New high quality radial velocity measurements of the RS CVn star 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
and the newly determined v sin i = 27.0
(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:
The inclination is high: .
The mass of the primary is between
(or more
probably
) and
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 .
Unfortunately, the atmospheric models are usually calculated in
steps of
logg=0.5. Our limits here indicate, that practically
the only gravity allowed is then log
. 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
(Dyck et al. 1996, consistent with Schmidt-Kaler 1982).
For
the inclination would only be
.
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 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
log
to describe the spectrum),
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.