Digitized Theses

1993

Dissertation

Degree Name

Doctor of Philosophy

Abstract

In this thesis, I shall investigate the dynamics of the envelopes of Be stars in the equatorial plane. Currently there is no general agreement as to what are the most important forces controlling the dynamics in and near the equatorial plane. I expect that my research will yield a significant improvement in the knowledge of the forces which are most important in controlling the dynamics of the circumstellar matter, and thus lead to a major advance in the understanding of Be stars. Recently, observations of the continuous energy distribution of several Be stars from infrared to radio wavelengths have been analyzed using simple models in order to determine both the density distribution and the radial component of velocity in the emitting matter. Earlier studies of spectral line profiles from Be stars also gave information about the velocity field. Both types of investigation have provided essential data for my study of the dynamics. In my approach I invert the equation of motion and solve for the unknown force or forces, F{dollar}\sb{lcub}\rm x{rcub}{dollar}(r), in addition to gravitation, rotation, and gas pressure gradient, required to produce a radial component of velocity having a functional form consistent with the data mentioned above. All of my investigations indicate that F{dollar}\sb{lcub}\rm x{rcub}{dollar}(r) has a similar characteristic shape. Beginning at the surface of the star, F{dollar}\sb{lcub}\rm x{rcub}{dollar}(r) initially decreases with increasing r, but less rapidly than does gravity, reaches a minimum at 10-100 stellar radii, and then increases again. It may continue to increase or reach a maximum and then decrease again, depending upon the detailed form of the velocity distribution used. This minimum in F{dollar}\sb{lcub}\rm x{rcub}{dollar}(r) and the subsequent increase beyond the minimum suggest either a change in the strength of F{dollar}\sb{lcub}\rm x{rcub}{dollar}, due perhaps to varying physical properties, and/or some other physical effect becoming important.;For a small sample of 3 Be stars and 3 Be-shell stars, I have noted that the minimum of F{dollar}\sb{lcub}x{rcub}{dollar}(r) for the Be stars is closer to the stellar surface than is that for the 3 Be-shell stars. This distinction may reflect the small sample size, or it may be a real physical effect, indicating that the denser part of the envelopes of Be-shell stars are more extensive than those of Be stars.;I next investigated how a weak radial magnetic field influences the dynamics. My investigation has shown that in a slowly expanding envelope, a weak magnetic field can influence the rotational velocity distribution quite effectively so that the centrifugal force is enhanced. For some cases the centrifugal force is so large that it can balance gravity.;I then considered another promising driving mechanism for Be star envelopes, namely, the radiation force due to the optically thin lines. I introduced two parameters, {dollar}\epsilon{dollar} and {dollar}\eta{dollar}, to describe this force, and constructed a weak line, radiation driven wind model. My calculations have shown that the model can also produce a slowly expanding wind with a terminal velocity of order of 60 km/s. When I used the velocity distribution and PM model to calculate the H{dollar}\alpha{dollar} profile, I obtained a reasonably good profile.

COinS