The 8.7 GHz hyperfine line of He-3(+) in galactic H II regions

Abstract

A search for the 8.7 GHz hyperfine line of <SUP>3</SUP>He<SUP>+</SUP> in galactic H II regions has yielded detections in three regions and significant upper limits for three others. The authors derive <SUP>3</SUP>He/H abundance ratios of 4×10<SUP>-5</SUP> in W43, 1×10<SUP>-4</SUP> in W51, and 5×10<SUP>-4</SUP> in W3. The upper limits found are 6×10<SUP>-5</SUP> in Orion A, and 2×10<SUP>-5</SUP> in W49 and M17S. These abundances have been corrected (by up to a factor of 2) for source structure using simple core-halo models. For these weak lines systematic errors can be much larger than random noise, so the authors have simultaneously observed many weak recombination lines in order to estimate the magnitude of the systematic errors. The implications of these results for the general chemical evolution of the Galaxy are discussed.