Symbiotic stars are interacting binary systems containing a cool giant whose wind is partly ionized by a hot companion, in most cases a hot white dwarf (e.g. Mikolajewska 1997). Mass transfer and accretion onto the hot component is necessary to explain its high temperature and luminosity (typical values are T105 K and L103 ), but affects only a small fraction of the mass lost by the giant. The rest distributes in some way around the binary system, forming an expanding gaseous nebula. Both the history and geometry of the red giant mass loss and the radiative and dynamical evolution of this circumbinary nebula can be strongly affected by the presence of the hot companion. In particular, phenomena relevant to a wide range of astrophysical topics can occur: aspherical mass deposition by the red giant envelope, collimated winds from accretion discs, colliding winds, mass loss induced by violent thermonuclear outbursts and its interaction with old circumstellar wind remnants.
Because of the energetic radiation field from the hot component and the shaping-exciting action of high velocity winds which are often observed in these systems, the circumbinary gas can show up as a large ionized nebula, which is observable in the light of characteristic emission lines. Such ionized nebulae have been discovered around several symbiotic systems, mostly those of the D-type, which contain a Mira variable as cool component, and contain dust. Being composed of the wind of an evolved AGB star and excited by a hot post-AGB star, these nebulae are very similar to planetary nebulae (PNe) in terms of excitation conditions and chemical abundances. The main distinction from PNe is expected in terms of smaller nebular masses, since only a fraction of the envelope of the AGB star was lost. These smaller masses might also imply shorter life-times (the nebulae should vanish in a shorter time). The fact that their morphology and kinematics are very similar to those of some PNe tells us that similar dynamical processes are likely to occur in both class of objects, and also that there could be some misidentifications in the literature (cf. Corradi 1995). In this respect, symbiotic stars provide a nice observable demonstration of the effects of binary interactions on the mass loss from evolved stars, providing clues to understand the origin of the shaping of certain morphological classes of PNe.
The ionized nebulae around symbiotic stars can be as large as 2 pc (BI Cru and V417 Cen, cf. Corradi & Schwarz 1997). Complete information is therefore obtained by adding the information which is provided, on the sub-arcsec scale, by radio and HST imagery for the innermost, bright regions of these systems, and by optical ground-based observations for the outermost, faint zones. Often these "outer reaches" are in fact too faint to be imaged with the HST.
In this paper, we report on results of a search for optically resolved nebulae around symbiotic stars from ground-based observations using some of the best imaging facilities available worldwide. In addition to individual papers reporting discovery of single objects, a search for resolved H nebulae was presented by Munari & Patat (1993) for a sample of 18 southern symbiotic stars and by Kohoutek (1997) for 5 objects.
© European Southern Observatory (ESO) 1999
Online publication: March 1, 1999