The infrared spectrum of the Be star γ Cassiopeiae ⋆

We present the 2.4−45 μm ISO-SWS spectrum of the Be star γ Cas (B0.5 IVe). The spectrum is characterised by a thermal continuum which can be well fit by a powerlaw Sν ∝ ν0.99 over the entire SWS wavelength range. For an isothermal disc of ionized gas with constant opening angle, this correponds to a density gradient ρ(r) ∝ r−2.8. We report the detection of the Humphreys (6-∞) bound-free jump in emission at 3.4 μm. The size of the jump is sensitive to the electron temperature of the gas in the disc, and we find T ≈ 9 000 K, i.e. much lower than the stellar effective temperature (25 000-30 000 K). The spectrum is dominated by numerous emission lines, mostly from H i, but also some He i lines are detected. Several spectral features cannot be identified. The line strengths of the H i emission lines do not follow case B recombination line theory. The line strengths and widths suggest that many lines are optically thick and come from an inner, high density region with radius 3-5 R∗ and temperature above that of the bulk of the disc material. Only the α, β and γ transitions of the series lines contain a contribution from the outer regions. The level populations deviate significantly from LTE and are highly influenced by the optically thick, local (disc) continuum radiation field. The inner disc may be rotating more rapidly than the stellar photosphere.