The phase-dependent Ca II K emission of zeta Aur K-type supergiants: chromospheric heating versus irradiation-induced emission

Zeta Aur systems, wide eclipsing binaries with K supergiants, have been given special attention in stellar astrophysics over the past 80 years, since their hot companions serve as probing light sources during chromospheric eclipse phases and provide us with unique information. On the other hand, at the same time their hot irradiation raises the question as to how representative the physical conditions found in zeta Aur type K supergiant chromospheres are for this class of objects. We here present, using high quality (20.000 resolution, s/n = 120 to 160) TIGRE-spectra, the Ca II K emission of the K4 II supergiants of zeta Aur and 32 Cyg in different, orbital phases, representing different degrees of irradiation by the hot companion. To extract the K giant spectra from each composite spectrum, we subtract each secondary's contribution by using a matching PHOENIX model spectrum as template. By the variation of the Ca II K emission with orbital phase, and by comparison with eclipse spectra of the K supergiants, we find that the irradiation induced contribution is of the order of at least 2.3 times larger than the intrinsic chromospheric emission caused by chromospheric heating processes common to all K supergiants.