Massive stars exploding in a He-rich circumstellar medium XII. SN 2024acyl: A fast, linearly declining Type Ibn supernova with early flash-ionisation features

This paper presents a comprehensive photometric and spectroscopic analysis of the fast, linearly declining Type Ibn supernova SN 2024acyl, revealing that its properties are best explained by ejecta interacting with a helium-rich circumstellar medium originating from a low-mass helium star in an interacting binary system.

The Gist

Here is an explanation of the paper "SN 2024acyl: A fast, linearly declining Type Ibn supernova with early flash-ionisation features," translated into simple, everyday language with creative analogies.

The Big Picture: A Cosmic Firework Show

Imagine a star that has lived a long, turbulent life and is now about to die. Usually, when a massive star dies, it explodes in a spectacular supernova. But this specific star, which gave birth to SN 2024acyl, didn't just explode; it exploded inside a thick, helium-rich fog it had created around itself.

Think of it like a firework being launched inside a thick cloud of helium gas. The explosion hits the gas immediately, creating a unique, bright, and fast-fading light show that astronomers call a Type Ibn supernova.

The Main Characters

• The Star (The Progenitor): This wasn't a typical giant star. The paper suggests it was likely a low-mass helium star that was part of a binary system (a pair of stars dancing around each other).
  • The Analogy: Imagine two dancers. One is the star that explodes, and the other is its partner. As they danced, the partner stole most of the exploding star's hydrogen "clothes," leaving it naked and made mostly of helium. This stripped star then exploded.
• The Surroundings (The CSM): Before exploding, the star had been coughing up a dense shell of helium gas.
  • The Analogy: It's like a person blowing up a giant, thick balloon around themselves right before they pop. When the person pops, the explosion hits the balloon immediately.

What Did We See? (The Observations)

Astronomers watched this event closely from Earth and space, and here is what they found:

1. The Flash (The "Flash-Ionisation")
Right at the very beginning, the explosion sent out a burst of intense light that hit the surrounding helium gas.

• The Analogy: Imagine a camera flash going off in a dark room filled with fog. The fog lights up instantly, glowing brightly for a split second before fading. This "flash" told astronomers that the star was surrounded by gas before it exploded.

2. The Light Curve (The Brightness)
The supernova got bright very quickly (in about 10 days) and then faded away in a straight, fast line.

• The Analogy: Most supernovae are like a slow-burning candle that glows for a long time. SN 2024acyl was more like a sparkler: it flared up instantly and then burned out quickly and steadily. This fast fade told scientists that the exploding material wasn't very heavy (low mass).

3. The Spectra (The Fingerprint)
When scientists split the light from the explosion into a rainbow (a spectrum), they saw specific lines.

• Helium: The dominant color was helium.
• Hydrogen: There was a tiny bit of hydrogen left over, like a few stray threads of the original "clothes" the star lost.
• The "Flash" Lines: Early on, they saw signs of highly charged carbon and nitrogen, confirming the "flash" theory.

The Detective Work: Solving the Mystery

The astronomers used computer models to figure out exactly what happened. They treated the light curve like a puzzle.

• The Mass: They calculated that the star didn't have much "stuff" left to explode. It was a lightweight, only about half the mass of our Sun (0.5 solar masses).
• The Energy: The explosion wasn't a massive, earth-shattering boom. It was a relatively "gentle" explosion compared to other supernovae, but it was powered by the collision of the debris with the helium shell.
• The Distance: It happened about 360 million light-years away, in the outskirts of a galaxy called CGCG 505-052. Because it was far out in the "suburbs" of the galaxy (where new stars aren't being born much), it supports the idea that this was an older, lower-mass star, not a brand-new, massive monster.

The Verdict: What Kind of Star Was It?

The paper discusses a few possibilities, but the evidence points to one main story:

The "Stripped Partner" Theory:
The most likely scenario is that this was a low-mass helium star in a binary system. Its partner star stole its hydrogen envelope, leaving it as a naked helium core. When this core finally ran out of fuel, it exploded. The explosion smashed into the helium shell the star had left behind, creating the unique "Type Ibn" signature.

• Why not a massive Wolf-Rayet star? Those are huge, heavy stars. SN 2024acyl was too light and too faint for that.
• Why not a merger? It's possible, but the "stripped partner" story fits the data better.

Why Does This Matter?

Supernovae are the universe's recycling plants. They create heavy elements and spread them across space to make new stars and planets.

• SN 2024acyl is special because it shows us a "middle ground" between different types of explosions. It proves that not all supernovae come from massive, lonely giants. Some come from smaller stars that had their lives changed by a partner.
• It helps astronomers understand the complex "breakups" and "stealing" that happen in binary star systems before they die.

Summary in One Sentence

SN 2024acyl was a fast, bright, helium-rich explosion caused by a small, stripped star that was likely robbed of its hydrogen by a partner star, leaving it to explode into a cloud of its own making.

Technical Summary

Here is a detailed technical summary of the paper "Massive stars exploding in a He-rich circumstellar medium XII. SN 2024acyl: A fast, linearly declining Type Ibn supernova with early flash-ionisation features."

1. Problem and Scientific Context

Type Ibn supernovae (SNe Ibn) are a rare subclass of core-collapse supernovae characterized by narrow helium (He I) emission lines and little to no hydrogen in their spectra, indicating interaction with a helium-rich circumstellar medium (CSM). While the general consensus suggests they originate from massive Wolf-Rayet (WR) stars, the diversity in their photometric and spectroscopic properties implies multiple progenitor channels may exist. These include:

• Massive WR stars in single or binary systems.
• Low-mass helium stars in interacting binaries.
• Merger-driven events or white dwarf interactions.
• Transitional objects between Type IIn (hydrogen-rich) and Type Ibn.

