arxiv: 2605.07827 · v1 · submitted 2026-05-08 · 🌌 astro-ph.HE

Recognition: 2 theorem links

· Lean Theorem

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

Ping Chen , Avishay Gal-Yam , Subo Dong , Renyue Cen , Boaz Katz , Kate Maguire , Steve Schulze , Jesper Sollerman

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Joseph P Anderson Ting-Wan Chen L. Galbany Mariusz Gromadzki Chang Liu Adam A.Miller Tom\'as E. M\"uller-Bravo Tanja Petrushevska Giuliano Pignata

Authors on Pith no claims yet

Pith reviewed 2026-05-11 01:50 UTC · model grok-4.3

classification 🌌 astro-ph.HE

keywords Type Ia supernovaehelium absorption91bg-like eventsdouble detonationnear-infrared spectroscopySN 2022anunburnt ejecta

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The pith

Persistent helium absorption in SN 2022an reveals unburnt material in the outer ejecta of a 91bg-like Type Ia supernova.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

This paper reports optical and near-infrared observations of the fast-declining supernova 2022an, confirming it as a standard 91bg-like Type Ia event. A narrow absorption feature near 1.037 microns persists from 30 to nearly 90 days after maximum light and is identified as blueshifted He I 1.083 microns at high velocity. The velocity and persistence cannot be explained by helium stripped from a companion star and instead indicate unburnt helium residing in the supernova's outer ejecta. This supplies the strongest evidence to date for helium-bearing ejecta in 91bg-like events and aligns with predictions from sub-Chandrasekhar double-detonation models that include a surface helium shell. The work underscores the diagnostic value of near-infrared spectroscopy for mapping ejecta composition in thermonuclear supernovae.

Core claim

The central claim is that the narrow, persistent absorption feature at 1.037 μm is the He I 1.083 μm line blueshifted by 1.3×10^4 km s^{-1} with FWHM of 2.1×10^3 km s^{-1}, supported by earlier optical He I lines at ~1.5×10^4 km s^{-1}. Because this velocity rules out external stripped helium from a companion, the feature must arise from unburnt helium in the outer ejecta, constituting the most compelling evidence yet for helium-bearing ejecta in a 91bg-like SN Ia and favoring sub-Chandrasekhar-mass double-detonation explosions with a surface helium shell.

What carries the argument

The narrow absorption feature near 1.037 μm identified as blueshifted He I 1.083 μm, whose velocity, width, and persistence distinguish internal unburnt helium from external companion-stripped material.

Load-bearing premise

The narrow 1.037 μm feature is correctly identified as blueshifted He I 1.083 μm rather than another species or artifact, and its velocity is too high to be stripped helium from a companion star.

What would settle it

A re-analysis showing the feature belongs to a different atomic species or velocity measurements matching those expected for companion-stripped helium would falsify the internal-ejecta interpretation.

Figures

Figures reproduced from arXiv: 2605.07827 by Adam A.Miller, Avishay Gal-Yam, Boaz Katz, Chang Liu, Giuliano Pignata, Jesper Sollerman, Joseph P Anderson, Kate Maguire, L. Galbany, Mariusz Gromadzki, Ping Chen, Renyue Cen, Steve Schulze, Subo Dong, Tanja Petrushevska, Ting-Wan Chen, Tom\'as E. M\"uller-Bravo.

**Figure 1.** Figure 1: The multiband light curves of SN 2022an and comparison with other 91bg-like SNe Ia. The bottom panel shows the full range of the light curves, while the top panels show zoomed-in views around the peak light. All the panels share the same color, symbol, and line-style scheme as shown in the legend of the bottom panel: colors distinguish different filters, symbols distinguish different instruments used for S… view at source ↗

**Figure 2.** Figure 2: Optical and NIR spectra of SN 2022an. The phases of the spectra are given with respect to the B-band maximum time of JD=2459582. A telluric spectrum is shown on the top of the plot, and those significant telluric regions are marked with vertical grey bands. The inset panel shows the zoom-in view around the detected absorption feature indicated by the black arrow, which, as discussed in the following sectio… view at source ↗

