On the baryometric status of ³He

Recent observations by Bania et al. measure ³He versus oxygen in Galactic H II regions, finding that ³He/H is within a factor of 2 of the solar abundance for [O/H] ≳ - 0.6. These results are consistent with a flat behavior in this metallicity range, tempting one to deduce from these observations a primordial value for the ³He abundance, which could join D and ⁷Li as an indicator of the cosmic baryon density. However, using the same data, we show that it is not possible to obtain a strong constraint on the baryon density range. This is due to (1) the intrinsically weak sensitivity of the primordial ³He abudance to the baryon density, (2) the limited range in metallicity of the sample, (3) the intrinsic scatter in the data, and (4) our limited understanding of the chemical and stellar evolution of this isotope. Consequently, the ³He observations correspond to an extended range of baryon-to-photon ratio, η = (2.2-6.5) × 10^-10, which diminishes the role of ³He as a precision baryometer. On the other hand, once the baryon-to-photon ratio is determined by the cosmic microwave background, D/H, or ⁷Li/H, the primordial value of ³He/H can be inferred. Henceforth, new observations of Galactic ³He can in principle greatly improve our understanding of stellar and/or chemical evolution and reconcile the observations of the H II regions and those of the planetary nebulae.