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Evidence for Asymmetric Ejecta and Circumstellar Material in SN 2023ixf Inferred from Extensive Nebular-phase Observations

Based on extensive optical, near-infrared, and James Webb Space Telescope observations of SN 2023ixf during its nebular phase, this study reveals evidence for asymmetric ejecta containing Ni-rich plumes and a disk-like circumstellar medium through the complex evolution of H $\alpha$ profiles and double-peaked infrared emission lines.

Original authors: Brian Hsu, Nathan Smith, K. Azalee Bostroem, Jeniveve Pearson, David J. Sand, Lindsey A. Kwok, Jennifer E. Andrews, Bhagya M. Subrayan, Griffin Hosseinzadeh, Manisha Shrestha, Conor Ransome, Moira And

Published 2026-07-07

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CC BY 4.0

Original authors: Brian Hsu, Nathan Smith, K. Azalee Bostroem, Jeniveve Pearson, David J. Sand, Lindsey A. Kwok, Jennifer E. Andrews, Bhagya M. Subrayan, Griffin Hosseinzadeh, Manisha Shrestha, Conor Ransome, Moira Andrews, Collin T. Christy, Yize Dong, Joseph Farah, Alexei V. Filippenko, Noah Franz, Jared A. Goldberg, Estefania Padilla Gonzalez, Claudia P. Gutiérrez, Emily Hoang, D. Andrew Howell, Saurabh W. Jha, Réka Kőnyves-Tóth, Michael Lundquist, Curtis McCully, Darshana Mehta, Nicolas E. Meza Retamal, Megan Newsome, Aravind P. Ravi, Jeonghee Rho, Giacomo Terreran, Stefano Valenti, Sergiy Vasylyev, X. -F. Wang, Kathryn Wynn, Yi Yang

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

Imagine a massive star, much heavier than our Sun, reaching the end of its life. Instead of fading away quietly, it explodes in a spectacular supernova. In May 2023, astronomers spotted one of these explosions, named SN 2023ixf, in a nearby galaxy called the Pinwheel Galaxy. Because it was so close, it gave scientists a rare, up-close look at the debris field left behind.

This paper is like a detailed forensic report on that debris field, written by a large team of astronomers. They watched the explosion evolve over nearly two years (from day 89 to day 749 after the blast), using powerful telescopes on Earth and the James Webb Space Telescope (JWST) in space.

Here is what they found, explained simply:

1. The Star Didn't Explode Evenly

When a star explodes, we often imagine a perfect sphere, like a balloon popping and sending shrapnel out in all directions equally. However, the authors found that SN 2023ixf was messy and lopsided.

The Analogy: Think of the explosion not as a smooth balloon popping, but as a firework that shoots out distinct, heavy "plumes" or jets of material in specific directions, rather than a uniform cloud.
The Evidence: By looking at the light coming from different chemical elements (like Oxygen, Magnesium, and Nickel), they saw that the material wasn't spread out evenly. Some elements were clumped together in specific spots, creating a "double-peaked" shape in the light, like a mountain range with two distinct peaks instead of one smooth hill. This suggests that the star shot out at least two large, heavy streams of nickel-rich material.

2. The Star Was Wearing a "Scarf" Before It Died

Before the star exploded, it wasn't just sitting there; it was shedding material, creating a cloud of gas and dust around it called Circumstellar Material (CSM).

The Analogy: Imagine the star was a person wearing a thick, uneven scarf. When the explosion happened, the blast wave hit this scarf.
The Discovery: The astronomers saw that the explosion didn't just blow through empty space. The fast-moving debris from the star crashed into this "scarf." Because the scarf wasn't a perfect sphere (it was likely shaped like a disk or a ring), the crash created a specific pattern of light.
The "Horns": As the debris hit this uneven scarf, it created bright, high-speed "horns" of light in the spectrum (a graph of the light). These horns are the signature of the explosion slamming into the material the star had shed earlier.

3. The Light Show Changed Over Time

The paper tracks how the "music" of the explosion changed as it got older.

