🔭 astrophysics

Detection of persistent helium absorption in the 91bg-like type Ia Supernova 2022an

This paper reports the detection of persistent helium absorption in the 91bg-like Type Ia supernova 2022an, providing compelling evidence for unburnt helium in the outer ejecta consistent with sub-Chandrasekhar-mass double-detonation models and highlighting the diagnostic power of near-infrared spectroscopy despite current gaps in theoretical simulations.

Original authors: Ping Chen, Avishay Gal-Yam, Subo Dong, Renyue Cen, Boaz Katz, Kate Maguire, Steve Schulze, Jesper Sollerman, Joseph P Anderson, Ting-Wan Chen, L. Galbany, Mariusz Gromadzki, Chang Liu, Adam A. Miller

Published 2026-05-11

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Ping Chen, Avishay Gal-Yam, Subo Dong, Renyue Cen, Boaz Katz, Kate Maguire, Steve Schulze, Jesper Sollerman, Joseph P Anderson, Ting-Wan Chen, L. Galbany, Mariusz Gromadzki, Chang Liu, Adam A. Miller, Tomás E. Müller-Bravo, Tanja Petrushevska, Giuliano Pignata

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 Mystery Solved

Imagine a supernova (a dying star exploding) as a massive, cosmic firework. For decades, astronomers have been trying to figure out exactly what kind of "firework" causes the specific Type Ia explosions they see. They know these explosions come from white dwarf stars, but they aren't sure if the star explodes because it ate too much material from a neighbor, or if it had a layer of helium on its skin that ignited first.

This paper is about a specific firework called SN 2022an. The astronomers found a very specific, glowing "smoke trail" in the explosion's light that had never been seen so clearly in this type of supernova before. That smoke trail is Helium.

The "91bg-like" Firework

First, the team identified what kind of explosion SN 2022an was. They call it a "91bg-like" supernova.

The Analogy: Think of standard Type Ia supernovas as bright, slow-burning fireworks that last a long time. The "91bg-like" ones are like the "dimmer, faster" cousins. They don't shine as brightly, and they fade away much quicker.
The Clue: These fast-fading explosions are thought to come from smaller white dwarf stars (about 85-90% the mass of our Sun) that are covered in a shell of helium. When the helium shell explodes, it triggers the main star to blow up.

The Smoking Gun: Finding the Helium

For a long time, astronomers suspected these smaller stars had helium shells, but they couldn't prove it. It was like trying to find a specific ingredient in a soup, but the soup was too dark and the flavors were too mixed up.

In SN 2022an, the team used powerful telescopes to look at the explosion's light in two ways:

Visible light (what our eyes see).
Near-Infrared light (a special kind of light just beyond what our eyes can see, like a night-vision camera).

The Discovery:
They found a very distinct "dip" or shadow in the light at a specific color (near 1.083 micrometers).

The Analogy: Imagine shining a flashlight through a foggy window. If there is a specific type of dust in the fog, it blocks a very specific color of the light, leaving a dark line in the rainbow.
The Result: This dark line matched perfectly with Helium. Even better, this "helium shadow" didn't disappear quickly. It stayed visible for nearly 90 days after the explosion peaked. Usually, these features vanish in a few weeks. This persistence was the key.

Why This Matters: The "Unburnt Fuel" Theory

The paper argues that this helium wasn't just floating around outside the star (like dust kicked up by the explosion). Instead, it was part of the star itself.

The Analogy: Think of a log fire. If you light a log that has a layer of unburnt wood on the outside, the fire starts on the outside and burns inward. Sometimes, a chunk of that outer wood doesn't burn completely and gets thrown out into the air.
The Science: The helium detected in SN 2022an was moving incredibly fast (about 13,000 km/s). This speed proves it was part of the star's outer shell that was blown off during the explosion, rather than being stripped off a neighbor star later. It was "unburnt fuel" that survived the blast.

The Missing Puzzle Piece

The paper points out a gap in our knowledge.

The Analogy: We have a recipe book (theoretical models) that says, "If you make a small star with a helium shell, it should produce helium smoke." But until now, we hadn't actually seen that smoke in a real explosion to confirm the recipe works.
The Gap: While the theory predicted this, the math models used by scientists haven't been updated to look at the specific time periods when SN 2022an was observed. So, while the observation matches the idea of the theory, the scientists can't yet say, "Our computer model predicted exactly this specific shape of the helium line." There is still work to be done to connect the observation perfectly with the math.

The Conclusion

SN 2022an is the strongest evidence to date that some Type Ia supernovas are indeed caused by helium-shell double detonations.

What it means: It confirms that small white dwarfs with helium skins exist and explode this way.
Why it's special: It shows that by looking at the "infrared" part of the light (the part we can't see with our eyes), we can find clues about the ingredients of the star that are invisible in normal light.

In short, the astronomers found a persistent helium fingerprint in a fast-fading supernova, proving that this specific type of cosmic explosion starts with a helium layer on a small white dwarf, just as some theories have long predicted.

