HYDRODYNAMICAL MODELS AND SYNTHETIC SPECTRA OF CIRCUMSTELLAR DUST SHELLS AROUND AGB STARS .1. STATIONARY SOLUTIONS

We present a sample of hydrodynamical steady state models of circumste llar gas/dust shells around late type giants together with computed sp ectral energy distributions (SEDs). In these models, the stellar wind is driven by radiation pressure on dust grains and subsequent momentum transfer to the gas molecules via collisions. Given the fundamental s tellar parameters (M, L*, T-eff), the mass loss rate ((M) over dot), and the dust properties, a self-consistent physical model of the circu mstellar gas/dust shell is obtained from the numerical solution of the coupled equations of hydrodynamics and radiative transfer. The comput ed outflow velocities and infrared fluxes of the circumstellar envelop es can be compared directly with the observed properties of stars on a symptotic giant branch. Plotting the positions of our steady state mod els in different IRAS two-color-diagrams, we confirm that, for fixed d ust properties, all models fall on a simple color-color relation with (M) over dot (or optical depth) as the only parameter. Surprisingly, w e find a good agreement between the synthetic spectra resulting from t he self-consistent hydrodynamical approach and those obtained from muc h simpler models based on a constant outflow velocity and ignoring dri ft of dust relative to the gas. Our models are compared with the resul ts of similar calculations by Netzer & Elitzur (1993). We find signifi cant differences which are probably the result of some unrealistic app roximations in the treatment of radiative transfer underlying the mode l calculations of Netzer & Elitzur. Moreover, our results demonstrate that, in general, gas pressure cannot be neglected for winds with rela tively low expansion velocities (u(e) < 30 km/s). For given stellar pa rameters and dust properties, the theoretical minimum (maximum) mass l oss rate decreases (increases) significantly when gas pressure is take n into account.