On the baryometric status of He3
Recent observations by Bania et al. (2002) measure He3 versus oxygen in Galactic HII regions, finding that He3/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 He3 abundance, which could join D and Li7 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 (i) the intrinsically weak sensitivity of the primordial He3 abundance to the baryon density; (ii) the limited range in metallicity of the sample; (iii) the intrinsic scatter in the data; and (iv) our limited understanding of the chemical and stellar evolution of this isotope. Consequently, the He3 observations correspond to an extended range of baryon-to-photon ratio, eta = (2.2 - 6.5) x 10^{-10}, which diminishes the role of He3 as a precision baryometer. On the other hand, once the baryon-to-photon ratio is determined by the CMB, D/H, or Li7/H, the primordial value of He3/H can be inferred. Henceforth new observations of Galactic He3, can in principle greatly improve our understanding of stellar and/or chemical evolution and reconcile the observations of the HII regions and those of the planetary nebulae.