Technical Summary: Detection of Persistent Helium Absorption in the 91bg-like Type Ia Supernova 2022an

Problem and Context
Type Ia supernovae (SNe Ia) are central to cosmology and binary evolution studies, yet the specific progenitor systems and explosion mechanisms remain debated. While the "sub-Chandrasekhar-mass" (sub-MCh) double-detonation scenario—where a surface helium shell detonates on a low-mass white dwarf, triggering a core carbon detonation—is a leading candidate for 91bg-like SNe Ia (low-luminosity, fast-declining events), direct observational evidence for the predicted unburnt helium in the ejecta has been elusive. Previous claims of helium in SNe Ia have been sparse, ambiguous, or attributed to circumstellar material rather than the ejecta itself. Theoretical models predict observable He I absorption features, particularly the 1.083 µm line in the near-infrared (NIR), but these have been difficult to confirm due to degeneracy with other ions (e.g., Mg II, C I) and limited wavelength coverage.

Methodology
The authors present multi-epoch optical and NIR observations of SN 2022an, a fast-declining SN Ia discovered in the lenticular galaxy NGC 4645B. The study involves:

Photometric Analysis: Constructing multiband light curves (BV gri JHKs) to characterize the supernova's evolution, peak luminosity, and decline rate. The authors utilized template-matching methods (SNooPy) and pseudo-bolometric light curve construction to estimate the synthesized $^{56}$ Ni mass and $\gamma$ -ray escape time.
Spectroscopic Analysis: Obtaining a sequence of optical and NIR spectra from approximately 5 days to 139 days after B-band maximum. The authors performed detailed line identification, focusing on a persistent absorption feature near 1.037 µm in the rest frame.
Identification Strategy: To identify the 1.037 µm feature, the authors tested alternative ion candidates (Mg II, C I, Fe II, Si I, S I) by checking for the presence of their expected companion transitions. They compared the observed spectra against non-local thermodynamic equilibrium (NLTE) radiative transfer models from previous studies (e.g., Blondin et al. 2018; Callan et al. 2025) and synthetic spectra of helium-shell double-detonation models.
Kinematic Analysis: Measuring the blueshift velocities and full width at half maximum (FWHM) of the absorption features to constrain the location and nature of the absorbing material.

Key Results

Classification: SN 2022an is confirmed as a typical 91bg-like SN Ia, exhibiting low peak luminosity, rapid decline, strong low-ionization features (enhanced Ti II), and a preference for an old host galaxy environment. Its photometric and spectroscopic properties align well with sub-MCh explosion models.
Persistent Helium Detection: The study reports the detection of a narrow absorption feature at 1.037 µm (rest frame) that persists from ~30 days to nearly 90 days after maximum light. This feature is identified as the blueshifted He I 1.083 µm line with a velocity of $\sim 1.3 \times 10^4$ km s $^{-1}$ and an FWHM of $\sim 2.1 \times 10^3$ km s $^{-1}$ .
Multi-line Confirmation: The identification is supported by the detection of multiple optical He I transitions (at $\lambda$ 5876, 6678, 7065, 7281) in earlier epochs at higher velocities ( $\sim 1.5 \times 10^4$ km s $^{-1}$ ). The absence of companion lines required for alternative identifications (e.g., Mg II 0.9227 µm) rules out other ions.
Kinematic Constraints: The high velocity of the helium ( $\sim 13,000$ –$15,000$ km s $^{-1}$ ) places it well outside the silicon layer and the photosphere. This velocity is inconsistent with helium stripped from a companion star (which would be confined to low velocities, $\lesssim 5,000$ km s $^{-1}$ ) or dynamically accelerated companions.
Model Comparison: While the detection aligns with the prediction of unburnt helium in sub-MCh double-detonation models, the observed line width is significantly narrower than that predicted by current NLTE models (Callan et al. 2025), suggesting a more confined velocity structure in the helium layer than currently simulated.

Significance and Claims
The paper claims that SN 2022an provides the "most compelling evidence to date" for helium-bearing ejecta in a 91bg-like SN Ia. The persistence of the absorption for months and the detection of multiple transitions strongly support the existence of unburnt helium in the outer ejecta, a key prediction of the helium-shell double-detonation scenario.

The authors emphasize that this discovery highlights the diagnostic power of NIR spectroscopy for probing the abundance and structure of supernova ejecta. However, they also note a significant gap between observation and theory: no modern helium-shell double-detonation calculations have been performed at the late epochs observed in SN 2022an to study the specific manifestation of helium at these times. Consequently, while the detection supports the sub-MCh channel, the precise physical conditions (mass, mixing, and geometry of the helium shell) required to reproduce the observed narrow, persistent features remain to be fully understood through dedicated multi-dimensional hydrodynamic and radiative-transfer calculations. The paper concludes that SN 2022an may represent either an extreme realization of a broader helium-bearing population or a case where favorable viewing geometry and data quality revealed a feature that is otherwise common but elusive.