Primordial Helium-3 Redux: The Helium Isotope Ratio of the Orion Nebula* \n \n
Ryan Cooke, P. Noterdaeme, James W. Johnson, Max Pettini, Louise Welsh, Céline Péroux, M. T. Murphy, David H. Weinberg
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.