A CORAVEL radial-velocity monitoring of S stars: Symbiotic activity vs. orbital separation. III.

Abstract

Orbital elements are presented for the Tc-poor S stars HR 363 (= HD 7351) and HD 191226. With an or- bital period of 4592 d (=12.6 y), HR 363 has the longest period known among S stars, and yet it is a strong X-ray source. Its X-ray flux is similar to that of HD 35155, an S star with one of the shortest orbital periods (640 d). This surprising result is put in perspective with other diagnos- tics of binary interaction observed in binary S stars. They reveal that there is no correlation between the level of bi- nary interaction and the orbital period. All these activity diagnostics moreover exhibit a strong time-variability. In the well-documented case of HR 1105, this time-variability appears to be a combination of orbital modulation and secular variation. A stream of gas from the red-giant wind, which is heated when funneled through the inner Lagrangian point, has been proposed as the source of the hard photons (Shcherbakov & Tuominen 1992). Dierent viewing angles of the stream during the orbital cycle ac- count for the orbital modulation, whereas long-term fluc- tuations of the mass-loss rate account for the secular vari- ations. Little dependence to the orbital separation is ex- pected for this kind of activity. If such streams are causing the activity observed in the other binary S stars as well, it would provide a natural explanation for the absence of correlation between orbital periods and activity levels, since the red-giant mass loss rate would be the dominant factor. The existence of such funneled streams is moreover predicted by smooth particle hydrodynamics simulations of mass transfer in detached binary systems.