Primordial helium-3 redux : the helium isotope ratio of the Orion nebula

Abstract

We report the first direct measurement of the helium isotope ratio, He-3/He-4, outside of the Local Interstellar Cloud, as part of science-verification observations with the upgraded CRyogenic InfraRed Echelle Spectrograph. Our determination of He-3/He-4 is based on metastable He I* absorption along the line of sight toward Theta(2)A Ori in the Orion Nebula. We measure a value He-3/He-4 = (1.77 +/- 0.13) x 10(-4) , which is just similar to 40% above the primordial relative abundance of these isotopes, assuming the Standard Model of particle physics and cosmology, (He-3/He-4)(p) = (1.257 +/- 0.017) x 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-circle dot and Lagarde et al. for M = 0.8-8 M-circle dot. We find that these simulations simultaneously reproduce the Orion and protosolar He-3/He-4 values if the calculations are initialized with a primordial ratio (He-3/He-4)(p) = (1.043 +/- 0.089) x 10(-4). Even though the quoted error does not include the model uncertainty, this determination agrees with the Standard Model value to within similar to 2 sigma. We also use the present-day Galactic abundance of deuterium (D/H), helium (He/H), and He-3/He-4 to infer an empirical limit on the primordial He-3 abundance, (He-3/H)(p) <= (1.09 +/- 0.18) x 10(-5), which also agrees with the Standard Model value. We point out that it is becoming increasingly difficult to explain the discrepant primordial Li-7/H abundance with nonstandard physics, without breaking the remarkable simultaneous agreement of three primordial element ratios (D/H, He-4/H, and He-3/He-4) with the Standard Model values.