1994A&A...290..609D

The near-IR excess emission of 144 Be stars is derived from visual and near-IR observations. The quasi-simultaneous nature of the observations provide colour excesses that are independent of temporal variations. Colour-colour diagrams are used to identify stars with excess colours markedly different from the bulk of the sample stars. The near-IR emission of four stars that have markedly different colours is attributed to the presence of a binary companion or thermal dust emission. The percentage of stars with a significant excess increases with wavelength. The excess emission increases with wavelength and the largest excesses occur in stars of earlier spectral type. The near-IR excess colours are examined and compared to theoretical excess colours calculated from a simple bremsstrahlung emitting disc model with a radial density distribution of the form ρ∝r^–β. The effect of model parameters on the excess colours is discussed. The observed excesses for the bulk of the stars are well fit by circumstellar discs with radii greater than ∼10R_* and with a density index β, in the range 2.0-5.0. This is very similar to the range of values previously determined by Waters et al. from IRAS far-IR observations. A small number of stars cannot be reconciled with discs with a constant density index out to 10R_*. It is argued that the circumstellar plasma around these stars has a change in structure at ∼2-10R_*. The possibilities of disc truncation or a change in the density index as the cause of the structure change are discussed.