SEARCH FOR RESOLVED H-ALPHA-NEBULAE AROUND SYMBIOTIC STARS AND THEIR FORMATION MECHANISMS

We present the results of Halpha-BVRI CCD observations of a randomly s elected sample of 18 southern symbiotic stars (8 D-, 7 S- and 3 yellow -type), aimed to search for spatially resolved HII circumstellar regio ns. Additional observations of He2-104 (the Southern Crab) have been p erformed in order to evaluate the sensitivity of the adopted instrumen tation. An extended circumstellar HII region has been discovered aroun d the D-type symbiotic He2-147. The shape is ellipsoidal with hints of a bipolar structure, elongated in the N-S direction, with dimensions of 8.1 x 6.0 arcsec (at 10% of peak intensity), which correspond to 27 ,000 x 20,000 AU at the system distance. The nebula radiates as much a s 75% of the Halpha emission of the central unresolved stellar source. The frequency of detection of extended nebulae around symbiotic stars is investigated as a function of the type of the cool giant. It is fo und that such nebulae are relatively common. D-type symbiotics exhibit nebulae with a much higher frequency (at least 5 times) and with much larger linear dimensions (approximately 200 times) then S-type object s. Simple statistical arguments suggest that very basic mechanisms lea d to the formation of these circumstellar nebulae. The nebulae around yellow-type symbiotics probably derive from the material ejected by th e hot component progenitor when evolving to the tip of the AGB phase. The nebulae around D-type objects are instead probably originated by m aterial lost during the mass transfer process from the cool giant to t he WD companion. Both mechanisms could operate in S-type symbiotics. T he investigation of nebulae around symbiotic stars is important in ord er to understand better the formation of axially symmetric planetary n ebulae. The presence of nebulae around symbiotic stars is shown not to be in contrast with their association with the precursors of type Ia supernovae.