Optical Emission-Line Spectra of Be Stars (Review Paper)

Abstract

AbstractRecent observational work on spectra obtained at optical and near-infrared wavelengths is reviewed for “ordinary” (non-supergiant non-peculiar) Be and Be-shell stars, with particular emphasis on comparison between high-resolution spectral measurements and current Be star models. Emission-line profiles are interpreted in terms of geometry, dimensions and dynamics of a dense (N ≃ 1011 cm-3), cool (T ≃ 104 k) line-emitting region in the circumstellar envelope, separated from the thin, hot expanding stellar wind region. Present observations support a rotating-disk model for the cool, line-emitting region. Indications pointing towards structuring of cool circumstellar envelope regions are reported. Significant spectral variations are detected both on short (less than one day) and long time scales (months to years). Typical cycle times for large-amplitude V/R variations and intervals between shell episodes depend on spectral type, increasing with decreasing effective temperature of the central stars and ranging from a few years for early-Be type stars to a few decades for late Be stars.