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Astron. Astrophys. 334, 845-856 (1998)

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3. The effect of hydrogen content

The aim of this section is to examine the effect of hydrogen content on [FORMULA] and to identify the group of WN stars that are entirely free of hydrogen. HKW have determined the temperatures [FORMULA] 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 [FORMULA]. However, they emphasised (as in Fig. 4) that the strong and weak line stars occupy different ranges of [FORMULA]. Fig. 6 shows the distribution of WN stars in the [FORMULA] vs. [FORMULA] [FORMULA] 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 [FORMULA] (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 [FORMULA] ; 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 [FORMULA] is not greatly affected.

[FIGURE] Fig. 6. The [FORMULA] vs. [FORMULA] diagram for WN stars with known distances. Symbols indicate the broad, b, and hydrogen subclasses: ha, h and o. [FORMULA] and [FORMULA] are from HKW, with corrections to [FORMULA] as described in Table 2 and Sect. 2.3. The error bars on single WR stars are [FORMULA] mag for [FORMULA] and [FORMULA] dex for [FORMULA] [FORMULA]. WR stars in binaries have larger uncertainties in both co-ordinates and are indicated by small error bars up, down and to larger [FORMULA] (the direction of the [FORMULA] correction that has not been made).
[FIGURE] Fig. 7. Relationship of [FORMULA] EW 5411 to [FORMULA] [FORMULA] for WN b stars. The ionisation subclasses are indicated with the same symbols as in Fig. 8.

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 [FORMULA].

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 [FORMULA] 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:

  • The hydrogen-containing envelope is extended, as shown by stellar models. An increasing temperature as the hydrogen layer diminishes and a sudden jump to larger core temperature (below the optically thick wind) is expected when the last of the hydrogen is removed from the star.
  • The closely correlated FWHM-EW relationships for each ionisation subclass (SSM96) have h-stars at the narrow-weak end proceeding through o to b-stars at the broad-strong end, suggesting a regular progression of decreasing hydrogen content.
  • The simple linear relationship between [FORMULA] and [FORMULA] is suggestive of, and in the right sense (decreasing luminosity with increasing temperature) to match the Mass-L- [FORMULA] relationship required by the interiors models for these (relatively) simple stars. From the present data, the relationship for b-stars is:
    [FORMULA] = 7 [FORMULA] [FORMULA] - 38.2
    and is used to derive the [FORMULA] of other b-stars, see Fig. 8 below. This relationship differs slightly from that originally suggested by HKW93 because of correction to the [FORMULA] of WR 136 (HKW95). All of HKW's derived values (used in later sections) have been corrected accordingly.

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.

[FIGURE] Fig. 8. Relationship between [FORMULA] and [FORMULA] EW 5411 for WN b stars on the assumption that the log [FORMULA] - [FORMULA] relationship (Sect. 3) applies to all b-stars. The ionisation subclasses are indicated for all points. The arrow indicates the slope of constant line-flux.

Comment is given in the footnotes 2 3 on a few WN b stars that (may) have unusual features.

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© European Southern Observatory (ESO) 1998

Online publication: June 2, 1998