1996A&A...310..813R

We present results of pointed ROSAT PSPC observations of 15 hybrid stars/candidates, which have been analyzed in a homogenous way. 7 of these stars were observed in X-rays for the first time. 12 out of 15 hybrid stars have been detected as X-ray sources, some of them close to the detection limit. We conclude that essentially all hybrid stars as defined by the simultaneous presence of transition region line emission and cool stellar winds are X-ray sources if exposed sufficiently deep. The X-ray luminosities of hybrid stars cover a range between 2x10²⁷ and ∼10³⁰erg/s. Their X-ray surface fluxes can be as low as ≃20 erg/cm²/s and thus considerably lower than those of normal luminosity class (LC) III giants. X-ray spectra of hybrid stars tend to be harder than that of normal LC III giants, moreover, the X-ray brightest stars have the hardest spectra. We find that for K II giants the normalized X-ray flux versus C IV flux obeys a power law with an exponent a=2.9, steeper than among normal giants (1.5). Hybrid K II stars are X-ray underluminous by a factor of 5 to 20 compared to LC III giants at the same level of normalized CIV flux f_CIV/f_bol; hybrid G supergiants are even more X-ray deficient. We reanalyze the CaII wind dividing line and find it vertical at B-V=1.45 for LC III giants. It is nearly horizontal between B-V=1.45 and 1.0 (at M_bol≃-2...-3), and not well defined for supergiants with B-V<1.0. We therefore suggest that possibly all LC II and Ib G and K giants are hybrid stars and that the "dividing line" concept in its simplest form is not valid for G/K giants brighter than M_bol≃-2. Hybrid stars are supposed to be evolved intermediate mass stars and their coronal activity may in principle be determined by the individual history of each star.