On the Baryometric Status of 3 He

UMN-TH-2106/02 TPI-MINN-02/21 astro-ph/0207583 July 2002 Recent observations by Bania et al. (2002) measure He versus oxygen in Galactic H II regions, finding that He/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 He abundance, which could join D and Li 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 He 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 He observations correspond to an extended range of baryon-to-photon ratio, η = (2.2− 6.5) × 10, which diminishes the role of He as a precision baryometer. On the other hand, once the baryon-to-photon ratio is determined by the CMB, D/H, or Li/H, the primordial value of He/H can be inferred. Henceforth new observations of Galactic He, 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.