1999/11/25 by S. Hony, Hony, S., L. B. F. M. Waters +17
Physics and Astronomy · #Astrophysical Phenomena and Observations #Astrophysics (astro-ph) #Astrophysics and Star Formation Studies #FOS: Physical sciences #Stellar, planetary, and galactic studies #astro-ph
paper · pdf · doi:10.48550/arxiv.astro-ph/9911470
8 pages, 5 figure, accepted by A&A
arxiv created 1999/11/25 · openalex publication_date 1999/11/25 · arxiv updated 2009/12/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
We present the 2.4-45 micrometer ISO-SWS spectrum of the Be star gamma Cas (B0.5 IVe). The spectrum is characterised by a thermal continuum which can be well fit by a power-law Snu ~ nu0.99 over the entire SWS wavelength range. For an isothermal disc of ionized gas with constant opening angle, this correponds to a density gradient rho(r) ~ r^(-2.8). We report the detection of the Humphreys bound-free jump in emission at 3.4 micrometer. The size of the jump is sensitive to the electron temperature of the gas in the disc, and we find T~9000 K, i.e. much lower than the stellar effective temperature (25000-30000 K). The spectrum is dominated by numerous emission lines, mostly from HI, but also some HeI lines are detected. Several spectral features cannot be identified. The line strengths of the HI\sc 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 Rstar and temperature above that of the bulk of the disc material. Only the alpha, beta and gamma 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 disk may be rotating more rapidly than the stellar photosphere.