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NLTE Spectral Modelling of the Nearby Stripped-Envelope Supernova 2024ehs

This paper presents a detailed NLTE spectral modelling study of the nearby Type IIb supernova 2024ehs using the \texttt{SUMO} code, revealing its distinct narrow light-curve peak and weak helium lines to infer a low ejecta mass, high expansion velocities, and a massive progenitor consistent with binary interaction scenarios.

Original authors: Wilmer Sand Hellman, Seán J. Brennan, Stan Bartmentloo, Christoffer Fremling, Anjasha Gangopadhyay, Anders Jerkstrand, Ragnhild Lunnan, Josiah Purdum, Steve Schulze, Jesper Sollerman

Published 2026-07-01

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CC BY 4.0

Original authors: Wilmer Sand Hellman, Seán J. Brennan, Stan Bartmentloo, Christoffer Fremling, Anjasha Gangopadhyay, Anders Jerkstrand, Ragnhild Lunnan, Josiah Purdum, Steve Schulze, Jesper Sollerman

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 the universe as a giant, cosmic stage. Most of the time, the stars are the actors, living out their long lives quietly. But occasionally, a star reaches its final act and explodes in a spectacular display known as a supernova. This paper is a detailed investigation into one specific "actor" that took the stage in March 2024: a star called SN 2024ehs.

Here is the story of what the astronomers found, explained without the heavy jargon.

The Mystery Guest

SN 2024ehs is a "stripped-envelope" supernova. Think of a star like a layered onion. Usually, the outer layer is hydrogen. When a star explodes, it throws all its layers out. But this particular star had its outer "skin" (the hydrogen) peeled off before it exploded, likely because it was in a tight dance with a companion star that stole the skin away.

Because it lost its skin, it belongs to a special club called Type IIb. These are the "chameleons" of the supernova world: they start looking like a hydrogen-rich star (Type II) but quickly morph into a helium-rich star (Type Ib) as the explosion fades.

The Performance: A Fast and Faint Show

The astronomers watched this star for about 10 months, tracking its brightness (light curve) and its "voice" (spectrum). Here's what they noticed:

The Flash: The star lit up quickly and then faded away faster than most of its peers. It's like a firework that explodes with a sharp pop and vanishes almost immediately, rather than a slow-burning flare.
The Missing Skin: Even though it's classified as a Type IIb (which should have some hydrogen left), the hydrogen signals were very faint. It's as if the star was so stripped down that it barely had any "skin" left to show.
The Silent Helium: Usually, these stars shout with bright helium lines. SN 2024ehs, however, was very quiet about its helium. The helium lines were weak and hard to hear.

The Detective Work: How They Figured It Out

To understand why this star behaved this way, the team used a supercomputer simulation called SUMO. Think of SUMO as a cosmic "flight simulator." The astronomers fed the computer different scenarios (different star masses, different amounts of radioactive fuel) to see which one matched the real-life data.

Here is what the simulation revealed:

The Star Was Light: The explosion came from a star that wasn't as heavy as the "heavyweights" in the galaxy. The team estimates the star's core was about 6 times the mass of our Sun before it exploded.
The Original Size: Before it lost its skin and shrank, the star was likely a massive giant, about 23 times the mass of our Sun.
The Fuel Tank: The explosion was powered by radioactive nickel (the "fuel" that makes the light shine). The team estimates there was about 0.1 solar masses of this nickel. This is a modest amount, which explains why the light faded so quickly—there wasn't a huge tank of fuel to keep the show going.
The Speed: The debris from the explosion was flying incredibly fast, at about 20,000 kilometers per second. That's fast enough to circle the Earth 500 times in a single second!

The "Broken" Voice

One of the most interesting findings was about the star's "voice" (its spectrum). In a typical Type IIb supernova, there is a specific line in the spectrum where hydrogen and helium overlap, creating a "broken" or split look.

In SN 2024ehs, this line looked strange. The astronomers found that the "broken" look was actually caused by a very weak helium signal trying to hide inside the hydrogen signal. By comparing the speed of the gas moving in different parts of the explosion, they realized that the helium was moving so fast and was so thin that it barely made a mark. It's like trying to hear a whisper in a hurricane; the wind (hydrogen) is so loud that the whisper (helium) is almost lost.

The Big Picture

This paper tells us that supernovae are more diverse than we thought. Not all "stripped" stars are the same. Some are heavy and slow; others, like SN 2024ehs, are lighter, faster, and have lost almost all their outer layers.

The study confirms that binary stars (stars in pairs) are likely the reason these stars get stripped. The companion star acts like a cosmic vacuum cleaner, stealing the hydrogen layers before the explosion happens.

Summary

In short, SN 2024ehs was a fast, faint, and fast-moving explosion from a star that had been stripped of its outer layers by a partner. It was a "lightweight" champion that burned its fuel quickly and left the stage sooner than the heavyweights. By studying it, astronomers are learning that the universe has many different ways for stars to end their lives, and the "stripped-envelope" family is full of unique characters.

