Recent Estimates of the Primordial Helium Abundance

Eric Mamajek (UR)

Last updated: 15 November 2010

Summary

I've reviewed the literature since 1990 regarding estimates of the primordial He abundance (Y_p; ⁴He specifically). For the purposes of stellar nucleosynthesis calculations that require a primordial He abundance (for metal fraction Z = 0), I adopt the predicted WMAP + standard Big Bang nucleosynthesis value of Y_p = 0.2486 from Cyburt, Fields, & Olive (2008). This appears to be near the median of published observational estimates of the primordial He abundance from the past two decades.

Description

Most published estimates of the primordial He abundance are from large numbers of extragalactic low-metallicity H II regions. Published estimates since 1990 have been slowly increasing from Y_p ~ 0.23 to 0.25 in recent years, despite papers in the 1990s confidently claiming that contemporary work was deriving the number at the third decimal place (see Figure 10 of Steigman 2007). A major review on the subject (before the recent Steigman reviews) summarizing the data up until the mid 1980s was Boesgaard & Steigman (1985), who adopted a best estimate of Y_p = 0.245 ± 0.003 from Kunth & Sargent (1983). Peimbert (1996) briefly reviewed the Y_p values published before 1996 and compared them to Big Bang nucleosynthesis theory.

There seems to be agreement (see e.g. Porter, Ferland, & MacAdam 2009) that systematic uncertainties in the theoretical He I emissivities appear to dominate the error budget when using these samples, and that simply observing more extragalactic H II regions will not improve future estimates. There are also estimates of the primordial He abundance from standard Big Bang nucleosynthesis calculations, using the best available cosmological parameters. Here, a major source of systematic uncertainty is due to discrepent published estimates of the neutron lifetime (see Peimbert, Peimbert, Carigi, & Luridiana 2010), which disagree at the ~1% level. However, the predicted abundances from Big Bang nucleosynthesis are now very precise given the strong constraints that WMAP and other projects has provided for relevant cosmological parameters like the the baryon-to-photon ratio.

To two significant figures, there seems to be general consensus among the observational and predicted estimates of the primordial He abundance of Y_p = 0.25. The median of the estimates published since 1990 is Y_p = 0.246 (± 0.007 ; 68%CL), however any estimate of the uncertainty in the observationally constrained Y_p is subject to these systematic errors due to uncertainties in He I emissivities, so the uncertainty is unknown. One can, however, estimate Y_p from standard Big Bang nucleosynthesis calculations (which are hopefully in agreement with the empirically constrained primordial He abundance!). The most precise predicted estimate of the primordial He abundance using WMAP cosmological data is Y_p = 0.2486 ± 0.0002 from Cyburt, Fields, & Olive (2008). This appear to be close to the most recent results from authors that have published multiple observational estimates over the years (e.g. Peimbert, Izotov, Thuan, Skillman, Olive, etc.), as well as the predicted estimates from Steigman. However, Cyburt et al. also remark that their nucleosynthesis calculation now produces a primordial ⁷Li abundance that is severely discrepent with the local primordial value estimated from metal poor stars.

Tabulated Data

I've highlighted in red those references that are obsoleted by more recent works by the same authors, with their most recent work in light green. The table is thought to be exhaustive for the period since 1990. Due to its importance to pre-1990 work, however, we have included the Kunth & Sargent (1983) estimate that was advocated in the review by Boesgaard & Steigman (1985).

Y_p	Reference	Sample/Notes
0.225 ± 0.013	Ribas et al. 2000	eclipsing binaries
0.228 ± 0.005	Pagel et al. 1992	HII galaxies
0.234 ± 0.002	Olive, Steigman, & Skillman 1997	low-Z extragal. H II regions
0.2345 ± 0.0026	Peimbert, Peimbert, & Ruiz 2000	NGC 346 H II regions
0.2384 ± 0.0025	Peimbert, Peimbert, & Luridiana 2002	5 low-Z extragal. H II regions
0.2391 ± 0.0020	Luridiana, Peimbert, Peimbert, & Cervino 2003	5 low-Z extragal. H II regions
0.240 ± 0.006	Steigman 2007 Steigman 2010	adopted observational estimate
0.2421 ± 0.0021	Izotov & Thuan 2004	82 low-Z extragal H II regions
0.243 ± 0.006	Cassisi, Salaris, & Irwin 2003	Zocali galactic globular clusters
0.244 ± 0.002	Izotov & Thuan 1998	45 low-Z extragal. H II regions
0.244 ± 0.006	Cassisi, Salaris, & Irwin 2003	Sanquvist galactic globular clusters
0.2443 ± 0.0015	Thuan & Izotov 2002	45 extragal HII regions
0.245 ± 0.012	Villanova, Piotto, & Gratton 2009	NGC 6752 globular cluster blue HB stars
0.245 ± 0.003	Kunth & Sargent (1983) adopted by Boesgaard & Steigman 1985	12 low-Z extragal. H II regions
0.2472 ± 0.0012	Izotov, Thuan, & Lipovetsky 1997	Benjamin He I emissivities
0.2477 ± 0.0029	Peimbert, Luridiana, & Peimbert 2007 adopted by Peimbert, Peimbert, Carigi, & Luridiana 2010	extragal. H II regions
0.248 ± NA	Steigman 2007	Standard Big Bang Nucleo.
0.2482 ± 0.0007	Steigman 2010	Standard Big Bang Nucleo., constrained from primordial D abundance
0.2486 ± 0.0002	Cyburt, Fields, Olive 2008	WMAP + standard Big Bang nucleo.
0.2486 ± 0.0085	Popa & Vasile 2008	CMB: WMAP5
0.249 ± 0.009	Olive & Skillman 2004	low-Z extragal. HII regions
0.25 ± +0.10-0.07	Ichikawa, Sekiguchi, & Takahashi 2008	CMB: WMAP5+ACBAR+BOOMERANG+CBI
0.250 ± 0.004	Fukugita & Kawasaki 2006	Izotov-Thuan low-Z extragal. H II regions
0.2516 ± 0.0011	Izotov, Thuan, & Stasinska 2007	Porter HeI_emissivities
0.2561 ± 0.0108	Aver, Olive, Skillman 2010	7 "high quality" extragal. H II regions
0.2565 ± 0.005	Izotov & Thuan 2010	86 low-Z extragal H II regions
0.28 ^+0.14_-0.15	Larson et al. 2010	WMAP
0.326 ± 0.075	Jarosik et al. 2010 Komatsu et al. 2010	WMAP+SDSS bary. acous. osc. + HST Ho