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Astron. Astrophys. 358, 665-670 (2000)

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1. Introduction

Recent observations with high angular resolution have accumulated convincing evidence that dust forming objects like the outflows of AGB stars, novae and supernovae ejecta, the hot winds of Wolf-Rayet stars or the outer atmospheres of R Coronae Borealis stars often exhibit a clumpy circumstellar environment. Prominent examples are the irregular, cloudy structures in the inner part of the circumstellar envelope of the infrared carbon star IRC+10216 (Weigelt et al. 1998; Haniff & Buscher 1998; Tuthill et al. 2000), and the patchy SiO maser spots inside the wind acceleration zone of the oxygen-rich Mira variable TX Cam (Diamond & Kemball 1999). A summary and discussion of such observations concerning AGB and post-AGB stars can be found e.g. in Lopez (1999).

The question arises whether the observed clumpiness of dusty media is simply a manifestation of pre-existing inhomogeneities or asymmetries in the condensing outflows (which merely become visible via dust formation), or whether the dust formation process itself introduces new and significant instabilities to the medium which cause the structure formation. In this paper we investigate the second possibility.

As will be argued below, a control loop of physical interactions can be identified in dust forming gases, where the non-local coupling via radiative transfer plays a crucial role. Investigations on similar radiative/thermal instabilities have been carried out by Spiegel (1957) and Trujillo Bueno & Kneer (1990). These works have mainly considered short-time, small-scale perturbations of stellar atmospheres in the diffusion limit, in particular concerning the sun. In this paper, we are interested in possible large-scale structure formation processes in the outflows of red giants, where a new and extremely temperature-sensitive source of opacity arises (the dust) which is exposed to directed, radially diluted star light.

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© European Southern Observatory (ESO) 2000

Online publication: June 8, 2000