The Be star phenomena I. General properties

Abstract

General properties of Be stars including their variabilities are considered. The Be stars are first defined as non-supergiant B stars with emission-lines including non-emission shell stars. The percentage fraction of Be/B is deduced based on a homogeneous sample of Be and shell stars and a rather high fraction is found over the whole spectral range with a peak at B2. A statistical study of rotational velocities is made with the aid of Uesugi & Fukuda'a catalogue transformed to the new Slettebak system. It is pointed out that the rotation properties of Be stars earlier than B2 are different from those of Be stars later that B3.

Statistics of Algol binaires are also examined in connection with the Be star phenomena. Although the HAlpha-emission survey is very incomplete at present, recent observations suggest a high fraction of Be and Ae stars among Algols, particularly for stars of longer orbital periods. Statistical grouping of Be binaries into short and long orbital periods is also suggested.

Hot plasma phenomena of Be stars appear in x-ray region, superionization in the UV spectral region, and in the mass-loss processes. A brief review is given with an attempt to find some properties depending on the spectral sequence or distinguished from normal OB stars. Mass loss rates from Be stars show no clear correlation with spectral sequence and rotational velocities.

Finally, a short review is given of the variabilities of Be stars, theor general properties and some typical examples of the long-term variations (>1 yr). The timescales of long-term variations are shorter in early-type Be stars thatn in later-type Be stars. Some characteristics features of E/C, V/R and brightness variations are also summarized.