Two-fluid models for stationary dust driven winds: I. Momentum and energy balance

D. Krüger, A. Gauger, and E. Sedlmayr

Astronomy & Astrophysics, 290, 573-589 (1994)

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Abstract:

Self-consistent two-fluid models of stationary dust driven winds around cool C-stars have been calculated. The dust and the gas component are coupled by the condensation of dust from the gas phase and grain-gas collisions and are described by separate sets of equations.

It was found that in the regime of substantial mass loss where purely dust driven winds are possible the dust grains move with their equilibrium drift velocity relative to the gas. Compared to one-fluid models the dust component is dynamically diluted, leading to a decreased radiative acceleration of the wind in the outer region.

We have included detailed energy exchange rates between gas and radiation field by vibrational transitions of CO and pure rotational transitions of dipolar molecules (CO, HCN, C_2H, C_3H, CS, SiS). Our models show that frictional heating by drifting dust grains raises the gas temperature considerably above its radiative equilibrium value. This leads to significant extension of the dust condensation zone.

A sequence of models with increasing luminosities shows that the drift velocity of the dust decreases for higher luminosities. Consequently the drift related effects are more pronounced at lower luminosities.

Keywords:

Stars: mass loss - circumstellar matter - stars: AGB - stars: carbon - hydrodynamics - radiation mechanisms: miscellaneous