**Figure 3.** Figure 3: B − V color curves of SN 2022an and other 91bglike SNe Ia including SN 1991bg (Filippenko et al. 1992c; Leibundgut et al. 1993b), SN 1999by (Garnavich et al. 2004a; H¨oflich et al. 2002a) and SN 2006mr(Krisciunas et al. 2017). The Galactic extinctions have been corrected for all the objects. scaled to match the observed optical photometry and interpolated NIR photometry (see the solid curves in [PITH_F… view at source ↗

**Figure 4.** Figure 4: The pseudo-bolometric light curve of SN 2022an (red) fit with the radioactive 56Ni decay model using the method outlined in Wygoda et al. (2019). The synthetic light curve of the SCH2p0 model (black) from Blondin et al. (2018) is shown for comparison. The three panels show the pseudo-bolometric light curves (top), the time weighted integrated luminosities (middle), and the 56Ni mass independent quantity d… view at source ↗

**Figure 5.** Figure 5 [PITH_FULL_IMAGE:figures/full_fig_p008_5.png] view at source ↗

**Figure 7.** Figure 7: Spectra of SN 2022an compared to those of corenormal Ia SN 2011fe (Pereira et al. 2013), transitional Ia SN 2015bp (Wyatt et al. 2021b), SN 1991bg (Filippenko et al. 1992b; Leibundgut et al. 1993a), and 91bg-like Ia SN 2005ke (Folatelli et al. 2013). The color stretch parameter sBV of the supernova and the phase after B-band maximum of the spectrum are given after the supernova name. The comparison spect… view at source ↗

**Figure 6.** Figure 6: Spectra of SN 2022an compared to those of normal Ia SN 2011fe (Pereira et al. 2013), transitional Ia SN 2015bp (Wyatt et al. 2021b), SN 1991bg (Filippenko et al. 1992b; Leibundgut et al. 1993a), and 91bg-like Ia SN 1999by (Garnavich et al. 2004a; H¨oflich et al. 2002a) and SN 2005bl (Taubenberger et al. 2008; Hachinger et al. 2009). The color stretch parameter sBV of the supernova and the phase after B-ban… view at source ↗

**Figure 8.** Figure 8: Spectra of SN 2022an compared to the SCH2p0 model spectra from Blondin et al. (2018) at similar phases. The model spectra have been scaled to match the integrated flux in wavelength 4000 – 8000 ˚A. and lies slightly redward of the S II line center. The inset panel displays the normalized profile, which is well described by a Gaussian with a full-width-at-halfmaximum (FWHM) of ∼ 70 ˚A. Given that the obser… view at source ↗

**Figure 9.** Figure 9: Line identification of the absorption feature in the NIR spectrum of SN 2022an. The +55.4-day X-Shooter spectrum and its smoothed version are shown in blue, with the original data shown in light color. The synthetic spectrum of the SCH2p0 model at a phase of +58.9 days after the B-band peak, from Blondin et al. (2018), is shown in grey for comparison. The absorption feature at around 1.037 µm is superposed… view at source ↗

**Figure 10.** Figure 10: (a) He I absorption features in the optical spectrum of SN 2022an (red line), obtained with SOAR/GHTS at +5.3 days relative to B-band maximum. The blue vertical dashed lines denote the identified He I lines at λ5876, λ6678, λ7065, λ7281. For comparison, spectra of SN 2020eyj, a Type Ia supernova interacting with helium-rich circumstellar material that exhibits He I emission (Kool et al. 2023), are shown i… view at source ↗

**Figure 11.** Figure 11: (a) Spectra of SN 2022an in red compared with model spectra from Dessart & Hillier (2015) (D15) in magenta, Boyle et al. (2017)(B17) in blue, and Callan et al. (2025)(C25) in green. For C25, both the full model spectra and those omitting He I line opacity are plotted with solid and dotted lines, respectively. The phase of each spectrum, relative to the B-band peak in days, is labeled. The vertical black l… view at source ↗

