Astron. Astrophys. 357, 572-580 (2000)

1. Introduction

Wolf-Rayet (WR) stars represent a late stage in the evolution of massive stars before the supernova explosion episode. WR stars are characterised by high effective temperatures (K), fast radiatively accelerated winds ( km s^-1), high mass loss rates ( yr^-1) which expel almost half of the initial ZAMS stellar masses to the surrounding medium (van der Hucht 1998). The winds of these objects are also the site of strong emission lines arising from excited atoms and ions and continuum free-free emission at infrared (IR) and radio wavelengths. WR stars experience three phases of evolution corresponding to different emission spectra: the WN phase is characterised by hydrogen burning in the core via the CNO cycle, and enhancement of helium and nitrogen as a result. The hydrogen abundance reflects the completeness of this process. The more evolved WC phase corresponds to helium burning in the core via the triple-alpha process accompanied by weak s-processes. WC stars are rich in helium and carbon, have varying amounts of oxygen and are deficient in hydrogen and nitrogen. Those with the most oxygen, whose spectra show conspicuous O VI emission lines, are classified WO stars. Therefore, the elemental composition at the stellar surface resulting from the various nuclear burning stages governs the wind chemistry and the type of dust formed in the outflow. As WC stars have no hydrogen left in their atmosphere, the wind elemental composition is dominated by helium, carbon and oxygen, mainly in their ionic forms due to the strong stellar radiation field.

Dust was first observed in WC stars of spectral type WC9 by Allen et al. (1972). Later photometric studies in the infrared by Williams et al. (1987, hereafter WHT) have shown that half of the WC8 and most of the WC9 stars were condensing dust in their wind steadily or episodically. The objects for which the dust was always observed had to be forming fresh grains on a continuous basis because dust particles were momentum-coupled to the fast stellar wind and thus being continuously carried away from the stars. WHT modelled the IR spectral energy distributions of the dust-making WC stars, finding that these could be well fitted with shells of amorphous carbon (AC) grains in radiative equilibrium with the stellar radiation field. Other condensates have also been considered with less success in reproducing the near-IR stellar spectra (Dyck et al. 1984). New observations with the Short Wavelength Spectrometer (SWS) on board the ISO satellite were conducted for various WC stars by van der Hucht et al. (1996) and full spectral energy distributions in the IR were obtained for these objects. Again, models by Williams et al. (1997) can reproduce well the ISO data using homogeneous shells of AC dust grains.

In addition to the IR emission, dust formation in WC stars has manisfested itself by eclipse-like variations recently attributed to episodic obscuration of starlight by blobs of dust in the line-of-sight very close to the star (Veen et al. 1998). These variations appear to be similar to those of the R Coronae Borealis (R CrB) stars but with smaller amplitudes and the mechanism triggering blob formation is still unknown. Comparison of the spectra of the archetypical dust-forming WC9 star WR104 (Ve2-45) during such an episode and under normal conditions showed that the dust was forming rather close to the star (Crowther 1997) and within the inner dust shells derived by WHT and Williams et al. (1997).

Silicon carbide clusters are responsible for the emission/absorption band at 11.3 µm in carbon-rich AGB stars. Such a feature is not detected in WC stars although silicon is present in the stellar wind as proved by the detection of ionic silicon forbidden IR lines in emission in the line forming region (Williams et al. 1997). The absence of SiC signature is puzzling and may hint at the fact that the chemical processes responsible for the formation of SiC in late-type stars may be hindered in the hydrogen-deficient WC wind.

In this paper, we extend the previous study of Cherchneff & Tielens (1995) and describe the first steps of dust nucleation in hydrogen-deficient environments using a chemical kinetic approach. Such a study has a more general relevance and could be applied as well to the He-burning zones of supernova ejecta and to R CrB stars. We do not attempt to construct a wind model for a WC star but we prefer testing the chemistry describing the formation of small carbon clusters and silicon-bearing species for the physical conditions encountered at the inner radius of the dust shells used in the models of WHT and Williams et al. (1997). In Sect. 2, we discuss the nature of carbon dust condensing in a hydrogen-poor environment and the potential precursors to the nucleation of silicon carbides. The chemical and stellar models are described in Sect. 3. Finally, results are presented in Sect. 4 and we discuss the possible wind geometries leading to dust nucleation and condensation and related issues in Sect. 5.

© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000
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