1 Introduction

A coherent trend of evolution in the IR spectra of carbonaceous dust has emerged in recent years (see Kwok et al. 1999; Guillois et al. 1996; Papoular et al. 1996; Buss et al. 1993, 1990 and references therein). The wealth of far IR spectra collected by the ISO satellite with good resolution and accuracy seems to confirm the conjecture that much of this evolution already takes place between the post-AGB and planetary-nebular stages of stellar life (Volk et al. 2001). On the other hand, terrestrial kerogens have comparable spectra, which undergo similar changes upon ageing or heating. It is the purpose of the present work to show that the parallel between available IR spectra of stellar dust and kerogen at successive stages of evolution makes it possible to approximately infer the composition and structure of the former, using the quantitative experimental data related to the corresponding properties of the latter.

It has long been known that the dominant constituent of the organic matter in meteoritic carbonaceous chondrites is similar to kerogen, which is the essential constituent of sedimentary rocks on Earth (see Kerridge 1990, p. 10). Kerogen is a family of highly disordered macromolecular organic materials made of C, H and O, and traces of N and S (hence the designation "CHON materials"). Similar materials, dubbed "CHON particles", were also discovered in comets (see Kissel et al. 1986). On the other hand, Pendleton et al. (1994) have produced an arresting fit of the 3.4-$\mu$ band observed towards the Galactic Centre with that of the organic extract of the Murchison meteorite, suggesting that kerogen might be a model of IS dust. This possibility had already been considered by Ehrenfreund et al. (1991), who looked for similarities between the spectra of meteorites and the GC, and those of several kerogens with different degrees of hydrogenation; unexpectedly, they concluded negatively, although one of their samples came quite close to the Murchison meteorite.

Figure 1: Van Krevelen diagram (after Durand 1980, p. 122). Each arrow represents the direction of loss of a particular gas molecule in the course of evolution. A rough sketch of the main structural elements is given on the left side for a few evolutionary stages.

The idea again surfaced in a paper by Tielens et al. (1996) but was apparently abandoned in favour of hydrogenated amorphous carbon HAC in Chiar et al. (2000). However, based on mid-IR spectra of the GC region, these authors find it necessary to include in their model some oxygen impurities, of which HAC is devoid in principle. One demonstration of the critical role of oxygen can be found in Sakata et al. (1987), by comparing the IR absorption spectra of oxidized vs. non-oxidized (as deposited) QCC (quenched carbonaceous compounds), a form of hydrogenated amorphous carbons, in their Fig. 1.

One notes that, in the works cited above, the presence of oxygen in kerogens was overlooked although the properties of these materials and, hence, their IR spectra are largely dependent on this element (see below). Neither is mention made of the parallel variety (similar evolution in time?) of kerogen and CS and IS spectra. Also, the explicit comparisons between kerogen and GC spectra were limited to the C-H stretch band (3.4 $\mu$m), in which oxygen is not involved.

Whilst the carrier of the 3.4-$\mu$ feature has long been thought to be produced in dense molecular clouds in the form of refractory organic mantles over silicate cores (see Sandford et al. 1991; Greenberg 1978), this model is brought into question by the recent finding that, unlike the 9.7-$\mu$m feature, the 3.4-$\mu$m feature towards Sgr A* (IRS 7) is not polarized as expected (Adamson et al. 1999). Also, $A_{\rm v}/\tau(3.4)=150$ towards the GC, as compared with 270, on average, for local ISM sources of various extinctions (Pendleton et al. 1994). This together with the spectral similarity of interstellar dust towards the GC and circumstellar dust around the PPN CRL 618 (Lequeux & de Muizon 1990; Chiar et al. 1998) also calls for a reassessment of the origin and fate of organic dust in space.

Figure 2: Typical IR absorbance spectrum of a type II kerogen, after Robin & Rouxhet (1976). The dashed areas are used to determine the band intensities, K (see Eq. (1)). A number of partial assignments of bands are enclosed in squares; see details in Sect. 3.

Finally, although Papoular et al. (1996) broached the kerogen case in their comparison of solid-state carbonaceous models, they did not dwell on spectral analogies with galactic dust.

The present work is an attempt to fill in some of these gaps by making use of the wealth of spectral, chemical and physical data in the kerogen literature. The availability of measured integrated absorbances for very different members of this family and for all the conspicuous IR features will alleviate the well known difficulty in the way of estimating celestial abundances of particular functional groups, which is due to the fact that the specific IR cross-section of a group depends on its environment, i.e. on the particular composition and structure of the material.