Early Days (The Battery): At first, the light was powered by a radioactive "battery" inside the explosion (created by the decay of Nickel). This made the light glow steadily.
Later Days (The Engine): As the radioactive battery ran down, the explosion's light didn't fade away as expected. Instead, it stayed bright. Why? Because the explosion kept hitting that "scarf" (the CSM). The friction and shock from this collision acted like a new engine, keeping the light show going.
The "Middle" Mystery: At a certain point, the light showed a weird mix. The authors realized this mix could be explained by combining the "battery" light from early on with the "engine" light from later on. It's like hearing a song that starts with a piano solo and ends with a drum solo; in the middle, you hear a blend of both.

4. Dust and Asymmetry

The team also looked at whether dust (tiny solid particles) formed inside the explosion, which can block light and make things look lopsided.

The Finding: While dust did form, the authors argue that the lopsided shape they saw wasn't just because dust blocked the view. Even accounting for dust, the explosion itself was inherently uneven. The "plumes" of heavy elements were real physical structures, not just an optical illusion caused by dust.

The Big Picture

The main conclusion of this paper is that supernovae are naturally chaotic and asymmetric.

The authors propose a mental image of SN 2023ixf like this:

The Core: The star exploded with a "jet" or "plume" of heavy nickel shooting out in at least two different directions.
The Shell: Surrounding the explosion was a disk-like ring of gas the star had shed before dying.
The Crash: The nickel plumes shot out and crashed into this ring, creating a complex, multi-layered light show that astronomers are only now able to decode with the help of the James Webb Space Telescope.

This study helps scientists understand that when massive stars die, they don't just leave behind a uniform cloud; they leave behind a complex, 3D sculpture of debris that tells the story of how the star lived and how it died.

Technical Summary: Evidence for Asymmetric Ejecta and Circumstellar Material in SN 2023ixf

Problem and Motivation
Core-collapse supernovae (CCSNe) are fundamental to cosmic evolution, yet the detailed distribution of material in their ejecta remains an unresolved problem. While modern 3D simulations predict generic asymmetries in stellar envelopes, cores, and circumstellar material (CSM), observational constraints on the final ejecta structure and elemental distribution are limited. Nebular-phase observations (epochs $\gtrsim$ 100 days post-explosion) offer a unique window into these structures as opacity drops and spectra become dominated by emission lines powered by radioactive decay and shock interaction. However, interpreting these lines is complicated by blending, dust attenuation, and the lack of mid-infrared (MIR) data for most events. This work addresses the need for high-cadence, multi-wavelength (optical to MIR) nebular observations to constrain the geometry and composition of the ejecta in the nearby Type II supernova (SN) 2023ixf.

Methodology
The authors present extensive optical and near-infrared (NIR) observations of SN 2023ixf from +89 to +749 days after explosion, supplemented by NIR and MIR spectroscopy from the James Webb Space Telescope (JWST).

Data Acquisition: Optical photometry and spectroscopy were obtained using a suite of ground-based facilities (Las Cumbres Observatory, MMT, LBT, Keck I, Bok, Lick, Gemini-N) covering the range from +89 to +749 days. NIR spectra were acquired using IRTF/SpeX and MMT/MMIRS. JWST NIRSpec and MIRI/LRS data were incorporated to extend coverage into the MIR and fill temporal gaps.
Data Reduction: Spectra were reduced using standard pipelines (e.g., FLOYDS, Binospec, MODS, LPipe, Spextool) and corrected for foreground and host-galaxy extinction. Flux calibration was performed by scaling spectra to contemporaneous photometry.
Analysis Techniques:
- Line Profile Evolution: The authors tracked the temporal evolution of prominent emission lines (H $\alpha$ , He I, [O I], Mg I, [Ni I], etc.), focusing on velocity shifts, profile shapes (single-peaked, double-peaked, asymmetric), and the emergence of high-velocity components.
- Empirical Modeling: To disentangle decay-powered and shock-powered emission, the authors scaled and summed line profiles from early epochs (decay-dominated) and late epochs (shock-dominated) to reconstruct intermediate epochs.
- Geometric Fitting: Selected emission lines were fitted with Gaussian and skewed Gaussian components to infer the kinematics and geometry of emitting regions. The analysis compared neutral species (Mg I, Na I, [Ni I]) with ionized species ([O I], [Ca II], [Ni II]) to probe stratification and ionization effects.
- Shock Power Calculation: The reverse-shock luminosity ( $L_{sh}$ ) was estimated using H $\alpha$ velocity limits and analytic formalisms for a radiative reverse shock interacting with a steady wind.