**Figure 12.** Figure 12: NIR spectra of SN 2022an compared to those of fast declining SNe Ia with sBV < 0.7. The vertical dashed lines show the rest-frame wavelengths of C I 1.069 µm, He I 1.083 µm, and Mg II 1.0927 µm in cyan, yellow and magenta, respectively. The magenta arrow indicates the absorption feature that is commonly attributed to Mg II 1.0927 µm line in literature. The cyan arrow indicates the absorption with disputab… view at source ↗

read the original abstract

We present optical and near-infrared observations of the fast-declining Type Ia supernova (SN Ia) 2022an. The photometric and spectroscopic properties identify it as a standard 91bg-like event; however, our data reveal a relatively narrow absorption feature with a full width at half maximum (FWHM) of 75 angstroms near $1.037\,\mu$m in the rest frame of the observed spectra that persists from around 30 days to nearly 90 days after maximum light. We attribute this feature to He I $1.083\,\mu$m line with a blueshifted velocity of $1.3\times10^{4}$ km s$^{-1}$ and a FWHM of $2.1\times10^{3}$ km s$^{-1}$, supported by the detection of multiple optical He I transitions in earlier epochs at a higher velocity around $1.5\times10^{4}$ km s$^{-1}$. The high velocity of the helium could not be explained by helium external to the progenitor at the explosion, such as the stripped surface helium from a companion star. The properties of the helium absorption in SN 2022an spectra instead point to unburnt material in the outer ejecta, thus providing the most compelling evidence to date for helium-bearing ejecta in a 91bg-like SN Ia. Such helium has been predicted for sub-Chandrasekhar-mass double-detonation explosions involving a surface helium shell. No theoretical calculations of modern helium-shell double detonation have been performed at epochs similar to those observed for SN 2022an to study the effect of helium on their spectra, revealing a gap between observations and theoretical calculations in understanding the manifestation of helium in SNe Ia. Nevertheless, the discovery of persistent helium absorption in SN 2022an demonstrates the diagnostic power of NIR spectroscopy for understanding thermonuclear supernova explosions by probing the abundance and structure of their ejecta.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

The paper reports a persistent narrow NIR absorption they identify as helium in SN 2022an, with velocity matching earlier optical He I lines, but the claim rests on identification without late-time models to back it up.

read the letter

The main thing to know is that this work presents multi-epoch optical and NIR spectra of the 91bg-like SN 2022an and flags a narrow feature near 1.037 microns that stays visible from roughly +30 to +90 days. They tie it to blueshifted He I 1.083 micron at 13,000 km/s and note that earlier optical He I lines sit at a similar but slightly higher velocity. The high velocity is used to argue against stripped companion material, pointing instead to unburnt helium in the outer ejecta of a sub-Chandrasekhar double-detonation explosion. They also flag the absence of any modern helium-shell models computed at these epochs, which is a straightforward admission of the current theoretical gap. That combination of data and honest caveat is the useful part. The classification as 91bg-like looks standard and the velocity consistency across wavelengths is the strongest observational thread. The soft spots are straightforward. The abstract gives FWHM and velocity numbers but no spectra, error bars, or alternative-line checks, so the identification cannot be verified from the text alone. More importantly, without any quantitative spectral synthesis at +30 to +90 days it is impossible to test whether unburnt outer helium actually produces a narrow, long-lived feature at exactly that velocity and strength. The attribution therefore stays at the level of plausible wavelength and velocity match rather than a direct model comparison. This paper is for people working on Type Ia progenitor channels and NIR diagnostics of thermonuclear events. A reader who follows 91bg-like objects or helium signatures would get the observational report and the clear call for new modeling. It is worth sending to peer review so the data can be examined and the modeling gap can be addressed by others.

Referee Report

2 major / 2 minor

Summary. The manuscript presents optical and NIR observations of SN 2022an, classifying it as a standard 91bg-like Type Ia supernova. It reports a narrow, persistent absorption feature (FWHM 75 Å) near 1.037 μm from +30 to +90 days post-maximum, identified as blueshifted He I λ1.083 μm at v ≈ 1.3×10^4 km s^{-1} (FWHM 2.1×10^3 km s^{-1}), supported by earlier optical He I lines at ~1.5×10^4 km s^{-1}. The authors argue this indicates unburnt helium in the outer ejecta rather than stripped companion material, providing evidence for helium-shell double-detonation progenitors, while noting the absence of relevant theoretical spectral models at these epochs.

