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 weak ly excited forbidden components in neutral helium, to predict which fo rbidden components should be observable in the He I spectrum of extrem e 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 broadeni ng data of Beauchamp et al. to calculate detailed synthetic spectra fo r 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 hel ium stars will provide a valuable test of current theories of the Star k broadening of neutral helium lines.