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

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. *Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO program(s) 107.22U1.001, 194.C-0833.