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Astron. Astrophys. 357, 241-254 (2000)

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4. Discussion and conclusions

Our high resolution spectra of SAO 85766 show that it corresponds, as of 1993, to an early B-type star (B1I) with numerous emission lines. The presence of nebular emission lines indicates the recent development of a low excitation nebula around the star.

The elemental abundance pattern of SAO 85766 is similar to that of other recently found hot post-AGB stars at high galactic latitudes (McCausland et al. 1992, Napiwotzki et al. 1994). The high galactic latitude, underabundance of metals, high radial velocity, detached cold circumstellar dust shell and the presence of low excitation nebula indicate that SAO 85766 is a old disk star in the post-AGB stage of evolution. The nebular abundances derived from the forbidden emission lines also lead to the same conclusions. The A5 spectral type assigned to SAO 85766 in 1940 in the HDE catalogue indicates that strong variations in the spectrum have taken place within the last 50 years as a result of rapid post-AGB evolution of the central star towards higher temperatures. Significant post-AGB mass loss may be the cause for this rapid evolution.

From the combination of the derived atmospheric parameters with the theoretical post-AGB evolutionary tracks we can estimate the mass and evolutionary status of SAO 85766. From the post-AGB evolutionary tracks in the log g - log Teff plane (Schönberner 1983, 1993) SAO 85766 should be a hot post-AGB star with a core mass of the order of 0.6 [FORMULA]. The observed abundance pattern of SAO 85766 is similar to that of Barnard 29 (Conlon et al., 1994, Moehler et al. 1998) which is a hot post-AGB star in the globular cluster M13. The underabundance of carbon in SAO 85766 and in other high latitude hot post-AGB stars is in agreement with the hypothesis pointed out by Iben (1991) that low mass stars may lose their envelopes and evolve blueward before thermal pulsing begins. However, in order for the star to have evolved so rapidly, it has to be a post-AGB star with a core mass of the order of 0.7 [FORMULA] (Blöcker 1995, Blöcker and Schönberner 1997). Arkhipova et al. (1999) also estimate the core mass to be about 0.7 [FORMULA], which translates to an initial mass of 5 [FORMULA]. Even if the core mass is 0.6 [FORMULA] (which seems to be too low in view of the rapid evolution) the initial mass would be about 2 [FORMULA]. The rapid post-AGB evolution of SAO 85766 indicates that its initial mass may be more than 2 [FORMULA] and the AGB phase of evolution was terminated only recently. According to Blöcker (1995) a post-AGB star of 0.7 [FORMULA] traverses the spectral interval F0 to B1 in about 100 years.

SAO 85766 was observed during 1972-73 by the Ultraviolet Sky Survey Telescope (S2/68) onboard the ESRO satellite TD-1. The UV fluxes of this star in 1973 are given in the Catalogue of Stellar Ultraviolet Fluxes (Thompson et al. 1978). The UV colours (m1565 - m2740) and (m1565-m2365) listed in this Catalogue indicate that in 1973 its spectral type was still A5I. The UV fluxes and colours clearly indicate that in 1973 it was not a B star. This implies that the changes observed in SAO 85766 from an A type spectrum to an early B type spectrum have occurred within the last 25 years as a result of the evolution of the central star towards higher temperatures.

Very rapid evolution has also been observed in the post-AGB star SAO 244567 (Hen 3-1357 = Stingray Nebula) (Parthasarathy et al. 1993, 1995; Bobrowsky et al. 1998). Hen 3-1357 has turned into a young planetary nebula within the last 20 years and the central star appears to be evolving rapidly into a DA white dwarf (Parthasarathy et al. 1993, 1995). SAO 85766 appears to be the second known case of rapid evolution of a hot post-AGB star. The high galactic latitude hot post-AGB stars LS II + 34 26 (Parthasarathy 1993b, 1994; García-Lario et al. 1997b) and LS IV -12 111 (Parthasarathy 1994; Conlon et al. 1993) are also IRAS sources with far-infrared colours similar to planetary nebulae and show nebular emission lines. They also show underabundance of carbon (Conlon et al. 1993; García-Lario et al. 1997b). The chemical composition SAO 85766 is very similar to that of LS IV -12 111 (Conlon et al. 1993). However, in the spectra of these two stars we do not see the permitted and forbidden emission lines due to Fe and other metals. As these stars evolve further towards higher temperatures it is possible that their spectra may show characteristics similar to those now observed in SAO 85766.

Some compact and young planetary nebulae have also been found to show permitted and forbidden emission lines of Fe. The spectrum of the compact planetary nebula HD 51585 (OY Gem) shows the so-called B[e] phenomenon. The spectrum of HD 51585 was described in detail by Jaschek et al. (1996), Arkhipova (1962) and Klutz and Swings (1977). Parthasarathy and Pottasch (1989) found it to be an IRAS source with far-IR colours similar to planetary nebulae. The spectrum of SAO 85766 appears to be very similar to that of HD 51585. Recently, Lamers et al. (1998) discussed the group of young planetary nebulae that show the B[e] phenomenon. Ciatti et al. (1974) suggested that some BQ[ ] stars must be evolving into planetary nebulae. Parthasarathy and Pottasch (1989) found several BQ[ ] and B[e] stars to show far-infrared IRAS colours similar to those of planetary nebulae. Their list included Hen 3-1357, and Hen 3-1475. Later, detailed observations of these objects confirmed that they are indeed in the young planetary nebula stage (Parthasarathy et al. 1993, 1995; Riera et al. 1995).

In the case of some BQ[ ] stars there is confusion on status between symbiotic stars and low excitation planetary nebulae (Lamers et al. 1998 and references there in). Symbiotic B[e] stars are interacting binaries with a cool giant and a hot compact object. They often show the TiO absorption bands in their spectra. There are no Tio bands in our high resolution spectra of SAO 85766. There is no observational evidence for any late-type companion and we conclude that SAO 85766 is not a symbiotic star. The far infrared (IRAS) colours of symbiotic stars are very different from that of planetary nebulae. The far infrared (IRAS) colours of SAO 85766 are similar to that of planetary nebulae and proto-planetary nebulae. The observed changes in the spectrum are the result of rapid post-AGB evolution of SAO 85766.

Imaging SAO 85766 in H[FORMULA], H[FORMULA] and in the forbidden emission lines displayed by this star with the HST WFPC2 may reveal the shape and physical properties of the incipient nebula and enable us to further understand the birth and early evolution of planetary nebulae.

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© European Southern Observatory (ESO) 2000

Online publication: May 3, 2000