The Chemical Evolution of Helium

We report on measurements of the He abundance toward the outer Galaxy H II region S 206 with the NRAO Green Bank telescope. Observations of hydrogen and helium radio recombination lines between 8 − 10GHz were made toward the peak radio continuum position in S 206. We derive He/H = 0.08459 ± 0.00088 (random)± 0.0010 (known systematic), 20% lower than optical recombination line results. It is difficult to reconcile the large discrepancy between the optical and radio values even when accounting for temperature, density, and ionization structure or for optical extinction by dust. Using only M17 and S 206 we determine ∆Y/∆Z = 1.41± 0.62 in the Galaxy, consistent with standard chemical evolution models. High helium abundances in the old stellar population of elliptical galaxies can help explain the increase in UV emission with shorter wavelength between 2000 and 1200 Å, called the UV-upturn or UVX. Our lower values of ∆Y/∆Z are consistent with a normal helium abundance at higher metallicity and suggest that other factors, such as a variable red giant branch mass-loss with metallicity, may be important. When combined with He abundances in metal poor galaxy H II regions, Magellanic cloud H II regions, and M17 that have been determined from optical recombination lines including the effects of temperature fluctuations, our radio He/H abundance ratio for S 206 is consistent with a helium evolution of ∆Y/∆Z = 1.6. A linear extrapolation to zero metallicity predicts a He/H primordial abundance ratio about 5% lower than that given by the Wilkinson Microwave Anisotropy Probe and standard Big Bang nucleosynthesis. The measured He abundances may be systematically underestimated by a few percent if clumping exists in these H II regions. Subject headings: ISM: abundances — HII regions: general — radio lines: ISM