The infrared spectrum of the Be star gamma Cassiopeiae

We present the 2.4-45 mu m ISO-SWS spectrum of the Be star gamma Cas (B0.5 IVe). The spectrum is characterised by a thermal continuum which can be wel l fit by a power-law S-v proportional to v(0.99) over the entire SWS wavele ngth range. For an isothermal disc of ionized gas with constant opening ang le, this correponds to a density gradient rho(r) proportional to r(-2.8) .W e reportthe detection of the Humphreys (6-infinity) bound-free jump in emis sion at 3.4 mu m The size of the jump is sensitive to the electron temperat ure of the gas in the disc, and we find T approximate to 9000 K, i.e. much lower than the stellar effective temperature (25000-30000 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 l ine strengths of the H r emission lines do not follow case B recombination line theory. The line strengths and widths suggest that many lines are opti cally 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 alph a, beta and gamma transitions of the series lines contain a contribution fr om the outer regions. The level populations de viate significantly from LTE and are highly influenced by the optically thick, local (disc) continuum r adiation field. The inner disc may be rotating more rapidly than the stella r photosphere.