Significance. If the line identification is robust, the result would be significant for linking 91bg-like events to sub-Chandrasekhar double-detonation models that predict unburnt outer helium. The multi-epoch NIR coverage and consistency across multiple He I transitions represent a clear observational strength, and the work correctly highlights the diagnostic value of NIR spectroscopy for ejecta structure. However, the lack of quantitative late-time spectral synthesis limits the strength of the unburnt-helium attribution relative to prior claims.

major comments (2)

[Abstract] Abstract: The central claim that the observed feature provides 'the most compelling evidence to date for helium-bearing ejecta in a 91bg-like SN Ia' is not fully supported. The manuscript explicitly states that 'no theoretical calculations of modern helium-shell double detonation have been performed at epochs similar to those observed,' so the attribution to unburnt outer helium rests solely on velocity arguments and earlier optical lines rather than direct comparison to synthetic spectra. This leaves open the possibility that the narrow 1.037 μm feature arises from a different species, ionization state, or unmodeled effect.
[Abstract] Abstract: The assertion that the helium velocity 'could not be explained by helium external to the progenitor at the explosion, such as the stripped surface helium from a companion star' is load-bearing for ruling out the alternative origin, yet the text provides no quantitative comparison (e.g., expected velocity or line profile from stripped-helium hydrodynamical models at +30–90 d). A specific reference or calculation showing the inconsistency is required to make this exclusion robust.

minor comments (2)

[Abstract] The abstract and main text should include at least one representative spectrum (with error bars and continuum subtraction) showing the 1.037 μm feature and the optical He I lines to allow independent assessment of the identification and FWHM measurement.
Add a brief discussion of possible contaminating lines or telluric artifacts near 1.037 μm in the observed frame, even if ultimately ruled out, to address potential alternative identifications.

Simulated Author's Rebuttal

2 responses · 1 unresolved

We thank the referee for their careful and constructive review of our manuscript. The comments highlight important points regarding the strength of our claims in the abstract, and we address each one below. We have revised the manuscript to moderate the language and add supporting discussion where feasible.

read point-by-point responses

Referee: [Abstract] Abstract: The central claim that the observed feature provides 'the most compelling evidence to date for helium-bearing ejecta in a 91bg-like SN Ia' is not fully supported. The manuscript explicitly states that 'no theoretical calculations of modern helium-shell double detonation have been performed at epochs similar to those observed,' so the attribution to unburnt outer helium rests solely on velocity arguments and earlier optical lines rather than direct comparison to synthetic spectra. This leaves open the possibility that the narrow 1.037 μm feature arises from a different species, ionization state, or unmodeled effect.

Authors: We agree that the original phrasing in the abstract implies a level of conclusiveness that exceeds what the available theoretical comparisons can support at these epochs. The line identification is based on the detection of multiple He I transitions at consistent high velocities across optical and NIR wavelengths, together with the feature's persistence and narrow profile. However, we will revise the abstract to state that the observations provide 'compelling evidence' for helium-bearing ejecta rather than 'the most compelling evidence to date,' while retaining the explicit mention of the gap in late-time spectral models. This change preserves the observational significance without overstating the direct link to specific progenitor models. revision: yes
Referee: [Abstract] Abstract: The assertion that the helium velocity 'could not be explained by helium external to the progenitor at the explosion, such as the stripped surface helium from a companion star' is load-bearing for ruling out the alternative origin, yet the text provides no quantitative comparison (e.g., expected velocity or line profile from stripped-helium hydrodynamical models at +30–90 d). A specific reference or calculation showing the inconsistency is required to make this exclusion robust.

