Dust cloud formation in stellar environments. II. Two-dimensional models for structure formation around AGB stars

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). Our 2D (axisymmetric) models combine a time-dependent description of the dust formation process according to Gail & Sedlmayr (1988) with detailed, frequency-dependent continuum radiative transfer by means of a Monte Carlo method (Niccolini et al.2003) in an otherwise static medium (v=0). 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.

16 pages, 8 figures, submitted to A&A

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• The trigger of the AGB superwind: the importance of carbon
• Tracing the tidal streams of the Sagittarius dSph, and halo Milky Way features, with carbon-rich long-period variables
• High-resolution spectroscopy of the R Coronae Borealis Star V Coronae Australis
• The Effects of Alfven Waves and Radiation Pressure in Dusty Winds of Late-Type Stars. II. Dust-Cyclotron Damping
• On the magnetic structure and wind parameter profiles of Alfven wave driven winds in late-type supergiant stars
• Evidence of a Binary-Induced Spiral from an Incomplete Ring Pattern of CIT 6
• Detection of an asymmetry in the envelope of the carbon Mira R Fornacis using VLTI/MIDI
• The Magnetised Bellows of Betelgeuse
• Mid-Infrared interferometry of dust around massive evolved stars
• A hybrid steady-state magnetohydrodynamic dust-driven stellar wind model for AGB stars
• The Keck Aperture Masking Experiment: Dust Enshrouded Red Giants