The optical spectrum of extreme helium stars: A hotbed of forbidden components of neutral helium

We develop a simple criterion, based on the general properties of weakly excited forbidden components in neutral helium, to predict which forbidden components should be observable in the He I spectrum of extreme helium stars. On the basis of this criterion, we find that nearly a dozen such components should be visible. We then use the He I broadening data of Beauchamp et al. to calculate detailed synthetic spectra for effective temperatures and gravities representative of these objects. All the forbidden components predicted by our criterion are present in the synthetic spectra. Observation of these features in extreme helium stars will provide a valuable test of current theories of the Stark broadening of neutral helium lines.