Helium-4 is the second most abundant nuclide in the universe, after hydrogen. In the post-BBN epochs the net effect of gas cycling though generations of stars is to burn hydrogen to helium, increasing the 4He abundance. As with deuterium, a 4He "plateau" is expected at low metallicity. Although 4He is observed in the Sun and in Galactic HII regions, the most relevant data for inferring its primordial abundance (the plateau value) is from observations of the helium and hydrogen recombination lines in low-metallicity, extragalactic HII regions. The present inventory of such observations is approaching of order 100. It is, therefore, 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. Especially in this situation, care must be taken with hitherto ignored or unaccounted for corrections and systematic errors.
In Figure 6 is shown a compilation of the data used by Olive & Steigman (1995) and Olive, Skillman, & Steigman (1997), along with the independent data from 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 ~ 1/30 of solar (for a solar oxygen abundance of O/H = 5 × 10-4; Allende-Prieto, Lambert, & Asplund 2001). A key feature of Figure 6 is that independent of whether there is a statistically significant non-zero slope to the Y vs. O/H relation, there is a 4He plateau! Since Y is increasing with metallicity, the relic abundance can either be bounded from above by the lowest metallicity regions, or the Y vs. O/H relation determined observationally may be extrapolated to zero metallicity (a not very large extrapolation, Y -0.001).
Figure 6. The 4He mass fraction, Y, inferred from observations of low-metallicity, extragalactic HII regions versus the oxygen abundance in those regions.
The good news is that the data reveal a well-defined primordial abundance for 4He. The bad news is that the scale of Figure 6 hides the very small statistical errors, along with a dichotomy between the OS/OSS and ITL/IT primordial helium abundance determinations (YP(OS) = 0.234 ± 0.003 versus YP(IT) = 0.244 ± 0.002). Furthermore, even if one adopts the IT/ITL data, there are corrections which should be applied which change the inferred primordial 4He abundance by more than their quoted statistical errors (see, e.g. Steigman, Viegas & Gruenwald 1997; Viegas, Gruenwald & Steigman 2000; Sauer & Jedamzik 2002, Gruenwald, Viegas & Steigman 2002 (GSV); Peimbert, Peimbert & Luridiana 2002). In recent high quality observations of a relatively metal-rich SMC HII region, Peimbert, Peimbert and Ruiz (2000; PPR) derive YSMC = 0.2405 ± 0.0018. This is already lower than the IT-inferred primordial 4He abundance. Further, when PPR extrapolate this abundance to zero-metallicity, they derive YP(PPR) = 0.2345 ± 0.0026, lending some indirect support for the lower OS/OSS value.
Recently, Peimbert, Peimbert, & Luridiana (2002; PPL) have reanalyzed the data from four of the IT HII regions. When correcting for the HII region temperatures and the temperature fluctuations, PPL derive systematically lower helium abundances as shown in Figure 7. PPL also combine their redetermined abundances for these four HII regions with the recent accurate determination of Y in the more metal-rich SMC HII region (PPR). These five data points are consistent with zero slope in the Y vs. O/H relation, leading to a primordial abundance YP = 0.240 ± 0.001. However, this very limited data set is also consistent with Y 40(O/H). In this case, the extrapolation to zero metallicity, starting at the higher SMC metallicity, leads to the considerably smaller estimate of YP 0.237.
Figure 7. The Peimbert, Peimbert, & Luridiana (2002) reanalysis of the Helium-4 abundance data for 4 of the IT HII regions. The open circles are the IT abundances, while the filled circles are from PPL.
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. For the comparisons between the predictions of SBBN and the observational data to be made in the next section, I will adopt the Olive, Steigman & Walker (2000; OSW) compromise: YP = 0.238 ± 0.005; the inflated errors are an attempt to account for the poorly-constrained systematic uncertaintiess.