Towards a Measurement of the Primordial Helium Isotope Ratio*

We report the discovery of two metastable neutral helium (He I *) absorbers in the Milky Way, and use the upgraded CRyogenic InfraRed Echelle Spectrograph (or CRIRES) on the Very Large Telescope to determine the helium isotope ratio, 3 He/ 4 He, along these sight lines. We have also obtained deeper observations of a third sight line to report a ≲4% precision measure of 3 He/ 4 He in the Orion Nebula. These data have allowed us to place a 2 σ limit on the time variability of He I * absorption in the Orion Nebula, d log 10 [ N ( He I ∗ ) / cm − 2 ] / d t ≤ 7.2 × 1 0 − 4 dex yr − 1 (<0.17% yr −1 ), suggesting that these absorbers are in radiative equilibrium. We compute new galactic chemical evolution models of the Milky Way, and use our observations to infer the primordial helium isotope ratio and a scaling factor for the yields reported by nucleosynthesis calculations. Based on the data and models that we report here, we infer a best-fit value ( 3 He/ 4 He) P = ( 1.1 5 − 0.21 + 0.24 ) × 1 0 − 4 , which agrees with Big Bang nucleosynthesis calculations that assume the Standard Model of particle physics in combination with the baryon density inferred from cosmic microwave background temperature fluctuations. We infer the stellar yield scale relative to the solar metallicity, y / Z ⊙ = 2.1 2 − 0.29 + 0.31 , which is somewhat higher than previously found. Finally, we note that the forthcoming generation of extremely large telescopes is poised to determine 3 He/ 4 He in more metal-poor environments, enabling a model-independent determination of the primordial value.