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Astron. Astrophys. 317, 859-870 (1997)

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1. Introduction

Studies of proto-planetary nebulae (PPNe) developed rapidly after the Infrared Astronomical Satellite (IRAS) mission. PPNe are objects in transition between asymptotic giant branch (AGB) stars and planetary nebulae (PNe). They are formed during evolution along the AGB when the mass of the H-rich envelope drops to very small values (about 10-3 [FORMULA] for a core mass of 0.60 [FORMULA] - see Schönberner 1983), mainly as a result of the large-scale mass loss process (nuclear reactions are less important during the formation of PNe). PPNe are very intriguing objects because of rapid changes in the stellar temperature and, consequently, in the physical conditions inside the ejected shell and the remnant stellar atmosphere. These changes could be responsible for the excitation of the 21 µm band (Kwok et al. 1989) which seems to be observed only during the post-AGB phase of evolution (although Henning et al. 1996 selected a few young stellar objects as new candidates with a 21 µm feature). Until now only nine PPNe are known (Kwok et al. 1995) to display the 21 µm emission band (Henning et al. proposed two more PPNe candidates with 21 µm features). Four of the sources known to have the 21 µm feature in their spectra were observed with the Kuiper Airborne Observatory (KAO) and were found to show a spectacular, very broad emission feature around 30 µm (Omont 1993; Cox 1993). This feature was previously known in a number of carbon-rich objects among AGB stars and PNe.

One such source is IRAS 22272 [FORMULA] 5435 (hereafter IRAS 22272) which has one of the strongest 30 µm features among the PPNe, accounting for about 20 % of its infrared bolometric luminosity (Omont et al. 1995a, b). IRAS 22272 [FORMULA] 5435 (known also as BD [FORMULA] [FORMULA] 2787 = HD 235858 = SAO 34504) is a source which was initially classified as an O-rich star because the shape of its IRAS Low Resolution Spectrum (LRS) suggests the presence of silicate absorption features at about 9.7 and 18 µm (see e.g. Pottasch & Parthasarathy 1988). The detection of CO emission (Zuckerman et al. 1986) followed by the re-detection of CO and other carbon-bearing molecules such as HCN, CS and CN (see Lindqvist et al. 1988) did not change the classification of IRAS 22272. In his paper devoted to the 21 µm feature, Kwok et al. (1989) called attention to the extreme carbon richness of the all 21 µm sources and refers to the observations later presented in Hrivnak & Kwok (1991) where they confirmed the C-richness of IRAS 22272 by detection of strong optical absorption bands of C [FORMULA] and C [FORMULA] (see also Hrivnak 1995). Further the comparison of the LRS spectrum of IRAS 22272 with those of IRAS 07134 [FORMULA] 1005 and IRAS 23304 [FORMULA] 6147 showed that the features in the LRS spectrum are not due to silicates, although they also are not typical of carbon-rich sources. Measurements of a large ratio of the millimeter HCN/CO line intensities (Omont et al. 1993 concluded that CO(1 [FORMULA] 0)/HCN [FORMULA] 5 and/or CO(2 [FORMULA] 1)/HCN [FORMULA] 12 seem to be a good indicator of C-richness and in the case of IRAS 22272 they found CO(1 [FORMULA] 0)/HCN [FORMULA] 3.9 and CO(2 [FORMULA] 1)/HCN [FORMULA] 4.8) and the detection of infrared (IR) features at 3.3, 3.4, 6.2, 6.9, 7.7 and 11.3 µm (Buss et al. 1990; Geballe et al. 1992; Buss et al. 1993) attributed to a mixture of polycyclic aromatic hydrocarbons (PAHs) and some sort of carbon clusters reinforced the conclusion that IRAS 22272 is C-rich. Also, recent analysis of a high-resolution optical spectrum for IRAS 22272 by Zas et al. (1995) clearly indicates the star is extremely carbon-rich (they estimated C/O [FORMULA] 12).

The main aim of this paper is to discuss the properties of the dust grains which could account for the spectral energy distribution (SED) of IRAS 22272 together with the unusual emission features observed in its spectrum. The paper is organized as follows. First, we describe the computer code and the input parameters which were used to study the energy distribution in post-AGB objects (Sect. 2). Next we discuss the dust optical properties which are adopted in the present modeling of IRAS 22272: the types of dust considered include PAHs, amorphous carbon, magnesium sulfides and an empirical opacity function which accounts for the 21 and 30 µm features (Sect. 3). In the subsequent Sects. (4 and 5), we present the results of the model and discuss in detail the implications of the derived parameters. Conclusions are given in Sect. 6.

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998