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

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5. Discussion

Including all the information in the literature, 71 symbiotic stars, 35 of S type (excluding the "yellow" ones), 21 of the D type, 9 "yellow" systems and 6 with unknown/uncertain IR classification were searched for extended optical nebulae. Note that the sample represents about one third of the total number of symbiotic stars known in the Galaxy to date.

8 D-type symbiotic stars (40[FORMULA] of the sample observed) have an extended optical nebula. Thus, as already remarked by Corradi & Schwarz (1997), the presence of a PN-like nebula is a common property of the symbiotic Miras. In principle, the nebulae around D-type symbiotics could be either the AGB remnant of the hot white dwarf (its PN), material from the Mira ionized by the hot component (and possibly also shaped by high velocity winds from the hot companion), or material ejected directly from the hot component during outbursts. We use the same statistical argument as in Corradi & Schwarz (1997) to show that these nebulae are not the PNe of the white dwarfs. If so, it would in fact imply that the white dwarf is in a very early post-AGB phase (few tens of thousands years after the envelope ejection, which is the typical life time of a PN). A young post-AGB star and a late AGB star would then co-exist in the same system. Considering the short evolutionary life times in these phases (e.g. Renzini 1993), this would imply an extremely small difference in the initial mass m between the two stars ([FORMULA]). And it is extremely unlikely that this applies to such a large fraction of the D-type systems. The nebulae have therefore to be composed of material lost by the Miras, likely mixed with the ejecta from the hot component (and thus are not genuine PNe). Note that this same argument of unlikely negligible mass differences between the two components can be used to prove that the hot component is not just a post-AGB star passing naturally through the phase of high luminosity/temperature in its early post-AGB evolution, but is an older object which must be continuously fed via mass accretion to explain and maintain the high luminosity and temperature which are typical of these compact stars.

Another important property of the nebulae around D-type symbiotic stars is their wide variety of shapes (see Tables 2 and 3) and sizes (cf. Corradi & Schwarz 1997). Elliptical or ring nebulae, collimated/bent jets, marked bipolar morphologies and irregular geometries are found. Without going into further details, it is clear that different dynamical processes must be taken into account in order to explain this complex and varied phenomenology. This is rich and challenging field of work for the future. Fast vs. slow wind interactions as proposed for the shaping of planetary nebulae (cf. Balick & Frank 1997and reference therein) might possibly also work to explain the shapes of some of the extended nebulae around symbiotic stars (with appropriate changes of the wind parameters and initial conditions), and especially those which most resemble bipolar PNe (e.g. He 2-104, R Aqr, BI Cru). Very likely, however, more ingredients (such as the action of magnetic fields, collimation by accretion discs, or the formation of non-azimuthally symmetric mass distribution due to the binary orbital motions) will have to be included to account for all the complex phenomenology observed in the outflows from symbiotic stars.

At variance with the D-type symbiotics, only 4 S-type systems out of 35 have an optically resolved nebula. For one of them (H 2-2), however, the detection needs further confirmation. Another one, HBV 475, has a small ([FORMULA]500 a.u.) high-excitation nebula, mainly distributed along the orbital plane (Schild & Schmid 1997). The nebula around CH Cyg (Corradi & Schwarz 1997) is also relatively small ([FORMULA]5000 a.u.) and is probably the result of the recent outburst phases. Finally, AG Peg has a complex multiple shell radio nebula (Kenny et al. 1991), which is partially detected in the optical (Fuensalida et al. 1988), which is ascribed to the interactions between slow and fast winds preceding and following its major outburst of 1850. We conclude that ionized, PN-like large nebulae are rare around S-types. Probably the main reason is that the mass loss from their cool giants (the main ingredient of the nebulae) is two orders of magnitude smaller than in D-types (Whitelock 1987, Kenyon et al. 1988). The scarcity of observable circumstellar material around S-type symbiotic stars has also implications on the suggested identification of these objects with the precursors of supernovae of type Ia (Munari & Renzini 1992). One of the key points here is the ability of the white dwarf companions to accrete enough material to reach the Chandrasekhar limit and explode by carbon deflagration. The higher the efficiency of mass accretion, the lower is the amount of material which is lost from the system and which might show up as a ionized circumstellar nebula (cf. Boffi & Branch 1995). Thus the fact that these nebulae are not observed, while it does not prove that the accretion rate is high enough, it certainly does not contradict the hypothesis that S-type symbiotics can evolve to the SN Ia phase.

As for yellow symbiotics, 2 out of 9 have an extended nebula. In this case, the argument as used for the nebulae around D-types does not hold, and the observed nebulae could be the AGB-remnant of the hot white dwarfs of these systems. If so, the hot components would be very young post-AGB stars surrounded by quite massive nebulae. This is consistent with the large size and kinematical age of V417 Cen (Corradi & Schwarz 1997).

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Online publication: March 1, 1999