1.4 The Infrared Excess of Be stars

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At infrared (IR) wavelengths the continuum of a Be star begins to be supplemented, then dominated, by emission interpreted as free-free and free-bound emission. An excess of IR radiation, relative to the normal stellar photosphere is observed (see Dachs and Wamsteker, 1982; Gehrz et al., 1974; Waters, 1986a, and also Figure 1.5).

The first detection of the IR excess was by Johnson (1966); Johnson (1967) concluded that this excess was due to IR emission from circumstellar material. Woolf et al. (1970) and Allen (1973) both formed the same conclusion from observations out to 10m and near-IR respectively, that the IR-excess could be generated from the same ionised circumstellar material that causes optical emission lines. Neither was able to discount the possibility that the excess was due to thermal dust emission. To conclusively distinguish between dust emission and free-free emission, near-IR (5-20m ) observations of a large sample of Be stars were carried out by Gehrz et al. (1974) who was then able to conclude that free-free radiation in a (T_disc10⁴K) disc-like circumstellar plasma is the most probable mechanism for producing infrared excesses.

This free-free and free-bound emission is indicative of an extended (typical radius of ~few × 1012cm ~ 10 R* ) dense (typical electron density 1 × 1011 cm-3 to 1 × 1012 cm-3; Waters, 1986b), ionized emitting region, and it is, currently, widely accepted that this is the case.

The number of Be stars seems to peak at ~B2 type stars (see e.g.,  Waters, 1986a, and Chapter 7), although no explanation is evident as to why this should be so. In addition the infrared wavelength region also contains many spectral lines of hydrogen, helium and other elements that can be used to obtain information concerning temperature, density and velocity structure of the disc (Waters et al., 2000).

The InfraRed Astronomical Satellite (IRAS) surveyed almost the entire sky at wavelengths of 12, 25, 60 and 100m providing a homogeneous set of observations of early-type stars (Cote and Waters, 1987). IRAS observations of Be stars are consistent with the circumstellar material being in a disc formation around the central star with the disc having a constant opening angle (see Waters, 1986b).

A continuum emission spectrum can be represented by a power law of the form F , over some frequency range, (where the slope is parameterised by , known as the “spectral index”). Studies of IRAS data found the spectral index of Be stars to be 0.6 - 1. Notably Wright and Barlow (1975) predict, from mass loss models, that for a uniform, spherically symmetric mass loss = 0.6.