3. Classification of 10 Å spectra

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.

sdB

(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).

• 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 al. sdB-O).
• sdB3. "dominant Balmer absorption along with pronounced HeI 4471 and often HeI 4026'' (Green et al. sdO(A)).
• 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 (Drilling 1986).

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 H.

sdOB

(intermediate HeII/HeI)

• sdOB1. "strong broad Balmer lines accompanied by weak HeI 4471 and HeII 4686 absorption'' (Moehler et al. sdOB)
• sdOB2. "dominated by HeI and HeII lines and generally showing hydrogen Balmer absorption'' (Green et al. sdO(B)).

sdO

(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.

   

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)

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

PG2215+151 sdO(C) sdO6:He4

4686/4713 not measured.