Survey of ionized helium in galactic H II regions

We have made high-frequency (10.5 and 14.7 GHz) recombination-line observations of the helium-to-hydrogen ratio, γ, in Sgr B2 and in seven other bright galactic H II regions. Measurements made one-half beamwidth off the peak of Sgr B2 show that the zone of helium ionization is extended. An examination of all data on this nebula shows no evidence for a dependence of γ on telescope beamwidth. However, the apparent helium abundance in Sgr B2, and in several other nebulae, appears to vary monotonically with the inverse of the principal quantum number of the observed transition: to first order γproportionaln-1. Nebulae that show a varying apparent helium abundance also show a variation, in the same sense, of the ratio of helium to hydrogen line widths: the ratio Δv(He)/Δv(H) decreases as n increases. Such correlations cannot result from any plausible nebular ionization structure, so it is likely that the observed helium ''anomalies'' arise in large part from difficulties in establishing a good instrumental baseline for the helium lines. Two effects, impact broadening and confusion by recombination lines from weak H II regions along the line of sight, can cause baselines to be systematically high, apparent abunances to be systematically low, and abundances to vary both with frequency and with distance from the galactic center. Sgr B2 is the extreme, but not the only, example of this phenomenon