Primordial Helium-3 Redux: The Helium Isotope Ratio of the Orion Nebula*

Cooke, Ryan J.; Noterdaeme, Pasquier; Johnson, James W.; Pettini, Max; Welsh, Louise; Peroux, Celine; Murphy, Michael T.; Weinberg, David H.

Abstract

We report the first direct measurement of the helium isotope ratio, 3He/4He, outside of the Local Interstellar Cloud, as part of science-verification observations with the upgraded CRyogenic InfraRed Echelle Spectrograph. Our determination of 3He/4He is based on metastable He i* absorption along the line of sight toward Θ2A Ori in the Orion Nebula. We measure a value 3He/4He = (1.77 ± 0.13) × 10−4, which is just ∼40% above the primordial relative abundance of these isotopes, assuming the Standard Model of particle physics and cosmology, (3He/4He)p = (1.257 ± 0.017) × 10−4. We calculate a suite of galactic chemical evolution simulations to study the Galactic build up of these isotopes, using the yields from Limongi & Chieffi for stars in the mass range M = 8–100 M⊙ and Lagarde et al. for M = 0.8–8 M⊙. We find that these simulations simultaneously reproduce the Orion and protosolar 3He/4He values if the calculations are initialized with a primordial ratio ${\left({}^{3}\mathrm{He}{/}^{4}\mathrm{He}\right)}_{{\rm{p}}}=(1.043\pm 0.089)\times {10}^{-4}$. Even though the quoted error does not include the model uncertainty, this determination agrees with the Standard Model value to within ∼2σ. We also use the present-day Galactic abundance of deuterium (D/H), helium (He/H), and 3He/4He to infer an empirical limit on the primordial 3He abundance, ${\left({}^{3}\mathrm{He}/{\rm{H}}\right)}_{{\rm{p}}}\leqslant (1.09\pm 0.18)\times {10}^{-5}$, which also agrees with the Standard Model value. We point out that it is becoming increasingly difficult to explain the discrepant primordial 7Li/H abundance with nonstandard physics, without breaking the remarkable simultaneous agreement of three primordial element ratios (D/H, 4He/H, and 3He/4He) with the Standard Model values.

Citation

Cooke, R. J., Noterdaeme, P., Johnson, J. W., Pettini, M., Welsh, L., Peroux, C., Murphy, M. T., & Weinberg, D. H. (2022). Primordial Helium-3 Redux: The Helium Isotope Ratio of the Orion Nebula*. Astrophysical Journal, 932(1), Article 60. https://doi.org/10.3847/1538-4357/ac6503

• Journal Article Type: Article
• Acceptance Date: Apr 5, 2022
• Online Publication Date: Jun 15, 2022
• Publication Date: Jun 10, 2022
• Deposit Date: Jul 12, 2022
• Publicly Available Date: Jul 12, 2022
• Journal: Astrophysical Journal
• Print ISSN: 0004-637X
• Electronic ISSN: 1538-4357
• Publisher: American Astronomical Society
• Peer Reviewed: Peer Reviewed
• Volume: 932
• Issue: 1
• Article Number: 60
• DOI: https://doi.org/10.3847/1538-4357/ac6503
• Public URL: https://durham-repository.worktribe.com/output/1198753