Radiative Transfer in Axisymmetric Circumstellar Envelopes.
Abstract
During the late stages of stellar evolution, many stars undergo rapid mass loss. The mechanism which initiates the mass loss and the geometry of the outflow are as yet unknown. Most models of the resulting circumstellar envelopes assume a spherically symmetric outflow. However, the observational evidence increasingly points toward an axisymmetric distribution of emission (and hence mass) from these objects. To interpret the observations and to place more constraints on the theories for the mass loss, detailed models of circumstellar envelopes are needed for axisymmetric geometries. I develop models of circumstellar dust shells which possess an axisymmetric distribution of dust. The transfer equation for the continuum is solved using an iterative technique. I then use these models to determine the resulting gas velocity structure and number density distribution in the envelope by assuming that the outflow is controlled by radiation pressure acting on the dust grains. Collisional and photodissociation of H _2O in the outflow produces OH molecules. The escape probability formalism and a modified form of the Sobolev theory are used to solve the equations of statistical equilibrium for the rotational level populations of OH. Line profiles are then computed for the 1612 MHz maser transition. From the continuum models I find that onedimensional visibility measurements at 10 and 20mum may be used to distinguish between "donutlike" and "disk like" distributions of silicate dust grains. From the maser models I find that many of the characteristics of observed profiles are reproduced. I also find that a latitudinal velocity gradient plays a key role in the line transfer and may help explain some of the deficiencies of current models of circumstellar masers.
 Publication:

Ph.D. Thesis
 Pub Date:
 1991
 Bibcode:
 1991PhDT.........6C
 Keywords:

 Physics: Astronomy and Astrophysics;
 Continuum Modeling;
 Cosmic Dust;
 Radiative Transfer;
 Stellar Envelopes;
 Stellar Evolution;
 Stellar Mass;
 Stellar Models;
 Density Distribution;
 Hydroxyl Radicals;
 Photodissociation;
 Probability Theory;
 Radiation Pressure;
 Silicates;
 Symmetry;
 Water;
 Weight Reduction;
 Astrophysics