LONG-TERM CHANGES IN EMISSION-LINE AND CONTINUUM SPECTRUM OF THE BE STAR GAMMA CASSIOPEIAE - H-BETA V/R AND IR CONTINUUM FLUX VARIATIONS

Long-term changes in the optical and IR continuum energy distribution of gamma Cassiopeiae are compared to the variability in the line shape of the Hbeta emission line. New near-IR observations are presented and combined with literature data, covering a period of 25 years. The IR data show only small variations in brightness and colour, with a trend for the K magnitude to decrease gradually. This brightening is also apparent in the V band photometry. The Hbeta line profile shows the familiar V/R variations with a typical time scale of 5+/-1 year. There is no correlation between the (strong) variations in the Hbeta line profile and the strength or shape of the IR continuum. Two models are considered that can explain the observed V/R variations: the expanding/contracting spherical shell model, and the non-axisymmetric disc model. In the spherical shell model periods of expansion give rise to a steepening of the IR continuum energy distribution because the radial density gradient of the gas increases. Conversely, periods of contraction give rise to a flattening of the IR continuum. In the disc model, no changes in the slope of the IR continuum are expected because the V/R variations are not the result of changes in the radial density gradient of the circumstellar gas. The energy distribution of gamma Cas and its variability is analysed using the S2/68 spectrophotometric data to establish the photospheric flux in the UV, and by fitting a Kurucz (1979) model atmosphere to the UV observations. The predicted optical and IR fluxes of the Kurucz model are compared to the observations in order to derive the excess emission. This excess radiation is modeled under the assumption it is due to free-bound and free-free emission from circumstellar gas. From the comparison between the two models and the observations it is concluded that the expanding/contracting shell model cannot account for the constancy of the IR continuum, and that the non-axisymmetiic disc model agrees best with the observations.