'' (Green et
- sdB3. "dominant Balmer absorption along with
pronounced HeI 4471 and often HeI 4026'' (Green et al.
- sdB4. "pure HeI absorption spectra, characterized by
the weakness or absence of hydrogen Balmer lines and
HeII 4686 while showing the singlet HeI 4388 about equal
in strength to the triplet HeI 4471'' (Green et al. sdO(D),
Moehler et al. He-sdB). It is significant that this is the same
definition as that given for extreme helium stars and binaries
- (intermediate HeII/HeI)
- sdOB1. "strong broad Balmer lines accompanied by
weak HeI 4471 and HeII 4686 absorption'' (Moehler et al.
- sdOB2. "dominated by HeI and HeII lines and
generally showing hydrogen Balmer absorption'' (Green et
- (largest HeII/HeI)
- sdO1. "a star in which HeII 4686 is seen, and which
has, relative to main sequence stars, strong Balmer lines
for the colour'' (Sargent & Searle sdO, not recognized by
Green et al.; Moehler et al. sdO)
- sdO2. "dominated by HeII absorption. Only a trace of
HeI 4471 may be present, and some Balmer absorption may
be blended with HeII Brackett lines'' (Green et al. sdO(C),
Moehler et al. He-sdO).
The two-dimensional sequence is illustrated schematically in Fig.
1 (click here). It should be noted that the Green et al. (1986)
classifications refer to 10 Å resolution spectra. It should also be
noted that this spectral classification system remains incomplete after
more than a decade of use. Standard stars observed at a specified
resolution and S/N have still to be established. In the following, we
shall adopt the Drilling notation to refer to 10 Å classifications.
4686/4713 not measured.
Object || Green || Moehler|| Us |
|| et al.|| et al. || |
PG0902+058 || sdO(D) || || sdO6:He3 |
PG0909+276 || sdO(A) || || sdO7:He1 |
PG0921+311 || sdO(B) || || sdO7:He3 |
HS1000+4704 || || || sdO6:He2 |
PG1127+019 || sdO(D) || || sdO6:He4 |
PG1230+067 || sdO(B) || || sdO6:He3 |
TON107 || sdO(D) || || sdO7:He2 |
PG1413+113 || sdO(D) || || sdO6:He4 |
PG1415+492 || sdO(D) || || sdO9:He4 |
PG1437+727 || sdO(D) || || sdO7?:He3 |
PG1441+407 || sdO(D) || || sdO5:He4 |
PG1526+440 || sdO(D) || || sdO7:He2 |
PG1544+488 || sdO(D) || He-sdB || sdO9:He4 |
PG1552+464 || sdO(D) || || DBA |
PG1554+408 || sdO(D) || || sdO7:He4 |
PG1559+222 || sdO(D) || || sdO7:He3 |
PG1600+171 || sdO(D) || || sdO5:He4 |
PG1607+173 || sdO(A) || || sdB:He2 |
PG1615+413 || sdO(D) || || sdO8:He3 |
PG1629+466 || sdO(D) || || sdO8:He3 |
PG1648+315 || sdO(D) || || sdO8:He2 |
PG1658+273 || sdO(D) || || sdO7:He4 |
PG1715+273 || sdO(D) || || sdO7:He3 |
HS1843+6343 || || || sdO8:He4 |
LS IV-14 116|| || || sdO7:He1 |
Table 2: Classification of "helium-rich''
PG2215+151 || sdO(C) || || sdO6:He4 |
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