A SEARCH FOR THE HE-I LAMBDA-5876 RECOMBINATION LINE FROM THE DIFFUSEINTERSTELLAR-MEDIUM

The He I lambda5876/Halpha line intensity ratio in the warm ionized co mponent of the interstellar medium is significantly less than that obs erved in traditional H II regions surrounding O stars. The observation s, carried out in two directions at low Galactic latitude, reveal that the helium is primarily neutral, n(He II)/n(He(total)) less-than-or-s imilar-to 0.27, within warm, low-density regions in which the hydrogen is primarily ionized. This implies that the spectrum of the diffuse i nterstellar radiation field that ionizes the hydrogen is softer than t hat from the Galactic O star population in the solar neighborhood. Spe cifically, the ratio of helium ionizing to hydrogen ionizing photons Q (He0)/Q(H0) less-than-or-similar-to 0.030, which corresponds to a spec tral type of O8 or later and, if representative of the interstellar ba ckground, challenges the O star ionization models.