[Paper Review] On the Primordial Helium Abundance and the DeltaY/DeltaO Ratio

M. Peimbert, V. Luridiana+2 more|arXiv (Cornell University)|Jan 10, 2007

Atomic and Subatomic Physics Research4 citations

TL;DR

This paper reviews the determination of the primordial helium abundance $Y_p$ from hydrogen and helium recombination lines in metal-poor extragalactic H ii regions, emphasizing systematic errors—especially temperature structure and collisional excitation—and shows that adopting $T(\text{He II})$ instead of $T(\text{O III})$ reduces $Y_p$ by 0.003–0.005. With updated atomic data, the $Y_p = 0.2474 \pm 0.0028$ result from Peimbert et al. (2007) now agrees with Big Bang nucleosynthesis predictions from WMAP, resolving prior tensions and supporting standard cosmology.

ABSTRACT

We present a review on the determination of the primordial helium abundance, Yp, based on the study of hydrogen and helium recombination lines in extragalactic H II regions. We also discuss the observational determinations of the increase of helium to the increase of oxygen by mass Delta Y/Delta O, and compare them with predictions based on models of galactic chemical evolution.

Motivation & Objective

• To resolve discrepancies in primordial helium abundance ($Y_p$) determinations from extragalactic H ii regions.
• To identify and quantify the dominant systematic errors affecting $Y_p$ measurements, particularly temperature structure and collisional excitation.
• To reconcile observational $Y_p$ with predictions from Big Bang nucleosynthesis (SBBN) using updated atomic physics and the $\Delta Y/\Delta O$ ratio.
• To improve constraints on galactic chemical evolution and non-standard nucleosynthesis by reducing uncertainties in $Y_p$.

Proposed method

• The authors perform a detailed error budget analysis for $Y_p$ using a sample of metal-poor H ii regions, identifying thirteen sources of uncertainty.
• They apply the maximum likelihood method to derive $T(\text{He II})$ from He i line intensities, replacing the commonly used $T(\text{O III})$ to reduce temperature structure bias.
• They use the relation $Y_p = Y - O \cdot \Delta Y/\Delta O$ to extrapolate $Y$ values to zero metallicity, relying on observational and model-based $\Delta Y/\Delta O$ estimates.
• They incorporate updated atomic data from Anderson et al. (2000, 2002) and Porter et al. (2005) to correct for recombination coefficient and collisional excitation effects.
• They compare their $Y_p$ result with the $Y_p$ derived from the WMAP baryon-to-photon ratio under SBBN assumptions to test consistency.
• They project future $Y_p$ precision, estimating a statistical error of $\sim 0.0020$ by 2010 using brighter, slightly more metal-rich H ii regions.

Experimental results

Research questions

• RQ1 What are the dominant systematic errors in the determination of the primordial helium abundance $Y_p$ from H ii regions?
• RQ2 Why do previous $Y_p$ determinations differ significantly, and how can these discrepancies be resolved?
• RQ3 How does the choice of temperature diagnostic ($T(\text{O III})$ vs. $T(\text{He II})$) affect $Y_p$ values?
• RQ4 To what extent do updated atomic physics data resolve the tension between observed $Y_p$ and SBBN predictions?
• RQ5 Can the $\Delta Y/\Delta O$ ratio be reliably determined from observations to enable accurate $Y_p$ extrapolation?

Key findings

• The most significant source of error in $Y_p$ determination is collisional excitation of H i Balmer lines, contributing an uncertainty of $\pm 0.0015$.
• Temperature structure is the second most important error source, with $T(\text{O III})$ yielding $Y_p$ values 0.003–0.005 higher than $T(\text{He II})$ due to incorrect temperature assumptions.
• The $Y_p$ value derived by Peimbert et al. (2007) is $0.2474 \pm 0.0028$, which agrees with the SBBN prediction from WMAP ($\Omega_b h^2 = 0.02233 \pm 0.00082$) within $1\sigma$.
• The use of $T(\text{O III})$ instead of $T(\text{He II})$ introduces a systematic bias of $\sim 0.003$–$0.005$, explaining much of the prior discrepancy with WMAP.
• Updated atomic data from Anderson et al. (2000, 2002) and Porter et al. (2005) resolved the need for 'new physics' previously suggested by Luridiana et al. (2003).
• Future $Y_p$ determinations could achieve a statistical error of $\sim 0.0020$ by observing brighter, slightly more metal-rich H ii regions, improving precision.