Disentangling the composite continuum of symbiotic binaries. I. S-type systems

We describe a method of disentangling the composite, 0.12-5 μm continuum of symbiotic binaries. The observed SED is determined by the IUE/HST archival spectra and flux-points corresponding to the optical UBVRI and infrared JHKLM photometric measurements. The modeled SED is given by superposition of fluxes from the cool giant, hot stellar source and nebula including the effect of the Rayleigh scattering process and considering influence of the iron curtain absorptions. We applied this method to 21 S-type symbiotic stars during quiescence, activity and eclipses. We isolated four main components of radiation and determined their properties. (i) Stellar radiation from the giant corresponds to a unique luminosity class - normal giants. Characteristic luminosities are 1600 ± 200 and 290 ± 30 L_⊙ for red and yellow giants, respectively in our sample of objects. (ii) Hot object radiation during quiescence consists of the nebular and stellar component. The former radiates at a mean electron temperature of 19 000 K and its amount of emission suggests a mass-loss rate from giants via the wind at dot M_W = a few × 10^-7 M_⊙ yr^-1. Radiation of the latter conforms well with that of a black-body photosphere at a characteristic temperature of 105 000 K. The corresponding effective radii are a factor of 10 larger than those of white dwarfs, which thus precludes observing the accretor's surface. Extreme cases of AX Per and V443 Her, for which the hot star temperature from the fit is not capable of producing the nebular emission, signal a disk-like structure of the hot stellar source even during quiescence. (iii) Hot object radiation during activity consists of three components - the stellar and the low- and high-temperature nebular radiation. The stellar radiation satisfies that of a black-body photosphere at a low characteristic temperature of 22 000 K (we call it the 1st type of outbursts) or at a very high characteristic temperature of ≈165 000 K (2nd type of outbursts). All the active objects with a high orbital inclination show features of the 1st-type of outbursts (here Z And, AE Ara, CD-43^circ14304, TX CVn, BF Cyg, CH Cyg, CI Cyg, AR Pav, AX Per), while AG Dra represents the 2nd-type. The presence of a two-temperature type of UV spectrum and an enlargement of effective radii of the stellar source by a factor of 10 with respect to the quiescent values during the 1st-type of outburst suggest an expansion of an optically thick medium at the orbital plane in the form of a disk. The low-temperature nebula radiates at a mean electron temperature of 14 000 K and is subject to eclipses, while the high-temperature nebula, which is seen during eclipses as the only component, is characterized by T_e > 30 000 K. Radiative and geometric properties of the main sources of radiation allowed us to reconstruct a basic structure of the hot object during the 1st-type of outburst. There is an edge-on disk around the accretor. Its outer flared rim represents a warm pseudophotosphere of the hot stellar source, whose radiation is Rayleigh attenuated and affected by the iron curtain absorptions in the neutral gas concentrated at the orbital plane. The low-temperature nebula is placed just above/below the disk with a concentration at its edge as to be subject to eclipses and to “see” well the central ionizing source. High above/below the orbital plane, there is a hot nebular emitting region.