Dust cloud formation in stellar environments

II. Two-dimensional models for structure formation around AGB stars

¹ Sterrewacht Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands e-mail: woitke@strw.leidenuniv.nl
² Zentrum für Astronomie und Astrophysik, TU Berlin, Hardenbergstrase 36, 10623 Berlin, Germany
³ Observatoire de la Côte d'Azur, Département Fresnel UMR 6528, BP 4229, 06034 Nice Cedex 4, France

Received: 9 February 2004
Accepted: 16 November 2004

Abstract

This paper reports on computational evidence for the formation of cloud-like dust structures around C-rich AGB stars. This spatio-temporal structure formation process is caused by a radiative/thermal instability of dust-forming gases as identified by Woitke et al. (2000, A&A, 358, 665). Our 2D (axisymmetric) models combine a time-dependent description of the dust formation process according to GailSedlmayr (1988, A&A, 206, 153) with detailed, frequency-dependent continuum radiative transfer by means of a Monte Carlo method (Niccolini et al. 2003, A&A, 399, 703) in an otherwise static medium (). These models show that the formation of dust behind already condensed regions, which shield the stellar radiation field, is strongly favoured. In the shadow of these clouds the temperature decreases by several hundred Kelvin, which triggers the subsequent formation of dust and ensures its thermal stability. Considering an initially dust-free gas with small density inhomogeneities, we find that finger-like dust structures develop which are cooler than the surroundings and point towards the centre of the radiant emission, similar to the “cometary knots” observed in planetary nebulae and star formation regions. Compared to a spherical symmetric reference model, the clumpy dust distribution has little effect on the spectral energy distribution, but dominates the optical appearance in near IR monochromatic images.