Helium-4 is the second most abundant nuclide in the Universe after hydrogen. In post-BBN evolution gas cycling though stars has its hydrogen burned to helium, increasing the 4He abundance above its primordial value. As with deuterium, a 4He "plateau" is expected at sufficiently low metallicity. Although 4He is observed in the Sun and in Galactic HII regions, the crucial data for inferring its primordial abundance is from observations of the helium and hydrogen emission (recombination) lines from low-metallicity, extragalactic HII regions. The present inventory of such regions studied for their helium content is approaching of order 100. Thus, it is not surprising that even with modest observational errors for any individual HII region, the statistical uncertainty in the inferred primordial abundance may be quite small. In this situation, care must be taken with hitherto ignored or unaccounted for corrections and systematic errors or biases.
In Figure 7 is shown a compilation of the data used by Olive & Steigman (1995) and Olive, Skillman, & Steigman (1997), along with the independent data set obtained by Izotov, Thuan, & Lipovetsky (1997) and Izotov & Thuan (1998). To track the evolution of the 4He mass fraction, Y is plotted versus the HII region oxygen abundance. These HII regions are all metal poor, ranging from ~ 1/2 down to ~ 1/40 of solar (for a solar oxygen abundance of O/H 5 × 10-4; Allende-Prieto, Lambert, & Asplund 2001). A key feature of Figure 7 is that for sufficiently low metallicity the Y versus O/H relation approaches a 4He plateau! Since Y increases with metallicity, the relic abundance can either be bounded from above by the lowest metallicity regions, or the Y versus O/H relation may be extrapolated to zero metallicity. The extrapolation is quite small, so that whether the former or the latter approach is adopted the difference in the inferred primordial abundance is small: |Y| 0.001.
Figure 7. The 4He mass fraction, Y, inferred from observations of low-metallicity, extragalactic HII regions versus the oxygen abundance derived from the same data. (Figure courtesy of K. A. Olive.)
While the data shown in Figure 7 reveal a well-defined primordial abundance for 4He, the scale hides the very small statistical errors as well as the tension between the two groups' helium abundances. Olive & Steigman (1995) and Olive et al. (1997) find YP = 0.234 ± 0.003, but Izotov et al. (1997) and Izotov & Thuan (1998) derive YP = 0.244 ± 0.002. Although it is difficult to account for all of the difference, much of it is traceable to the different ways the two groups correct for the contribution to the emission lines from collisional excitation of neutral helium and also to Izotov and collaborators rejecting some helium emission lines a posteriori when they yield "too low" an abundance. Furthermore, for either data set, there are additional corrections for temperature, for temperature and density fluctuations, and for ionization, which when applied can change the inferred primordial 4He abundance by more than the quoted statistical errors (see, e.g., Steigman, Viegas, & Gruenwald 1997; Viegas, Gruenwald, & Steigman 2000; Gruenwald, Steigman, & Viegas 2002; Peimbert, Peimbert & Luridiana 2002; Sauer & Jedamzik 2002).
For example, Peimbert et al. (2002) recently reanalyzed the data from four of the Izotov & Thuan (1998) HII regions, employing their own HII region temperatures and accounting for temperature fluctuations. Peimbert et al. (2002) derive systematically lower helium abundances, as shown in Figure 8. From this very limited sample Peimbert et al. suggest that the Izotov & Thuan (1998) estimate for the primordial 4He mass fraction might have to be reduced by as much as ~ 0.007. Peimbert et al. go further, combining their redetermined helium abundances for these four HII regions with an accurate determination of Y in a more metal-rich HII region (Peimbert, Peimbert, & Ruiz 2000). Although these five data points are consistent with zero slope in the Y - O/H relation, leading to a primordial abundance YP = 0.240 ± 0.001, this extremely small data set is also consistent with Y 40(O/H), leading to a smaller primordial estimate of YP 0.237.
Figure 8. The Peimbert et al. (2002) reanalysis of the 4He abundance data for four of the Izotov & Thuan (1998) HII regions. The open circles are the Izotov & Thuan (1998) abundances, while the filled circles are from Peimbert et al. (2002).
It seems clear that until new data address the unresolved systematic errors afflicting the derivation of the primordial helium abundance, the true errors must be much larger than the statistical uncertainties. In an attempt to account for this, here I follow Olive, Steigman, & Walker (2000) and adopt a compromise mean value along with a larger uncertainty: YP = 0.238 ± 0.005.