3. The effect of hydrogen content
The aim of this section is to examine the effect of hydrogen content on and to identify the group of WN stars that are entirely free of hydrogen. HKW have determined the temperatures for most galactic WR stars by model fitting to the helium lines. They found (as confirmed in the previous section) that the ionisation subclasses are not clearly separated by either luminosity L or effective temperature . However, they emphasised (as in Fig. 4) that the strong and weak line stars occupy different ranges of . Fig. 6 shows the distribution of WN stars in the vs. diagram, with the stars represented according to their broad, b, and hydrogen subclasses: ha, h and o. This is similar to Fig. 7 of HKW93 and to Fig. 4 of HKW95 but is confined to the stars with known (including the improvements described in the previous section and Table 2) and includes the spectral separation of b, o, h (including (h)) and ha stars. Binaries analysed by HKW have not been corrected for the companion, which will increase the derived ; however, those included in the diagram have luminosity ratios of O/WR less than 1 (see Table 2) and model fits by Howarth & Schmutz (1992) indicate that, so long as the WR is the brighter member of the pair, the resulting is not greatly affected.
Fig. 6 shows some interesting features. As well as the clear separation of the broad line b-stars, we also see a clear separation of the hydrogen subclasses: ha, h and o. The ha-stars separate clearly (as in Fig. 5), with a flat slope at high luminosity. The difference between the h and o-stars is within the error bars for an individual point, however the average separation appears to be significant with the o-stars systematically hotter than the h-stars at the same .
The h-star point on the ha line is WR 89 (WN 8h+abs) indicating that it is probably an ha star. WR 87 (WN 7+abs), which is in the same cluster with nearly 1 mag fainter, falls with the other h-stars and the absorption lines are probably due to a companion.
On the basis of Fig. 6, we suggest that all narrow line stars (both o and h-stars) contain some hydrogen, with the difference that only the h-stars have sufficient hydrogen to be spectroscopically detectable; i.e. that the o-stars constitute the "missing" stars at low H/He in Fig. 2. We also suggest that the b-stars have no hydrogen; i.e. they represent the completely hydrogen free phase of the WN stars. There are several reasons for this suggestion:
WR 149 (WN 5o) is included by HKW95 as a strong-line star because it has a thick atmosphere, a property that is probably shared by all (or most) b-stars. (See the footnote in Sect. 2.2 regarding the stars WR 3 and 46.) WR 149 has a lower FWHM 4686 (20 Å) and larger EW 5411 (39 Å) than any other WN 5o star (see SSM96, Fig. 17). This is a combination that is exceptional in a population where these two parameters are extremely well correlated. While WR 149 fails the b-star criteria, we included it as an interesting object which appears closely related to the "fully developed" b-stars. Note that it falls in the intermediate region between b and non-b stars in Fig. 4.
© European Southern Observatory (ESO) 1998
Online publication: June 2, 1998