Search for resolved H-alpha nebulae around symbiotic stars and their formation mechanisms.

We present the results of Hα-BVRI CCD observations of a randomly selected sample of 18 southern symbiotic stars (8 D-, 7 S- and 3 yellow-type), aimed to search for spatially resolved HII circumstellar regions. Additional observations of He2- 104 (the Southern Crab) have been performed in order to evaluate the sensitivity of the adopted instrumentation.

An extended circumstellar HII region has been discovered around 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 8.1×6.O arcsec (at 10% of peak intensity), which correspond to 27,000×20,000 AU at the system distance. The nebula radiates as much as 75% of the Hα 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 found 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 (∼200 times) then S-type objects.

Simple statistical arguments suggest that very basic mechanisms lead to the formation of these circumstellar nebulae. The nebulae around yellow-type symbiotics probably derive from the material ejected by the hot component progenitor when evolving to the tip of the AGB phase. The nebulae around D- type objects are instead probably originated by material lost during the mass transfer process from the cool giant to the WD companion. Both mechanisms could operate in S-type symbiotics.

The investigation of nebulae around symbiotic stars is important in order to understand better the formation of axially symmetric planetary nebulae.

The presence of nebulae around symbiotic stars is shown not to be in contrast with their association with the precursors of type Ia supernovae.