The Effect of Optical Depth in the Spectrum of Helium (triplets) in Nebulae

Relative intensities of emission lines in the trtplet system of neutral He are determined as a function of the optical depth of an atmosphere in lambda 3889. The atmosphere considered is one of low density, comparable to nebulae, to minimize the importance of effects due to electron collisions. The range of optical depth considered is limited so that only lines originating from the metastable 23S level show effects of optical depth. Thus THETA (l0830) < 10, since above this value the population of the 23P level becomes large enough such that the optical depth in lines arising from that level cannot be ignored. A comparison with the He observations of 5 well-known nebulae shows the effects of optical depth in 3 and probably 4 of these nebula. (auth)