A SIMPLE-MODEL OF PHASE-CHANGING VARIATION BETWEEN BE STARS AND SHELL STARS

Phase changing variations between Be and shell stars are considered from the viewpoint of the formation of shell absorption lines in the envelopes of these stars. Typical shell stars are characterized by large optical depths of the envelopes in the Hα line τ(Hα) in a range of 2000 to 5000, whereas the envelopes of typical Be stars are optically thinner with the values of τ(Hα)≦100. This infers that the envelopes of Be stars should be fully expanded as compared to those of shell stars, so as to reduce the optical thickness. Spectral formation in shell stars shows that their envelopes are well condensed near the equatorial planes forming disks or rings. In this paper a simple model of transformation from a disk envelope of shell star to a spherical envelope of Be star is considered to show the relative volume and volume emission measures of the envelopes in both phases. The phase change variations observed in Pleione and in other Be and shell stars are discussed based on this simple consideration. Some implications of the present model in the linear polarization, IR-excess, UV spectra and the radiation field of the envelope are also discussed briefly. © 1990 Kluwer Academic Publishers.