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Towards a measurement of the primordial helium isotope ratio

This paper reports the discovery of new helium absorbers and high-precision measurements of the $^{3}$ He/ $^{4}$ He ratio in the Milky Way and Orion Nebula, which are used to constrain galactic chemical evolution models and infer a primordial helium isotope ratio consistent with Standard Model Big Bang nucleosynthesis predictions.

Original authors: Ryan J. Cooke (Centre for Extragalactic Astronomy, Durham University), James W. Johnson (Carnegie Observatories), Pasquier Noterdaeme (Institut d'Astrophysique de Paris), Max Pettini (Institute of Ast

Published 2026-05-04

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Original authors: Ryan J. Cooke (Centre for Extragalactic Astronomy, Durham University), James W. Johnson (Carnegie Observatories), Pasquier Noterdaeme (Institut d'Astrophysique de Paris), Max Pettini (Institute of Astronomy, University of Cambridge), Louise Welsh (Centre for Extragalactic Astronomy, Durham University), Aldric Wong (Centre for Extragalactic Astronomy, Durham University), Celine Peroux (European Southern Observatory, Laboratoire d'Astrophysique de Marseille)

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: A Cosmic Time Capsule

Imagine the universe as a giant kitchen. About 13.8 billion years ago, during the first few minutes after the "Big Bang" (the start of the cooking), the universe baked a specific recipe of ingredients. Most of the helium in the universe was baked then, specifically two types: a heavy version called Helium-4 and a much lighter, rarer version called Helium-3.

The ratio of these two ingredients (how much Helium-3 exists for every Helium-4) is a "fingerprint" of the early universe. Scientists call this the primordial helium isotope ratio. If we can measure this ratio accurately today, we can check if our understanding of how the universe began is correct.

The Problem: A Needle in a Haystack

The problem is that Helium-4 is everywhere, while Helium-3 is incredibly rare—about 10,000 times less common. It's like trying to find a single specific grain of sand on a beach, but that grain is also hiding inside a giant boulder.

Furthermore, the "heavy" helium (Helium-4) and the "light" helium (Helium-3) are so similar that they are hard to tell apart. Usually, they are mixed together in a way that makes them look like one big blob of gas.

The Solution: The "Metastable" Trick

The researchers in this paper found a clever way to spot the rare Helium-3. They looked for a specific state of helium called "metastable."

Think of a helium atom like a ball on a hill. Usually, the ball rolls down to the bottom (the ground state). But sometimes, the ball gets stuck in a little dip halfway down the hill. It stays there for a while before rolling down. This "stuck" state is the metastable state.

The team found that in certain hot, dense clouds of gas near bright stars (like in the Orion Nebula), helium atoms get stuck in this "dip." When they are stuck there, they absorb light in a very specific way. By using powerful telescopes, the team could see the "shadow" these atoms cast on the starlight. Because the heavy and light versions of helium absorb light at slightly different "colors" (wavelengths), the team could finally count them separately.

What They Did: Three Cosmic "Sightlines"

The team pointed the Very Large Telescope (VLT) in Chile at three different stars to look through the gas clouds in front of them:

Θ2A Ori: A star in the famous Orion Nebula.
HD 319718: A star in a cluster called Pismis 24.
Her 36: A star in the Lagoon Nebula.

They used two different instruments on the telescope:

UVES: To look at the "heavy" helium (Helium-4) in visible light.
CRIRES+: A super-sensitive infrared camera to catch the faint signal of the "light" helium (Helium-3).

Key Findings

1. The Gas Clouds are Stable (They Don't Wiggle)
Before measuring the ratio, the team wanted to make sure the gas clouds weren't changing shape or moving too fast, which would ruin the measurement. They looked at the Orion Nebula cloud twice, eight years apart.

The Result: The cloud was as steady as a rock. The amount of helium didn't change, and the temperature didn't fluctuate significantly. This proved that the gas is in a calm, balanced state, making the measurements reliable.

2. Measuring the Ratio
They successfully measured the ratio of Helium-3 to Helium-4 in all three locations.

The Result: They found that in our galaxy (the Milky Way), there is slightly more Helium-3 than the "Big Bang recipe" predicts.
Why? Stars act like factories. While the Big Bang made the initial helium, stars have been making more Helium-3 over billions of years. So, the gas in our galaxy is a mix of the original "Big Bang" helium and the "Star-made" helium.

