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

International audienceWe report the first direct measurement of the helium isotope ratio, $^{3}$He/$^{4}$He, outside of the Local Interstellar Cloud, as part of science-verification observations with the upgraded CRyogenic InfraRed Echelle Spectrograph. Our determination of $^{3}$He/$^{4}$He is based on metastable He i* absorption along the line of sight toward Θ$^{2}$A Ori in the Orion Nebula. We measure a value $^{3}$He/$^{4}$He = (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, ($^{3}$He/$^{4}$He)$_{p}$ = (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 $^{3}$He/$^{4}$He values if the calculations are initialized with a primordial ratio . 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 $^{3}$He/$^{4}$He to infer an empirical limit on the primordial $^{3}$He abundance, , which also agrees with the Standard Model value. We point out that it is becoming increasingly difficult to explain the discrepant primordial $^{7}$Li/H abundance with nonstandard physics, without breaking the remarkable simultaneous agreement of three primordial element ratios (D/H, $^{4}$He/H, and $^{3}$He/$^{4}$He) with the Standard Model values