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

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.