Authors: We acknowledge that the manuscript would benefit from a more explicit quantitative argument on this point. The observed helium velocity of ~1.3×10^4 km s^{-1} is substantially higher than the orbital velocities and post-interaction speeds expected for stripped companion material in hydrodynamical simulations of Type Ia events. We will add a concise discussion in the revised text, including a reference to relevant stripped-material models (such as those examining velocity distributions of companion ejecta), to demonstrate the inconsistency with the observed high-velocity, narrow feature. This addition will strengthen the exclusion of the external-helium scenario without altering the core observational results. revision: yes

standing simulated objections not resolved

The absence of published late-time (+30 to +90 d) spectral synthesis calculations for modern helium-shell double-detonation models, which prevents direct quantitative comparison of the observed feature to synthetic spectra.

Circularity Check

0 steps flagged

No circularity: purely observational line identification and velocity argument

full rationale

The manuscript reports NIR and optical spectra of SN 2022an, measures a narrow absorption at 1.037 μm (FWHM 75 Å) persisting +30 to +90 d, identifies it as blueshifted He I 1.083 μm at v ≈ 1.3×10^4 km s^{-1}, and notes consistency with earlier optical He I lines at slightly higher velocity. The claim that this indicates unburnt outer helium (rather than stripped companion material) rests on the observed velocity and line persistence, not on any fitted parameter, self-citation chain, or equation that reduces the conclusion to its own inputs. No theoretical spectral synthesis or model fitting is performed; the paper explicitly notes the absence of relevant late-time helium-shell double-detonation calculations. The derivation chain is therefore self-contained observational reporting with no load-bearing circular step.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The claim rests on standard spectroscopic line identification and the astrophysical assumption that high-velocity helium must originate internally rather than from a companion.

axioms (1)

domain assumption The 1.037 μm absorption is blueshifted He I 1.083 μm
Wavelength coincidence plus consistency with optical He I lines at similar velocity.

pith-pipeline@v0.9.0 · 5733 in / 1307 out tokens · 57563 ms · 2026-05-11T01:50:32.083344+00:00 · methodology

discussion (0)

Lean theorems connected to this paper

Citations machine-checked in the Pith Canon. Every link opens the source theorem in the public Lean library.

IndisputableMonolith/Foundation/RealityFromDistinction.lean reality_from_one_distinction unclear

?

unclear
Relation between the paper passage and the cited Recognition theorem.

We attribute this feature to He I 1.083 µm line with a blueshifted velocity of 1.3×10^4 km s^{-1} ... point to unburnt material in the outer ejecta ... sub-Chandrasekhar-mass double-detonation explosions involving a surface helium shell.
IndisputableMonolith/Cost/FunctionalEquation.lean washburn_uniqueness_aczel unclear

?

unclear
Relation between the paper passage and the cited Recognition theorem.

No theoretical calculations of modern helium-shell double detonation have been performed at epochs similar to those observed for SN 2022an

What do these tags mean?

matches: The paper's claim is directly supported by a theorem in the formal canon.
supports: The theorem supports part of the paper's argument, but the paper may add assumptions or extra steps.
extends: The paper goes beyond the formal theorem; the theorem is a base layer rather than the whole result.
uses: The paper appears to rely on the theorem as machinery.
contradicts: The paper's claim conflicts with a theorem or certificate in the canon.
unclear: Pith found a possible connection, but the passage is too broad, indirect, or ambiguous to say the theorem truly supports the claim.

Reference graph

Works this paper leans on

12 extracted references · 12 canonical work pages

[1]

2026, A&A, 707, A91, doi: 10.1051/0004-6361/202555976 Barkhudaryan, L

Alburai, A., Galbany, L., Burgaz, U., et al. 2026, A&A, 707, A91, doi: 10.1051/0004-6361/202555976 Barkhudaryan, L. V., Hakobyan, A. A., Karapetyan, A. G., et al. 2019, MNRAS, 490, 718, doi: 10.1093/mnras/stz2585 Bildsten, L., Shen, K. J., Weinberg, N. N., & Nelemans, G. 2007, ApJL, 662, L95, doi: 10.1086/519489 Blondin, S., Dessart, L., & Hillier, D. J. ...

work page doi:10.1051/0004-6361/202555976 2026
[2]

The images were reduced with the LCOGT/BANZAI pipeline (McCully et al. 2018). We downloaded the reduced frames from the Las Cumbres Observatory science archive. NTT/EFOSC2—We observed two epochs of photo- metric data inBVbands with the ESO Faint Object Spectrograph and Camera version 2 (EFOSC2; Buzzoni et al

work page 2018
[3]

