The Chemical Evolution of Helium
  We report on measurements of the 4He abundance toward the outer Galaxy HII region S206 with the NRAO Green Bank telescope. Observations of hydrogen and helium radio recombination lines between 8-10 GHz were made toward the peak radio continuum position in S206. We derive 4He/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 S206 we determine dY/dZ = 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 Angstrom, called the UV-upturn or UVX. Our lower values of dY/dZ 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 4He abundances in metal poor galaxy HII regions, Magellanic cloud HII regions, and M17 that have been determined from optical recombination lines including the effects of temperature fluctuations, our radio 4He/H abundance ratio for S206 is consistent with a helium evolution of dY/dZ = 1.6. A linear extrapolation to zero metallicity predicts a 4He/H primordial abundance ratio about 5% lower than that given by the Wilkinson Microwave Anisotropy Probe and standard Big Bang nucleosynthesis. The measured 4He abundances may be systematically underestimated by a few percent if clumping exists in these HII regions. 
Dana S. Balser