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

TL;DR

This study measures the helium isotope ratio 3He/4He in the Orion Nebula, compares it with primordial and galactic values, and uses models to understand galactic chemical evolution, supporting the Standard Model of cosmology.

Contribution

First direct measurement of 3He/4He outside the Local Interstellar Cloud, combined with galactic evolution simulations to constrain primordial helium isotope ratios.

Findings

Measured 3He/4He in Orion Nebula: (1.77±0.13)×10^{-4}

Galactic models reproduce observed ratios with primordial ratio ~1.043×10^{-4}

Primordial 3He abundance constrained to <(1.09±0.18)×10^{-5}

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 (CRIRES). Our determination of 3He/4He is based on metastable HeI* absorption along the line-of-sight towards Tet02 Ori A in the Orion Nebula. We measure a value 3He/4He=(1.77+/-0.13)x10^{-4}, which is just ~40 per cent above the primordial relative abundance of these isotopes, assuming the Standard Model of particle physics and cosmology, (3He/4He)_p = (1.257+/-0.017)x10^-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 (2018) for stars in the mass range M=8-100 M_sun and Lagarde (2011,2012) for M=0.8-8 M_sun. We find that these simulations simultaneously reproduce the Orion and protosolar 3He/4He values if the calculations are initialized with a primordial ratio (3He/4He)_p=(1.043+/-0.089)x10^-4. Even though the quoted error does not include the model uncertainty, this determination agrees with the Standard Model value to within ~2sigma. 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, (3He/H)_p < (1.09+/-0.18)x10^-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 non-standard physics, without breaking the remarkable simultaneous agreement of three primordial element ratios (D/H, 4He/H, and 3He/4He) with the Standard Model values.