Towards a Measurement of the Primordial Helium Isotope Ratio
Abstract
We report the discovery of two metastable neutral helium (He I*) absorbers in the Milky Way, and use the upgraded CRyogenic InfraRed Echelle Spectrograph on the Very Large Telescope to determine the helium isotope ratio, 3He/4He, along these sightlines. We have also obtained deeper observations of a third sightline to report a ≲4% precision measure of 3He/4He in the Orion Nebula. These data have allowed us to place a 2σ limit on the time-variability of He I* absorption in the Orion nebula, dlog10[N(HeI∗)/cm−2]/dt≤7.2×10−4 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 (3He/4He)P=(1.15+0.24−0.21)×10−4, which agrees with Big Bang nucleosynthesis calculations that assume the Standard Model of particle physics in combination with the baryon density inferred from the cosmic microwave background temperature fluctuations. We infer the stellar yield scale relative to the solar metallicity, y/Z⊙=2.12+0.31−0.29, which is somewhat higher than previously found. Finally, we note that the forthcoming extremely large telescopes are poised to determine 3He/4He in more metal-poor environments, to secure a model-independent determination of the primordial value.
Related
- The LBT Y p Project. IV. A New Value of the Primordial Helium Abundance
- EMPRESS. XV. A New Determination of the Primordial Helium Abundance Suggesting a Moderately Low Y P Value
- Search for subsolar primordial black holes in low mass-ratio binaries with LIGO-Virgo O2 data and implications for the primordial black hole dark matter fraction
- Simulation of the electron escape ratio for keV alpha-particle ionization tracks in liquid helium
- High-resolution transmission spectroscopy of KELT-18b. Metastable helium detection at 1083 nm
BibTeX
@article{cooke2026towards,
title = {{Towards a Measurement of the Primordial Helium Isotope Ratio}},
author = {Ryan J Cooke and James W Johnson and Pasquier Noterdaeme and Max Pettini and Louise Welsh and Aldric Wong and Céline Péroux},
journal = {The Astrophysical Journal},
year = {2026},
month = may,
doi = {10.3847/1538-4357/ae66f8},
eprint = {2605.00122},
archivePrefix = {arXiv},
url = {https://hal.science/hal-05764768v1},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Towards a Measurement of the Primordial Helium Isotope Ratio
AU - Ryan J Cooke
AU - James W Johnson
AU - Pasquier Noterdaeme
AU - Max Pettini
AU - Louise Welsh
AU - Aldric Wong
AU - Céline Péroux
PY - 2026
DA - 2026/05/28
JO - The Astrophysical Journal
DO - 10.3847/1538-4357/ae66f8
UR - https://hal.science/hal-05764768v1
AB - We report the discovery of two metastable neutral helium (He I*) absorbers in the Milky Way, and use the upgraded CRyogenic InfraRed Echelle Spectrograph on the Very Large Telescope to determine the helium isotope ratio, 3He/4He, along these sightlines. We have also obtained deeper observations of a third sightline to report a ≲4% precision measure of 3He/4He in the Orion Nebula. These data have allowed us to place a 2σ limit on the time-variability of He I* absorption in the Orion nebula, dlog10[N(HeI∗)/cm−2]/dt≤7.2×10−4 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 (3He/4He)P=(1.15+0.24−0.21)×10−4, which agrees with Big Bang nucleosynthesis calculations that assume the Standard Model of particle physics in combination with the baryon density inferred from the cosmic microwave background temperature fluctuations. We infer the stellar yield scale relative to the solar metallicity, y/Z⊙=2.12+0.31−0.29, which is somewhat higher than previously found. Finally, we note that the forthcoming extremely large telescopes are poised to determine 3He/4He in more metal-poor environments, to secure a model-independent determination of the primordial value.
SN - 0004-637X
SN - 1538-4357
ER -