Interpretation of anomalous helium abundance derived from radio recombination line observations of nebulae

Abstract We solve the ionization and thermal balance problem for homogeneous and inhomogeneous models of gaseous nebulae. The relative intensities of helium to hydrogen radio recombination lines (RRL) are calculated for these models at various positions and widths of the antenna beam. A class of inhomogeneous models of nebulae is found in which, due to the hardening of the spectrum in the periphery of the H II–He II region, the ratio of the integral intensities of the He and H RRLs, y +, can reach relatively high values near the nebula boundary (y + ≥ 20% in spite of the relative He abundance y o = 8%). This maximum observed value of y + decreases with increasing width of the antenna beam. New calculations of the interstellar extinction curve in the wavelength range 100–3500A are employed. Contrary to previous studies, we conclude that the presence of dust can enhance the effect of hardening.