Hydrodynamical models and synthetic spectra of circumstellar dust shells around AGB stars ^*

I. Stationary solutions

¹ Astrophysikalisches Institut Potsdam, D-14473 Potsdam, Germany e-mail: MSteffen@aip.de
² Institut für Astronomie und Astrophysik der Universität Kiel, D-24098 Kiel, Germany
³ Nicolaus Copernicus Astronomical Center, PL-87-100 Torun, Poland
⁴ Department of Physics and Astronomy, University of Calgary, Calgary, AB T2N 1N4, Canada
⁵ Max-Planck-Gesellschaft, AG “Dust in star forming regions”, D-07745 Jena, Germany e-mail: sascha@georg.astro.uni-jena.de
⁶ Astrophysikalisches Institut Potsdam, D-14473 Potsdam, Germany e-mail: Deschoenberner@aip.de

Send offprint request to: M. Steffen

Received: 9 December 1996
Accepted: 5 February 1997

Abstract

We present a sample of hydrodynamical steady state models of circumstellar gas/dust shells around late type giants together with computed spectral 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 stellar parameters (, , ), the mass loss rate (), and the dust properties, a self-consistent physical model of the circumstellar gas/dust shell is obtained from the numerical solution of the coupled equations of hydrodynamics and radiative transfer. The computed outflow velocities and infrared fluxes of the circumstellar envelopes can be compared directly with the observed properties of stars on asymptotic giant branch. Plotting the positions of our steady state models in different IRAS two-color-diagrams, we confirm that, for fixed dust properties, all models fall on a simple color-color relation with  (or optical depth) as the only parameter. Surprisingly, we find a good agreement between the synthetic spectra resulting from the self-consistent hydrodynamical approach and those obtained from much simpler models based on a constant outflow velocity and ignoring drift of dust relative to the gas. Our models are compared with the results of similar calculations by Netzer & Elitzur (1993). We find significant differences which are probably the result of some unrealistic approximations in the treatment of radiative transfer underlying the model calculations of Netzer & Elitzur. Moreover, our results demonstrate that, in general, gas pressure cannot be neglected for winds with relatively low expansion velocities ( km/s). For given stellar parameters and dust properties, the theoretical minimum (maximum) mass loss rate decreases (increases) significantly when gas pressure is taken into account.