Technical Summary: NLTE Spectral Modelling of the Nearby Stripped-Envelope Supernova 2024ehs

Problem and Context
Stripped-envelope supernovae (SESNe), particularly Type IIb, occupy a transitional classification between Type II (hydrogen-rich) and Type Ib (helium-rich, hydrogen-poor) events. Their progenitors are thought to lose their outer hydrogen envelopes via strong stellar winds or binary mass transfer. However, the exact boundaries between sub-types, the role of mixing in hiding residual hydrogen, and the specific progenitor masses required to produce observed spectral features remain areas of active investigation. This study addresses these uncertainties by presenting a detailed analysis of SN 2024ehs, a nearby Type IIb supernova discovered in March 2024 in the galaxy NGC 3443. The object exhibits a narrow light-curve peak, rapid decline, and notably weak helium lines, distinguishing it from typical Type IIb events and necessitating a rigorous investigation into its physical properties and progenitor history.

Methodology
The authors utilized a comprehensive dataset spanning ten months of photometric and spectroscopic observations.

Observations: Photometry in $g$ , $r$ , and $o$ bands was obtained from the Zwicky Transient Facility (ZTF) and the Asteroid Terrestrial-impact Last Alert System (ATLAS). Spectroscopy was acquired using the ALHAMBRA Faint Object Spectrograph and Camera (ALFOSC) on the Nordic Optical Telescope (NOT) for the photospheric phase and the DBSP spectrograph on the Palomar 200-inch telescope for the nebular phase (7 months post-peak).
Analysis Techniques:
- Light Curve Analysis: The light curve was analyzed for peak magnitude, decline rates, and comparison with standard radioactive decay tails ( $^{56}$ Co $\to$ $^{56}$ Fe).
- Spectral Evolution: Absorption line velocities (specifically H $\alpha$ and He I $\lambda$ 5876) were measured by fitting parabolas to absorption features to track velocity evolution.
- Radiative Transfer Modelling: The authors employed the SUMO (SUpernova MOnte carlo) code, a non-local thermodynamic equilibrium (NLTE) Monte Carlo radiative transfer code.
  - Nebular Modelling: SUMO models with varying pre-explosion masses, $^{56}$ Ni masses, and mixing values were compared to the 7-month post-peak spectrum. The models were scaled based on distance ( $d = 23.8$ Mpc) and estimated $^{56}$ Ni mass.
  - Photospheric Modelling: A grid of four SUMO models was created to investigate the weak feature in the H $\alpha$ emission, specifically varying the velocity of the lowest velocity helium-rich material ( $V_{He}$ ).

Key Results

Light Curve Properties: SN 2024ehs reached an absolute peak magnitude of $-17.37$ in the $r$ -band. The light curve displayed a narrow peak and a steep decline in the tail phase ( $\sim 1.82$ mag/100d), significantly steeper than the standard $^{56}$ Co decay rate (0.98 mag/100d). This suggests inefficient $\gamma$ -ray trapping and a low ejecta mass. No initial shock-cooling phase was observed, implying a compact progenitor without a surrounding shell.
Spectral Characteristics: The spectra are dominated by hydrogen features, confirming a Type IIb classification, yet the He I $\lambda$ 6678 absorption is unusually weak. The H $\alpha$ line developed a "boxy" or top-hat profile rather than the typical P-Cygni shape. The expansion velocities were high, with hydrogen absorption features measured at $\sim 20,000$ km s $^{-1}$ in the earliest spectra.
Progenitor and Ejecta Parameters:
- $^{56}$ Ni Mass: Based on the light curve placement between SN 1993J and SN 2020acat, the $^{56}$ Ni mass is estimated to be just below $\sim 0.1$ M $_{\odot}$ (specifically $\sim 0.09$ M $_{\odot}$ in the models).
- Progenitor Mass: Nebular spectral modelling indicates the best fit with a model having a helium core mass of $\sim 6$ M $_{\odot}$ . This corresponds to a zero-age main-sequence (ZAMS) mass of $\sim 23$ M $_{\odot}$ and a pre-explosion core mass of 4.45 M $_{\odot}$ .
- Composition and Velocity: The ratio of [O I] $\lambda\lambda$ 6300,6364 to [Ca II] $\lambda\lambda$ 7291,7323 supports the $\sim 6$ M $_{\odot}$ helium core model. The study also notes an unusually strong [Fe II] $\lambda$ 7155 line. Photospheric modelling suggests a link between the expansion velocity and the relative strength of different element lines, with the weak helium feature potentially attributed to the specific velocity structure of the helium-rich layers.

Significance and Claims
The paper claims that SN 2024ehs represents a distinct variant within the Type IIb population, characterized by a compact progenitor, low ejecta mass, and high expansion velocities. The authors assert that the nebular spectral models provide strong evidence for a progenitor with a $\sim 6$ M $_{\odot}$ helium core, consistent with binary interaction scenarios rather than single-star Wolf-Rayet evolution.

The study highlights the diversity of stripped-envelope supernovae, noting that the "broken" H $\alpha$ emission typical of Type IIb events can manifest in a wide range of appearances, potentially linked to the relative velocities of spectral lines. The authors conclude that while the current modelling refines the understanding of SN 2024ehs, further modelling is necessary to precisely constrain $^{56}$ Ni mass estimates and to fully understand the physical mechanisms driving the evolution of such high-velocity, low-mass ejecta systems. The work underscores the importance of binary interactions in the formation of these progenitors.