The Chemical Evolution of Helium

We report on measurements of the 4He abundance toward the outer Galaxy HIIregion S206 with the NRAO Green Bank telescope. Observations of hydrogen andhelium radio recombination lines between 8-10 GHz were made toward the peakradio continuum position in S206. We derive 4He/H = 0.08459 +/- 0.00088(random) +/- 0.0010 (known systematic), 20% lower than optical recombinationline results. It is difficult to reconcile the large discrepancy between theoptical and radio values even when accounting for temperature, density, andionization structure or for optical extinction by dust. Using only M17 and S206we determine dY/dZ = 1.41 +/- 0.62 in the Galaxy, consistent with standardchemical evolution models. High helium abundances in the old stellar populationof elliptical galaxies can help explain the increase in UV emission withshorter 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 athigher metallicity and suggest that other factors, such as a variable red giantbranch mass-loss with metallicity, may be important. When combined with 4Heabundances in metal poor galaxy HII regions, Magellanic cloud HII regions, andM17 that have been determined from optical recombination lines including theeffects of temperature fluctuations, our radio 4He/H abundance ratio for S206is consistent with a helium evolution of dY/dZ = 1.6. A linear extrapolation tozero metallicity predicts a 4He/H primordial abundance ratio about 5% lowerthan that given by the Wilkinson Microwave Anisotropy Probe and standard BigBang nucleosynthesis. The measured 4He abundances may be systematicallyunderestimated by a few percent if clumping exists in these HII regions.Comment: 19 pages, 3 figures (accepted for publication in the AJ