COMPARISON OF THEORY TO OBSERVATION

5. CONCLUSIONS

5.1 COMPARISON OF THEORY TO OBSERVATION

In examining the fits for individual lines, one sees that two of the lines are not well reproduced by the theoretical calculations. The Hell 4686A line is found in emission in the Of stars, and Auer and Mihalas [7] showed that this could not be explained by fluorescence arising from the overlap of the hydrogen Balmer and helium Pickering series. They concluded that the emission was likely a result of atmospheric extension. The analysis here shows that the theoretical calculations produce a line that is too strong even for normal O stars.

Auer and Mihalas also noted that the Hel 5876A line was somewhat stronger in their small sample of stars than was predicted by their theoretical work, although the extent and severity of the discrepancy was unclear. The analysis here confirms the discrepancy. It is found that the effect is strongest in the cooler and more luminous stars in the sample; it is most pronounced in BD +41

°

3804, HD 13022, HD 194280, HD 229234, HD 194094, HD 207198, and 19 Cephei, all of which have Teff

<

36000K and log g

<

3.7. This discrepancy probably arises from the geometrical dilution effect described by Ghobros [18], as noted by Voels et al. [47]. However, the effect seems not to be significant for other neutral helium lines. As a check, a second fit to the observed profiles for BD +41

°

³⁸⁰⁴ was made with the Hel 6678A, 5876A, and 4471A lines omitted. The change in the fit parameters was not significant.

Because the theory poorly reproduces the Hell 4686A and Hel 5876A lines, these were given one-tenth the normal weight in the

x

² fit.

Another discrepancy between theory and observations is that the theory predicts emission lines of Hel 6678A and 4922A in the hottest stars that are not observed. This is most evident for 9 Sagit- tarii, which is slightly hotter than 50000K. Since 9 Sagittarii appears to have a rather high surface

131 5.2 HELIUM ABUNDANCES

One disturbing tendency in the fitted parameters is the almost perfect correlation between low gravity (or high luminosity) and high He/H (Figure 7 ). Although the more luminous stars lose mass at a great rate, so that they are the most likely members of the sample to show a large He/H, it seems very unlikely that all the high-luminosity stars should show large He/H, since the associations chosen are not all of the same age. These fitted abundances most likely reflect a systematic failure of the theory rather than the actual compositions of the stars involved.

However, such a failure of the theory is not suggested by the quality of the fits, which are generally quite good. In this respect, the "normal" 0 stars analyzed differ from the Of stars that were dropped from the sample; attempts to analyze the Of stars by the method described yielded obviously poor fits, with, e.g., indications that the cores of the Balmer lines are much shallower in the observed profiles than in any of the theoretical profiles.

It is of note that Voels et al. [47] do not find a large value for He/H for the O supergiants they have analyzed ( although two of their objects, ( Pup and a Cam, show somewhat enhanced He). The only substantial difference between their models and the models used here is the inclusion of the effects of wind blanketing. It is surprising that neglect of wind blanketing should result in systematic overestimates of He/H, particularly since some of the O supergiants for which Voels et al. find cosmic He/H show negligible effects from the wind blanketing. Unfortunately, there is no overlap between their sample of objects and the sample of objects analyzed here, which makes a direct comparison difficult. Likewise, they do not present tables of profiles and their illustrations containing profiles do not have labels on the Y-axis. Thus no direct comparison is possible.

A final point that should be raised is that many of the objects with low values for logg are off the original model grid, and the values for the stellar parameters are extrapolated. In the case of the two lowest-gravity objects, HD 194280 and BD +41

°

3804, the model grid was extended to Teff=28000K and logg=3.75 for He/H of0.20 and 0.50, to reduce the amount of the extrapolation.

The result was that the estimated value of He/H actually increased, which would seem to rule

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133

out the extrapolation of the theoretical profiles as the cause for the overestimation of He/H. (The parameters given for these objects are those obtained with the extended model grid.)

5.2.1 Semi-Empirical Helium Abundances

If the correlation of He/H with log g is, in fact, the result of a failure of the theory, we may account for this failure by fitting a curve through the plotted points in the (logg, He/H) plane and using it to reduce all values for He/H determined by our method of analysis. The data shown are well represented by the formula, log(He/H) = l.1264-0.4791logg. The reduced helium abundances calculated using this formula and assuming a cosmic He/H of 0.10 are given in the table of estimated parameters under the column labeled "(He/H)0 , " along with association averages. Figure 8 plots these corrected values against effective temperature.

When this reduction is made, we find that there is no object in the sample with an obviously high value for (He/H)0 • The three stars near log ^g = 3.8, He/H=0.4, HD 242908, HD 12993, HD 193595, are the likeliest candidates for a moderate helium overabundance ((He/H)0

~

0.19). It may be significant that HD 12993 is certainly a blue straggler, while the other two stars are also possibly blue stragglers (being the hottest objects in each of their respective associations). Although this apparent helium overabundance may be a temperature effect, the normal abundances for HD 40800, HD 242935, HD 35619, HD 42088, #4 OB2 Cyg, and A Ori argue against this.

One may also conclude from the plotted points and association averages and probable errors that there is no significant difference in the primordial helium abundance between the associations studied. This agrees with the results obtained by Wolf and Heasley [50]but does not agree with those of Nissin [38].