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Astron. Astrophys. 332, 928-938 (1998)

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1. Introduction

The late AGB and post-AGB phases are among the least understood in current low- and intermediate-mass stellar evolution theory, mainly because of the coarse understanding of the mass-loss mechanism(s) and internal convection. The theoretical understanding of the total chemical evolution and certainly the chemical enrichment due to the 3rd dredge-up phenomena are therefore still poorly understood (e.g. Lattanzio et al. 1996).

One of the tracers of the chemical evolution of the star is the chemical content of the dusty circumstellar envelope which is formed by the high mass-loss episode(s) during the late AGB phase. The infrared spectral region is often used to infer the envelope composition since not only the bulk of radiation is emitted in the IR but also because the IR-spectra are characterized by the chemistry. The main signature of O-rich dusty environments are silicate features at 9.7 µm and 18 µm while a C-rich chemistry is characterized by the presence of the SiC emission at 10-12 µm and/or the presence of 3.3, 6.2, 7.7 and 11.3 µm features which are usually attributed to the polycyclic aromatic hydrocarbons (PAHs). The spectrographs on board of ESA's Infrared Space Observatory (ISO) are revealing now a wealth of detail concerning these 2 chemical types of circumstellar environments around evolved stars (see e.g. the ISO dedicated November 1996 issue of A&A).

Recently the detection of a strong broad emission feature at 21 µm accompanied by unusually strong 3.4-3.5 and 6-9 µm emission features in several post-AGB stars has been reported (Kwok et al. 1989; Hrivnak & Kwok 1991a, b; Kwok et al. 1995; Geballe et al. 1992; Henning et al. 1996). While the carrier of the broad feature is not yet identified (e.g. Begemann et al. 1996 and references therein), there are several observational indications that it is only present in a C-rich circumstellar chemistry: the 21 µm stars usually also display strong PAH features at lower wavelengths; Hrivnak (1995) studied low resolution spectra and revealed the presence of circumstellar C2 and C3 and also Bakker et al. (1995, 1996, 1997) detected the optical circumstellar carbon molecules around 21 µm stars; Omont et al. (1993) deduced a C-rich environment from the large HCN/CO millimetre-line ratio. Till now the 21 µm feature is only observed in post-AGB stars and young PN's and it is not clear whether the carrier is only produced during the transition from the AGB to the PN phase or already produced on the AGB but not excited.

Since the discovery of Justannont et al. (1996) that also HD 187885 may display a small 21 µm feature, all known the post-AGB stars with photospheres showing the yields of an efficient 3rd dredge-up (Van Winckel 1997 and references therein), also display the 21 µm feature in their IR spectrum. The study of the stellar photospheres of the 21 µm stars is therefore not only interesting for the study of the carrier and excitation mechanism of the 21 µm feature itself, but also in a broader context of stellar (chemical) evolution of low and intermediate mass stars.

In this paper, we report on a detailed chemical study of the stellar photospheres of two 21 µm post-AGB stars: IRAS 22223+4327 and IRAS 04296+3429. The two objects are optically faint (V = 9.6 and 14.2 respectively) with a total optical flux which is smaller than the total infrared flux. Their spectral energy distribution is double-peaked, with visible and near-infrared components due to the reddened photosphere and a far-infrared component caused by the detached dust shell (Hrivnak & Kwok 1991a). The huge IR excess may indicate a relatively high initial mass. In the IRAS colour-colour diagram, the two stars are located in the region of post-AGB stars in between the late AGB stars and young PN's (Omont et al. 1993). We used two bright massive supergiants HR 1017 ([FORMULA] Per) and HR 1865 ([FORMULA] Lep) of similar spectral type (F5Ib and F0Ib respectively) as reference objects (see Table 1).


Table 1. The atmosphere models for HR 1865, HR 1017, IRAS 22223+4327 and IRAS 04296+3429.

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© European Southern Observatory (ESO) 1998

Online publication: March 30, 1998