Symbiotics are evolved low-mass binaries consisting of a hot and very luminous (up to some 1000 L_{odot}) star and a cold RGB or AGB star. Evolutionary tracks suggest that the hot star is reborn due to accretion. On the other hand, observational evidence of fast stellar winds and jets in certain symbiotics lead to the suggestion that at least some symbiotics should be colliding wind binaries (Kwok & Purton, 1979, Wallerstein et al., 1984). Indeed, HST high resolution spectra of the symbiotic novae AG Pegasi presented by Nussbaumer et al. (1995) clearly show P-Cygni profiles, indicating a wind of about 1000 km/s from the hot star. Based on X-ray observations, Mürset et al. (1997) suggested that some other symbiotic systems show colliding wind signatures as well.

A variety of models have been invoked to further investigate the colliding winds scenario for symbiotics. Quantitative models, however, are rare, due to the sheer number of involved processes. NLTE radiative transfer, 3D hydrodynamics, as well as heat transfer by fast thermal electrons and X-ray photons contribute to the thermal and ionization structure in symbiotic nebulae. So far, 3D hydrodynamical models have revealed that the fast wind from the hot component can substantially re-distribute the material lost by the red star. The mass outflow from the system is extremely spirally shaped. The material is accelerated to values clearly above normal values of red star winds. Highly ovrercompressed shells form in the wind-wind interaction zone of the two winds. Hot, X-ray emitting plasma is efficiently generated by the strong shocks of the interaction zone. Photo-ionization models show that the high-density shells efficiently contribute to the emission spectrum. The asphericity of the outflows is also one key-ingredient which determines the ionization and thermal structure of the circumstellar material. Other ingredients are the temperature and luminosity of the hot star.

Symbiotics have a link to aspherical planetary nebulae. Therefore, we briefly address some new developments in modeling such nebulae on the basis of wide binary systems. Finally, we discuss differences between accretion and colliding wind models for symbiotics and try to show how such models can be distinguished on the basis of observed spectra.

References:

Kwok, S. and Purton, C.,R., 1979, ApJ, 283, 187-195.
Mürset, U., Wolf, B., and Jordan, S., 1997, A&A 319, 201-210.
Nussbaumer, H., Vogel, and M. Schmutz, W., 1995, A&A 293, L13-L16.
Wallerstein, G., Willson, L.,A., Salzer, J., and Brugel, E., 1984, Pasp, 133, 137-153.