Astron. Astrophys. 336, L25-L28 (1998)

3. Discussion and conclusions

From the results obtained for four RS CVn stars the following common conclusions can be drawn.

Spot lifetimes. Active regions determining active longitudes exist permanently, though their areas and/or temperature are periodically changing. Then, the relations for spot lifetimes should be revised. Lifetimes of active regions of many years should give new constraints for the global magnetic field models for this type of stars.

Active longitude orientation. In three cases out of four, active longitudes have no preferable orientation with respect to the line of centres in the binary. In other words, they are not synchronized with the orbital motion. However, in the case of Gem they are synchronized, and located in the line of centres of the binary. Nevertheless, this problem should be studied with other stars from the point of view of the present results.

Activity cycles. All stars show similar behaviour: one longitude is usually more active than the other, and the changing of the activity between the longitudes is cyclic with periods of years. Switch of the activity happens on a much shorter time scale, during a few months. Periodical repetition of the switches allows to consider them as new tracers of the activity cycles of the spotted stars. The cyclic activity discovered seems to be basically different from that in the Sun, since the structure is strongly nonaxisymmetric. Also, the almost linear phase migration of the active regions suggests a very small, if at all, latitude motion of the spots. Although photometry gives no information about magnetic polarity, one can suppose that switching moments correspond to changing polarity of the magnetic field in the active regions and, probably, special symmetric redistribution of the spots on the stellar surface.

Magnetic dynamo models. By analogy with sunspots, active regions on the surfaces of RS CVn stars may be accociated with magnetic field structures, generated by a dynamo operating in the convective envelopes. Simplified theoretical calculations by Moss & Tuominen (1997) showed that synchronized close late-type binaries can be expected to exhibit large-scale nonaxisymmetric magnetic fields with maxima at the longitudes corresponding to the two conjunctions. Such a case is observed for  Gem, while the other three stars show noticeable non-synchronization of the active longitudes and, therefore, no preferred orientation. The above calculations showed also that the field maxima are symmetrically located in both hemispheres relative to the orbital plane. All stars discussed here seem to have inclinations of their rotational axes of (Stawikovski & Glebocki 1994; Glebocki & Stawikovski 1994; Berdyugina et al. 1998a). Thus, the discovered two permanent active regions on the surfaces of four RS CVn stars seem to be one half of the total magnetic structure of the stars. The other two appear to be located in the invisible parts of the stellar surfaces, near the invisible poles of the stars, since surface imaging of rapidly rotating late-type stars shows in general mainly high-latitude spots. Proximity of the spots to the polar regions has also been found with flux tube calculations for single active stars by Schüssler et al. (1996). They showed that the mean latitude of the magnetic flux emergence shifts towards the poles for increasingly rapid rotation and deep convective zones. Obviously, the presence of the secondary component and the orbital motion induce additional forces which should affect the flux tube trajectories. They could also somehow provoke the observed migration of the active longitudes and their periodic switching, especially with the presence of some non-synchronized rotation. The latter can be caused e.g. by differential rotation of the outer layers or a different rotation rate of the stellar core. Evidently, new dynamo and flux-tube simulations for binaries with magnetic activity are necessary.

© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998
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