Radiation from Gas Envelopes Around Be Stars

Abstract

Be stars [1] are in the mass range 5–20 M_O and have a surface temperature between 10,000 – 30,000 °K. They show large rotational velocities - between 100 and 500 km.s^-1. These stars sometimes have emission lines. These emission lines build up in intensity, persist for a while and disappear gradually. The star then becomes a B star with normal absorption lines. This phenomenon repeats in a quasi-periodic fashion with periods of a few years to a few tens of years. The emission lines are mostly Balmer lines, but sometimes also contain He I and low excitation metal lines. The appearance of the emission lines is interpreted as due to formation of a gas envelope around the Be star - the gas envelope is in the equatorial region in the form of a ring. The formation of the ring is related to the high rotation velocity - destabilisation forces [2] lead to ejection of a gas ring from the equatorial regions.