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

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1. Introduction

From an analysis of IRAS data several cool and hot post-Asymptotic Giant Branch (post-AGB, hereafter) supergiants have been found (Parthasarathy and Pottasch 1986; Pottasch and Parthasarathy 1988; Parthasarathy 1993a). They seem to form an evolutionary sequence evolving from cool post-AGB stars towards hot post-AGB stars and then into planetary nebulae (Parthasarathy 1993b; Parthasarathy et al. 1993, 1995). The evolution from B-type post-AGB stars into young planetary nebulae can sometimes be rather rapid. One such example is the post-AGB star SAO 244567 (Hen 3-1357 = Stingray Nebula) (Parthasarathy et al. 1993, 1995; Bobrowsky et al. 1998) which has turned into a planetary nebula within the past 20 years. SAO 85766 seems to be another case of rapid post-AGB evolution.

SAO 85766 (HDE 341617 = BD +24o 3337 = IRAS 18062+2410) is an IRAS source with far-infrared colours similar to planetary nebulae. It is a high galactic latitude star (l=51o; b=+20o). Based on the IRAS colours and high galactic latitude it was first classified as a post-AGB star (Volk and Kwok 1989; Parthasarathy 1993a. More recently it has been found to be a transition object from the AGB to the PN phase (García-Lario et al. 1997a; Arkhipova et al. 1999) which has significantly evolved within the past 50 years. According to the HDE catalogue its spectral type in 1940 was A5 indicating an effective temperature of 8500 K. The spectral energy distribution obtained in 1985-1987 indicates, however, that it was a Be star less than 50 years later (Downes and Keyes, 1988). The red-sensitive objective prism spectra taken in 1978-1984 shows H[FORMULA] emission (Stephenson, 1986) and its visual magnitude was estimated to be 11.4 while the BD and SAO catalogues give magnitudes of 8.8 and 9.6 respectively. In this paper we present an analysis of the high resolution spectrum of SAO 85766 and its low excitation nebula, as of 1993.

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© European Southern Observatory (ESO) 2000

Online publication: May 3, 2000