The basic data reduction, including bias subtraction, flat-field correction, and astrometric solu- tion, was performed using the PESSTO pipeline (Smartt et al

on the ESO 3.58m New Technology Tele- scope (NTT). The basic data reduction, including bias subtraction, flat-field correction, and astrometric solu- tion, was performed using the PESSTO pipeline (Smartt et al. 2015b). We also obtained four epochs of acquisi- tion images in theVband from the spectroscopic obser- vations. Magellan/IMACS—We obtained three e...

work page 2011
[4]

We performed PSF photometry with thedaophottask inIRAF(Tody 1986, 1993)

on the NTT telescope. We performed PSF photometry with thedaophottask inIRAF(Tody 1986, 1993). The host galaxy background flux was modelled using an isophote model and iter- atively subtracted from the image (see the method in Chen et al. 2022). We utilize the ATLAS All-sky Stel- lar Reference Catalog (ATLAS-REFCAT2; Tonry et al

work page 1986
[5]

to derive the photometric zero-point of the optical photometry. Before being used for photometric calibra- tion of our target, the ATLAS-REFCAT2 magnitudes of the reference stars in the fields are first converted to the JohnsonBVand Sloan-grizbands using the trans- formations given in Tonry et al. (2012). Reference stars in the field of view from the 2MAS...

work page 2012
[6]

All these photometry procedures were performed using thepmpyeasypipeline (Chen et al

were used to derive the photometric zeropoint for theJ HKsbands. All these photometry procedures were performed using thepmpyeasypipeline (Chen et al. 2022). The photometry data for SN 2022an are summarized in Table 1 for theBV gribands and in Table 2 for the J HKbands. Thegrimagnitudes are reported in the AB system, while theBV J HKmagnitudes are based o...

work page 2022
[7]

Image Subtraction (No reference flux added)

from the ASAS-SN Sky Patrol 3 (Kochanek et al. 2017). We adopted the “Image Subtraction (No reference flux added)” photometry method, which performs aperture photometry on the coadded image- subtracted data for each epoch but does not add the source flux from the reference image to the light curve. The result is present in Table

work page 2017
[8]

SOAR/GHTS—SN 2022an was observed on 2022 Jan- uary 7 by the Goodman High Throughput Spectrograph (GHTS; Clemens et al

The details of the follow-up campaign and data reduction are described below. SOAR/GHTS—SN 2022an was observed on 2022 Jan- uary 7 by the Goodman High Throughput Spectrograph (GHTS; Clemens et al

work page 2022
[9]

400 SYZY

′′0 wide slit and the “400 SYZY” grating. The spectrum4 was first presented in the classification report (Jacobson-Gal´ an et al. 2022). We obtained the raw data from the Las Cumbres Ob- servatory Science Archive 5 and performed the data re- duction. We reduced the spectrum withIRAF(Tody 1986, 1993), including bias subtraction, flat-field correc- tion, co...

work page 2022
[10]

on 2022 Febru- ary 26 and March 31, through a DDT program (Pro- gram ID: 108.23MS, P.I.: P. Chen). All observations were performed in nodding mode and with 1.′′0/0.′′9/0.′′9 wide slits (UVB/VIS/NIR). The observations covered the entire spectral range of the X-shooter spectrograph from 3000 to 24800˚A. We first removed cosmic rays with 24Chen Ping T able 2...

work page 2022
[11]

We obtained another epoch of IMACS spectroscopy of SN 2022an on 2022 May

work page 2022
[12]

The FIRE spectrum was taken with the long-slit mode, and the data were reduced with the IDL pipelinefirehose (Simcoe et al

Both FIRE and IMACS are mounted on the 6.5m Magellan Baade telescope. The FIRE spectrum was taken with the long-slit mode, and the data were reduced with the IDL pipelinefirehose (Simcoe et al. 2013). The IMACS spectra were reduced withIRAF(Tody 1986, 1993), including bias subtraction, flat-field correction, cosmic-ray removal, wavelength cal- ibration (u...

work page 2013