¹ Astronomical Institute Anton Pannekoek, University of Amsterdam, Kruislaan 403, NL-1098 SJ Amsterdam, The Netherlands
² Instituut voor Sterrenkunde, K.U. Leuven, Celestijnenlaan 200B, 3001 B-Heverlee, Belgium
³ Department of Physics & Astronomy, University of Western Ontario, London, Ontario N6A 3K7, Canada
⁴ Integral Science Operations, Astrophysics Division, Space Science Department of ESA, ESTEC, P.O. Box 299, NL-2200 AG Noordwijk, The Netherlands
⁵ SRON Laboratory for Space Research, PO Box 800, NL-9700 AV Groningen, The Netherlands

We present the 2.4-45 m ISO-SWS spectrum of the Be star  Cas (B0.5 IVe). The spectrum is characterised by a thermal continuum which can be well fit by a power-law S  ^0.99 over the entire SWS wavelength range. For an isothermal disc of ionized gas with constant opening angle, this correponds to a density gradient (r)  r^-2.8. We report the detection of the Humphreys (6-) bound-free jump in emission at 3.4 m. 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 , and 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.

2000, A&A, 355, 187