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6. Summary and discussion

 

New high quality radial velocity measurements of the RS CVn star tex2html_wrap_inline2367 Gem have been used to derive the orbital parameters of this single-lined binary. Essentially, the orbit given by Bopp & Dempsey (1989) is confirmed; however, the orbit is most probably circular.

The orbital parameters and the rotational parameters tex2html_wrap_inline2369 and the newly determined v sin i = 27.0tex2html_wrap2409 (confirming the result by Eaton 1990) are combined to construct the region of allowed combinations of the inclination, the two masses and the gravity of the primary. A few plausible assumptions have to be made during this process:

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that the primary is spherical and does not fill its Roche-lobe;
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that the rotational axis of the primary and the orbital axis are parallel;
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that the secondary is less massive than tex2html_wrap_inline2377.

The first assumption is made plausible by the fact that the RV curve is stable over the century of data, i.e. there seems to be no interaction between the components. The second assumption is based on the circularized orbit and the seemingly synchronized rotation; the third on the invisibility of the secondary in the high S/N SOFIN spectra and the additional assumption that it is a main-sequence star.

The inclination is high: tex2html_wrap_inline2379. The mass of the primary is between tex2html_wrap_inline2381 (or more probably tex2html_wrap_inline2383) and tex2html_wrap_inline2385 which is normal for a K1-giant (Schmidt-Kaler 1982), at least when it belongs to the old disk population (Eker 1992; Eggen 1993). Its radius, however, seems to be smaller than the ``normal'' values for stars of this type.

The limits on the gravity are especially interesting: The gravity is in principle an observable; it can be obtained from careful modelling of the spectrum. As can be seen in Fig. 4 (click here) an accurate determination of the gravity leads to a very narrow range for the allowed values of tex2html_wrap_inline2387. Unfortunately, the atmospheric models are usually calculated in steps of tex2html_wrap_inline2389logg=0.5. Our limits here indicate, that practically the only gravity allowed is then logtex2html_wrap_inline2393. The next lower gravity is almost certainly excluded, because the mass of the primary becomes too low for its spectral class or the Roche-lobe limit would be violated. Should we, however, find that a higher gravity is needed to describe the spectrum, this would also contradict our limits. The most uncertain assumption made is that the secondary is a main-sequence star. For a higher gravity in the primary, both stars have to be more massive. Since under these conditions the secondary cannot be a main-sequence star (otherwise it would have been seen at least in the SOFIN spectra) and it cannot be a white dwarf (Ayres et al. 1984) we need to conclude that the secondary had to be a neutron star. Note that this would also mean, that the primary's radius is then allowed to be at its normal value of tex2html_wrap_inline2395 (Dyck et al. 1996, consistent with Schmidt-Kaler 1982). For tex2html_wrap_inline2397 the inclination would only be tex2html_wrap_inline2399. This might be another check, because according to Piskunov et al. (1996) (largely) different i can be distinguished by the quality of surface imaging.

The fact that during the history of RV curves for tex2html_wrap_inline2403 Gem all new orbital solutions tend to confirm (with increasing refinement) the previous solutions is a sign that there is no interaction between the two stars. If the secondary would be a neutron star, there is therefore not much hope to see any sign of it. If, however, indirect evidence for it is found (e.g. by the necessity of a logtex2html_wrap_inline2405 to describe the spectrum), tex2html_wrap_inline2407 Gem would turn out to be indeed a very special RS CVn-star.

Acknowledgements

This work has made use of the SIMBAD data base, operated at CDS, Strasbourg, France. Z. Eker has kindly made his unpublished individual RV-measurements available to us. We thank the referee, B.W. Bopp, for his valuable comments. R.D. would like to thank A. Bruch for very helpful discussions concerning the Roche-lobe limit; he gratefully acknowledges further discussions with S. Jankov and O. Vilhu. The project ``Late-type stars: activity, magnetism, turbulence'' is supported by the EC Human Capital and Mobility Network, No. ER-BCHRXCT940483.


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