3. Calculating the Original Recipe
Since the gas in our galaxy has been "contaminated" by stars, the team had to do some math to figure out what the original ratio was before any stars existed. They used a computer model of how the Milky Way evolves (how stars are born, live, and die) to subtract the "star-made" helium from their measurements.

The Big Discovery: After doing the math, they calculated the primordial ratio (the original Big Bang recipe) to be approximately 1.15 parts of Helium-3 for every 10,000 parts of Helium-4.
The Verdict: This number matches perfectly with the predictions made by the "Standard Model" of physics. This model uses the known laws of particle physics and the density of the universe (measured from the Cosmic Microwave Background) to predict what the universe should look like. The fact that their measurement matches the prediction is a huge win for our understanding of the universe's birth.

4. The "Star Factory" Scale
They also figured out how much helium stars produce compared to what we thought. They found that stars are producing helium at a rate about 2.1 times higher than some previous estimates suggested. This helps astronomers understand how efficiently stars turn gas into new elements.

Why This Matters

This paper is like finding a pristine, unopened time capsule from the first day of the universe. By proving that the helium ratio matches our theoretical predictions, the scientists confirm that our understanding of the Big Bang and the laws of physics is on the right track.

They also showed that with better telescopes (like the upcoming Extremely Large Telescopes), we can look at even older, less "polluted" gas in other galaxies to get an even clearer picture of the universe's very first moments.

1. Problem Statement

The primordial abundance of light elements, forged during Big Bang Nucleosynthesis (BBN), serves as a critical test for the Standard Model of particle physics and cosmology. While the primordial abundances of deuterium (D/H), helium-4 ( $^4$ He/H), and lithium-7 ( $^7$ Li/H) have been measured with high precision, the primordial ratio of helium-3 to helium-4 ( $^3$ He/ $^4$ He) remains poorly constrained.

The primary challenges in measuring $^3$ He/ $^4$ He are:

Abundance Disparity: $^4$ He is approximately 10,000 times more abundant than $^3$ He.
Spectral Resolution: The isotope shift between the two isotopes is small ( $\lesssim 15$ km s $^{-1}$ for optical lines and $\sim 36.6$ km s $^{-1}$ for the near-infrared line).
Detection Difficulty: Direct radio detection of the $^3$ He $^+$ 8.7 GHz spin-flip transition is limited to a few H II regions and is currently too weak for extragalactic detection.
Stellar Contamination: In the Milky Way, stellar nucleosynthesis produces additional $^3$ He, obscuring the primordial value. Disentangling the primordial component from stellar yields requires precise measurements across different galactic environments and robust chemical evolution models.

2. Methodology

Observations

The authors utilized the Very Large Telescope (VLT) to obtain high-resolution spectroscopy of three sightlines in the Milky Way:

$\Theta^2$ A Ori: A previously known absorber in the Orion Nebula.
HD 319718: A new discovery in the open cluster Pismis 24.
Her 36: A new discovery in the Lagoon Nebula (M8).

Instrumentation:

UVES (Optical): Used to observe the metastable neutral helium (He i*) transitions at $\lambda 3188$ Å and $\lambda 3889$ Å. These lines are optically thin (for $\lambda 3188$ ) and serve as the reference for $^4$ He column density.
CRIRES+ (Near-Infrared): The upgraded CRIRES spectrograph was used to observe the metastable He i* transition at $\lambda 1.083$ $\mu$ m. This line contains the $^3$ He isotope shift relative to the stronger $^4$ He line.

Data Reduction & Analysis:

Time-Variability Check: The authors compared archival UVES data (2014) with new data (2022) for $\Theta^2$ A Ori to test if the metastable He i* column density varies over time (a potential issue given the 130-minute lifetime of the metastable state).
Profile Fitting: They employed the alis (Absorption LIne Software) package to perform simultaneous fits of the stellar continuum, He i* absorption lines, telluric lines, and wavelength solutions.
Broadening Models: The analysis assumed thermal broadening dominates over turbulent broadening ( $b_{turb} \approx 0$ ), allowing the separation of $^3$ He and $^4$ He components based on their mass-dependent thermal widths.
Chemical Evolution Modeling: The authors used the VICE (Versatile Integrator for Chemical Evolution) code to model the Galactic Chemical Evolution (GCE) of the Milky Way. They integrated a grid of models varying the primordial ratio $(^3\text{He}/^4\text{He})_P$ and a stellar yield scaling factor ( $y/Z_\odot$ ).

