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Introduction

This paper is a continuation of our series of papers on the investigation of stars which are believed to be in the post-Asymptotic Giant Branch (post-AGB) stage of evolution (Klochkova 1995; Zacs et al. 1995, 1996; Klochkova and Panchuk 1996a, 1998; Klochkova et al. 1997a; Klochkova and Mishenina 1998). The post-AGB stars (hereafter also referred to as proto-planetary nebulae - PPNe) being in the transition phase from AGB to planetary nebulae offer an opportunity to study in detail a chemical composition which has undergone changes due to nucleosynthesis and mixing processes in the course of the stars evolution. Here we present new results for the peculiar supergiant with a large infrared excess IRAS04296+3429.

On the 12/25/60 ${\mu}$m colour-colour diagram from the IRAS data infrared source IRAS04296+3429 (hereafter IRAS04296), associated with a faint carbon-rich star (Omont et al. 1993; Loup et al. 1993) classified as type G0Ia by Hrivnak et al. (1994), is located in the region occupied by planetary nebulae, non-variable OH/IR stars, and proto-planetary nebulae (Iyengar and Parthasarathy 1997). The object IRAS04296 belongs to the small group of sources which show a spectral feature around 21${\mu}$m (Kwok et al. 1989). This feature is seen only for some post-AGB objects and has not been detected either in the preceding (AGB) nor in succeeding (PN) evolution stage. Note that a search for new 21${\mu}$m emitters by means of ISO SWS observations among candidates selected by Henning et al. (1996) failed to give any detections (Henning, private communication). Using a medium-resolution (3Å) optical spectrum, Hrivnak (1995) found that IRAS04296 is a strongly reddenned (E(B-V)=1.3) G-star with features indicative of high luminosity, with molecular absorption features of ${\rm C_2}$ and rarely observed features of ${\rm C_3}$ of circumstellar origin and quite strong absorption lines of s-process elements (Ba, Sr, Y) indicating that outer layers of the atmosphere of IRAS04296 have been enriched by products of nucleosynthesis. Therefore this star is very well suited for the study of detailed chemical abundances which have been changed by the third dredge-up. Indeed, Decin et al. (1998), using high resolution spectra, obtained the chemical abundance pattern for this object and concluded that its metal-deficient, carbon-rich atmosphere has large overabundances of s-process elements.

In Sect.2 we describe our observational material for IRAS04296 and discuss its molecular features, comparing them with the corresponding spectrum of the Hale-Bopp comet. Sect.3 is devoted to presentation of the main parameters and detailed analysis of the chemical composition of IRAS04296 derived from our optical spectra. The next section presents modelling of spectral energy distribution for this source with the aim to get insight into its physical parameters (mainly to determine the stellar effective temperature which is crucial for the chemical composition estimation). Finally, in Sect.5 we discuss the results obtained and compare them to the results for related objects.