Balmer Decrement and the Structure of Be-Star Envelopes

Abstract

The observed Balmer decrements D ₃₄=F(Hα)/F(Hβ)of Be stars are collected in an unified system and compared with theoretical decrements based on the non-LTE solution of the radiation field. It is shown that the Balmer decrements are closely related to the structure of stellar envelope which is characterized by the mean dilution factor, optical thickness of the envelope and spectral subclass of the star. This is in contrast to the explanation of Slettebak et al. (1992) and of Dachs et al. (1990), both of which claimed the decrement to be a function of electron density in the envelope, based on the nebular approach of the decrements.