Infrared and H-alpha emission from Be stars

Abstract

It was noted earlier by several authors that the infrared excess Hα emission observed from Be stars are not compatible with the idea that both emissions occur from these same optically thin ionised region. We show here that the emissions can be reconciled, if self –absorption of the Hα emission in the ionised region, which is optically thick for Hα emission, is considered. We further show by using the observed Paschen line intensity, for which the ionised region is optically thin, that the infrared excess and the hydrogen line emissions do indeed come from the same ionised region formed around the Be star. We show that the observed infrared emission requires Lyman continuum flux much larger than given by the present model atmosphere calculations. We discuss two possibilities: (i) Be star emit Lyman continuum fluxes more copiously than given model atmosphere calculation (ii) A binary compact companion to the Be star exists, which supplies the necessary ionizing photons. Recent EUVE satellite observations indicate that the first possibility may be correct.