THE HI INFRARED LINE SPECTRUM FOR BE STARS WITH LOW-DENSITY DISCS

We present theoretical H alpha and HI infrared recombination line calc ulations for low-density discs around B stars. Such a disc shows no vi sible emission in H alpha, while the HI IR recombination lines are in emission. This phenomenon has been found in the spectrum of the B0.2V star, tau Sco and could be simulated with a simple disc model. As an e xtension of that particular case we calculate the entire IR HI line sp ectrum of a normal B star surrounded by a low-density disc with a theo retical curve of growth for HI IR line fluxes, which we introduce as a tool for studying low-density discs. We find that IR emission lines m ay be detectable for densities up to about 10(-14) gcm(-3) which is a factor 10(2) - 10(3) lower than typically found in Be stars. For diffe rent spectral types, B0, B2, B5 and B8 we determined the density range for which emission is prominent in the IR recombination lines but not in Ha alpha.