On the baryometric status of 3He

Recent observations by Bania et al. measure He-3 versus oxygen in Galactic H II regions, finding that He-3/H is within a factor of 2 of the solar abundance for [O/H] greater than or similar to -0/6. These results are consistent with a at behavior in this metallicity range, tempting one to deduce from these observations a primordial value for the He-3 abundance, which could join D and Li-7 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 He-3 abundance 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 He-3 observations correspond to an extended range of baryon-to-photon ratio, eta = (2.2-6.5) x 10(-10), which diminishes the role of He-3 as a precision baryometer. On the other hand, once the baryon-to-photon ratio is determined by the cosmic microwave background, D/H, or Li-7/H, the primordial value of He-3/H can be inferred. Henceforth, new observations of Galactic He-3 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.