OBSERVATIONS OF H, HE, AND C RADIO LINES IN THE SOURCE W3A AT 8 MM - THE HELIUM PROBLEM

The abundance of ionized helium by number has been measured from the ( H, He) 56 alpha and 57 alpha radio recombination lines: y(+) = (9.7 +/ - 0.8)%. We propose a model for the ionization structure which allows for a hardening of the emission from the star in the H II region, heli um ionization through the metastable level, and the influence of dust; it shows that the value of y(+) measured from the radio recombination lines may be greater than the actual helium abundance y(0). The model yields y(0) approximate to 8.9% in W3A. It provides an explanation fo r the enhancement of y(+) of up to 40% at the edge of the source obser ved at high angular resolution (Roelfsema and Goss 1991). The electron temperature of the H II region has been measured: T-e = (7540 +/- 300 ) K. The detection of C56 alpha and C57 alpha, the first radio recombi nation lines of carbon at millimeter wavelengths, is reported.