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Astron. Astrophys. 343, 841-846 (1999)

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6. Conclusions

We have presented an optical search for extended nebulae around symbiotic stars. The frequent occurrence of these nebulae (40[FORMULA] of the observed sample) among symbiotic Miras, as well the paucity of objects for systems with normal giants, is confirmed.

Several things remain to be done. A complete search among D-types would possibly detect new nebulae. Even for those objects in which no nebula was found in the present survey, coronographic observations and careful subtraction of the point-spread function might reveal faint nebulae, as done in the case of RX Pup (Paresce 1990). In particular, some objects should be definitely reobserved. One is RR Tel, for which a deep [NII ] image would be needed. Another one is H 2-2 (see Sect. 3). AG Peg would also need new H[FORMULA] images (none were taken during the present survey) as well as deeper [NII ] exposures, to confirm the nebula detected by Fuensalida et al. (1988) and to look for optical counterparts of the multiple-shell radio nebula found by Kenny et al. (1991).

Detailed studies of individual nebulae are also necessary. Spatiokinematical modelling will set constraints on the geometry, dynamics, and age of the outflows, providing basic information on the mass loss history of the systems in the last few thousand years; information which is not obtainable any other way. Radio and HST imagery will also measure the apparent expansion of the nebula on time scales of a few years for objects as far as 1 kpc (giving detailed 3-D information, and in some cases a fundamental parameter: the distance). Study of the physico-chemical abundances in the extended nebulae, to be compared with the stellar abundances, will also be possible with the new generation of 10m-telescopes even for the faintest objects.

The relative orientation of the nebulae and the orbital planes is a basic piece of information to understand the mass loss processes in symbiotic stars. In this respect, spectropolarimetric studies seems to be a promising way to determine the orbital parameters even for wide binaries as the D-type symbiotics (Schild & Schmid 1996). Alternatively, it has been beautifully shown by Hollis et al. (1997b) in the case of R Aqr that in the closest objects a direct observations of the two components of the binary system is possible by comparing high-resolution images at 7 mm in the SiO maser (associated with the extended atmosphere of the cool giant) and in the nearby continuum (mainly produced close to the hot companion).

Finally, theoretical studies are needed to explain the complex phenomenology of the observed outflows. Especially, a theoretical framework which considers the relationships between the processes in the innermost regions of the systems (accretion discs, magnetic fields, binary influence on the red giant mass loss, mass release during outbursts) and the large scale jets, bipolar nebulae, rings, etc., would provide new important constraints on the formation and evolution of symbiotic stars and of related objects (PNe, novae, supernovae).

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© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999