Astron. Astrophys. 345, 244-248 (1999)

6. Conclusions

Based on LTE model atmospheres and both low and high-resolution spectra, we present abundance analysis of a sample of 6 standard stars and 21 He-rich stars. Having used a standard synthetic spectrum method we achieved the following progress results:

• a lack of fast rotators

• given the atomic data a possible diverse pattern of abundances from under-solar up to over-solar values

• an underabundance of carbon in He-rich stars with respect to standard stars

• a solar abundance for the heavier element, magnesium

The results above suggest that observed CNO abundances do not entirely fulfil the predictions of the model proposed by Michaud et al. (1987), i.e. predicted solar values, and that this model should include not only the stellar wind but also the detailed magnetic geometry as well as the interaction of mass loss with it, as a next step to further progress. The agreement is reached however for the coolest He-rich stars, since they tend to have the same carbon abundance as standard stars, following the model predictions. The general carbon underabundance is also in agreement with the results of Hunger & Groote (1993) and paper V of Kilian (1994) as well as the nearly solar magnesium abundance. Another interesting result is that even normal stars like HD 122980 seem to have non-solar abundances (Fig. 1). The abundances are based on LTE model atmospheres and semi-empirical gf values from Kurucz & Peytremann (available e.g. in the CCP7 library) and may be less accurate than if they were based on full NLTE treatment, but they seem really different from one "standard" star to the other, even if their absolute value is less reliable. Kilian (1992) also found significant abundance variations among normal B stars which, however, are hotter on average than ours. The intercomparison of equivalent widths wherever possible with her paper is within 10-15. She reached the statement that interstellar matter was well mixed on large scales, but inhomogeneous on small scales. Owing to the limited wavelength region of high-resolution data, the error bars and finally the LTE treatment, we cannot proceed to further considerations on the abundance spread. In particular, these limitations exceed another source of uncertainities such as semi-empirical atomic data. On the other hand, slow rotation of He-rich stars turned out to be a significant fact, in comparison with the rotational velocities of normal B type stars. Thus, slow rotation is characteristic of all magnetic main-sequence stars. We hope to obtain better abundance estimates in the near future, on the basis of new échelle spectra and of a more realistic interpretation, including NLTE treatment.

© European Southern Observatory (ESO) 1999

Online publication: April 12, 1999
helpdesk.link@springer.de