Key Results

H $\alpha$ Evolution and CSM Interaction: The H $\alpha$ profile exhibits complex evolution. Initially blueshifted, it develops high-velocity "CSM horns" (emission at $\sim \pm 7000$ km s $^{-1}$ ) starting around +199 days, consistent with the outer ejecta interacting with low-density CSM. By +600 days, the heating source transitions from radioactive decay to shock interaction.
Decomposition of Line Profiles: The H $\alpha$ profile at intermediate epochs (e.g., +375 days) can be empirically reconstructed by scaling and summing an early decay-powered component and a late shock-powered component. This reveals an intermediate-width component ( $\sim \pm 3000$ km s $^{-1}$ ) that emerges as decay fades, interpreted as ejecta crashing into swept-up, initially aspherical dense CSM (likely a disk or torus).
Asymmetric Ejecta Geometry:
- Double-Peaked Neutral Lines: Neutral transitions of intermediate-mass elements (Mg I 1.504 $\mu$ m, Na I 2.206 $\mu$ m) and iron-group elements ([Ni I] 3.12 $\mu$ m) exhibit clear double-peaked profiles between +200 and +374 days. The peaks are separated by $\sim 2000-3000$ km s $^{-1}$ .
- Symmetric Ionized Lines: In contrast, ionized species ([Ar II], [Ne II], [Ni II], [Co II]) generally show broader, more symmetric, single-peaked profiles centered near zero velocity.
- Interpretation: The authors posit that the double-peaked profiles arise from an intrinsically asymmetric distribution of Ni-rich material (at least two large plumes) that heats the ejecta inhomogeneously. The symmetric ionized lines may trace warmer, more diffuse regions or be smoothed by resolution effects.
Blueshifted Oxygen and Carbon: [O I] $\lambda\lambda$ 6300, 6364 and [C I] $\lambda$ 8727 show persistent blueshifts ( $\sim -1600$ km s $^{-1}$ ) throughout the nebular phase. This suggests an asymmetric distribution of O-rich material or dust attenuation, though the weak wavelength dependence of the asymmetry favors intrinsic geometry over dust extinction as the primary driver.
Shock Power Dominance: Calculations of the reverse-shock luminosity indicate that by +600 days, shock interaction dominates the heating of the ejecta, consistent with the flattening of the optical light curve.

Significance and Claims
The paper claims that SN 2023ixf provides robust evidence for intrinsic asymmetries in both the ejecta and the circumstellar material of a Type II supernova.

Ejecta Asymmetry: The double-peaked profiles of neutral metal lines support a geometry where Ni-rich material is ejected in large, asymmetric plumes, consistent with 3D hydrodynamic simulations of core-collapse mechanisms. This challenges simple spherical or purely bipolar models, suggesting a more complex, turbulent distribution of heavy elements.
CSM Geometry: The evolution of the H $\alpha$ profile and the emergence of intermediate-width components support a model where the progenitor possessed a dense, aspherical CSM (e.g., a disk or torus) that was swept up by the forward shock.
Methodological Advance: The study demonstrates the power of combining extensive ground-based optical/NIR monitoring with JWST MIR spectroscopy to disentangle the effects of dust, ionization stratification, and geometric asymmetry in CCSN ejecta. The authors note that the similarity of these findings to SN 2024ggi suggests that intrinsic asymmetry may be a common feature of Type II supernovae, though further 3D simulations and observations are required to confirm this generality.

The authors remain modest regarding the definitive nature of the ejecta geometry, acknowledging that dust formation and resolution effects could influence the observed line profiles, and call for future self-consistent modeling to fully disentangle these factors.

1. The Star Didn't Explode Evenly

2. The Star Was Wearing a "Scarf" Before It Died

3. The Light Show Changed Over Time

4. Dust and Asymmetry

The Big Picture

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