3. Key Contributions

Discovery of New Absorbers: The paper reports the first detection of metastable He i* absorption in two new sightlines: HD 319718 and Her 36.
Time-Stability Confirmation: By comparing observations separated by 8 years, the authors confirmed that the He i* absorption in the Orion Nebula is stable, validating the use of non-simultaneous optical and IR data for isotope ratio measurements.
Improved Precision: The authors improved the precision of the $^3$ He/ $^4$ He measurement for $\Theta^2$ A Ori by a factor of $\sim 2$ compared to previous work (Cooke et al. 2022), achieving $\sim 3.6\%$ precision.
Joint GCE Analysis: They combined new observational data with a multi-zone GCE model that includes radial migration and gas ejection to simultaneously constrain the primordial ratio and the stellar yield scale.

4. Results

Observational Measurements

The authors measured the $^3$ He/ $^4$ He ratio for three sightlines at different galactocentric radii ( $R_{gal}$ ):

$\Theta^2$ A Ori ( $R_{gal} \approx 8.4$ kpc): $(1.855 \pm 0.067) \times 10^{-4}$ (Precision: 3.6%)
HD 319718 ( $R_{gal} \approx 6.5$ kpc): $(2.13 \pm 0.22) \times 10^{-4}$ (Precision: 11%)
Her 36 ( $R_{gal} \approx 6.9$ kpc): $(3.98 \pm 0.84) \times 10^{-4}$ (Precision: 21%)

Note: The Her 36 measurement is slightly higher than the others and may be affected by blending or continuum uncertainties, but it is statistically consistent within errors.

Variability Limits

The study placed a 2 $\sigma$ upper limit on the time-variability of the He i* column density in the Orion Nebula:
$\frac{d \log_{10}[N(\text{He i}^*)/\text{cm}^{-2}]}{dt} \leq 7.2 \times 10^{-4} \text{ dex yr}^{-1} (< 0.17\% \text{ yr}^{-1})$
This suggests the absorbers are in radiative equilibrium.

Primordial Ratio and Yield Scale

Using MCMC analysis on the VICE model grid:

Primordial Ratio: The best-fit primordial helium isotope ratio is:
$(^3\text{He}/^4\text{He})_P = (1.15^{+0.24}_{-0.21}) \times 10^{-4}$
This value is consistent with Standard Model BBN predictions ( $(1.262 \pm 0.017) \times 10^{-4}$ ) derived from CMB baryon density.
Stellar Yield Scale: The inferred scaling factor for stellar yields relative to solar metallicity is:
$y/Z_\odot = 2.12^{+0.31}_{-0.29}$
This is somewhat higher than previous estimates (which often favored $y/Z_\odot \approx 1$ ), suggesting that current nucleosynthesis models may underestimate metal yields or that gas ejection rates in the Milky Way are lower than assumed.

5. Significance

Cosmological Validation: The derived primordial ratio supports the Standard Model of particle physics and the baryon density inferred from the Cosmic Microwave Background (CMB), reinforcing the consistency of BBN theory.
Galactic Chemical Evolution: The study demonstrates that $^3$ He/ $^4$ He is a powerful tracer for breaking degeneracies in GCE models between stellar yield scales and gas ejection rates. The elevated yield scale ( $y/Z_\odot \approx 2.1$ ) implies that stellar populations may be more efficient at producing metals than previously thought, or that the Milky Way has retained more gas than standard models predict.
Future Prospects: The paper highlights that the current measurements are limited to metal-rich environments in the Milky Way. Future observations with Extremely Large Telescopes (ELTs) in metal-poor environments (e.g., Magellanic Clouds, dwarf galaxies) are essential to reduce the covariance between the primordial ratio and the yield scale, potentially allowing for a model-independent determination of $(^3\text{He}/^4\text{He})_P$ .

In summary, this work represents a significant step forward in measuring the primordial helium isotope ratio by combining high-precision spectroscopy of metastable helium with advanced galactic chemical evolution modeling, successfully constraining both cosmological parameters and stellar nucleosynthesis yields.