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

We present theoretical H alpha and HI infrared recombination line calculations for low-density discs around B stars. Such a disc shows no visible 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 extension of that particular case we calculate the entire IR HI line spectrum of a normal B star surrounded by a low-density disc with a theoretical 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 may 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 different 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.