The specific nature of the progenitor systems for SNe Ibn remains an open question, particularly regarding the origin of the dense CSM and the presence of residual hydrogen in some events. SN 2024acyl presents a unique case study due to its specific light-curve morphology and early spectral features.

2. Methodology

The authors conducted a comprehensive multi-wavelength study of SN 2024acyl, discovered by ATLAS on December 1, 2024.

• Observations:
  • Photometry: Extensive multi-band (UV to optical) light curves were obtained using facilities including Swift/UVOT, ATLAS, Pan-STARRS, the Nordic Optical Telescope (NOT), the 2.4m Lijiang Telescope (LJT), and the Mephisto telescope.
  • Spectroscopy: 12 optical spectra were collected from early phases (−3.7 days) to late phases (+42.8 days) using instruments such as EFOSC2 (NTT), YFOSC (LJT), Kast (Shane), DOLORES (TNG), and GMOS-N (Gemini-North).
• Data Analysis:
  • Light Curve Modeling: The authors employed the MOSFiT (Modular Open Source Fitter for Transients) code to fit the multi-band light curves. They utilized a hybrid model combining Radioactive Decay (RD) of $^{56}\text{Ni}$ and Circumstellar Interaction (CSI). They tested various density profiles for the ejecta ($n$) and CSM ($s=0$ for shell-like, $s=2$ for wind-like) using nested sampling (dynesty) to derive posterior distributions for physical parameters.
  • Spectral Modeling: Observed spectra were compared with non-local thermodynamic equilibrium (non-LTE) radiative transfer models generated by CMFGEN. Specifically, they used models from Dessart et al. (2022) tailored for low-mass helium-star progenitors (model he4p0).
  • Comparative Analysis: SN 2024acyl was compared against a sample of other SNe Ibn (e.g., SN 2006jc, SN 2010al, SN 2019cj) and transitional events (e.g., SN 2018gjx) to contextualize its photometric and spectroscopic evolution.

3. Key Contributions and Results

Photometric Properties

• Light Curve: SN 2024acyl rose to a peak absolute magnitude of $M_o = -17.58 \pm 0.15$ mag in $\sim 10.6$ days. It exhibited a fast, linear post-peak decline ($\gamma_{0-60(V)} = 0.097 \pm 0.002$ mag day$^{-1}$), characteristic of SNe Ibn but faster than many typical cases.
• Luminosity: The peak optical pseudobolometric luminosity was $(3.5 \pm 0.8) \times 10^{42}$ erg s$^{-1}$, with a total radiated energy of $(5.0 \pm 0.4) \times 10^{48}$ erg. It is sub-luminous compared to the average SN Ibn ($M_r \approx -19$ mag) but brighter than the faintest known SN Ibn (SN 2023utc).
• Color Evolution: The $B-V$ color evolved rapidly from blue to red, then blued slightly before reddening again, showing heterogeneity compared to other SNe Ibn.

Spectroscopic Evolution

• Early Phase (Flash Ionization): Early spectra (pre- and post-maximum) showed a blue continuum with narrow P-Cygni He I lines and prominent flash-ionization features (C III, N III, He II). These features indicate the presence of a dense, pre-existing CSM ionized by the shock breakout or early ejecta-CSM interaction.
• Composition: The spectra were dominated by He I lines throughout. However, H$\alpha$ was detected throughout the evolution, becoming more prominent at late phases. This confirms the CSM is helium-rich but contains a residual amount of hydrogen.
• Velocity: Narrow He I components indicated CSM velocities of $\sim 1000-1300$ km s$^{-1}$. Broader components (ejecta) reached $\sim 5800-6400$ km s$^{-1}$.

Physical Parameter Estimation (MOSFiT Modeling)

The best-fit model (assuming a shell-like CSM, $s=0$, and ejecta density $n=10$) yielded:

• Ejecta Mass ($M_{ej}$): $0.49^{+0.11}{-0.09} M{\odot}$ (Low mass).
• Kinetic Energy ($E_k$): $0.06^{+0.01}_{-0.01} \times 10^{51}$ erg (Low energy).
• $^{56}\text{Ni}$ Mass ($M_{Ni}$): $0.018 M_{\odot}$.
• CSM Mass ($M_{CSM}$): $0.51^{+0.05}{-0.04} M{\odot}$.
• CSM Geometry: Inner radius $R_0 = 17.8^{+3.6}_{-3.0}$ AU, density $\rho_{CSM} \approx 8.3 \times 10^{-12}$ g cm$^{-3}$. The CSM is a relatively thin shell ($\sim 6.5$ AU thick).

Spectral Modeling

Comparison with CMFGEN models (he4p0, corresponding to a He-ZAMS mass of $4 M_{\odot}$and pre-SN mass of$3.16 M_{\odot}$) successfully reproduced the spectral evolution, particularly the dominance of He I lines and the presence of Fe II and Ca II features at late times. The models suggested a compact, dense shell formed by the interaction of low-mass ejecta with a He-rich CSM.

4. Significance and Progenitor Scenarios

The study proposes that SN 2024acyl is likely the explosion of a low-mass helium star ($\sim 3-4 M_{\odot}$) that evolved within an interacting binary system.

• Binary Channel: The low ejecta mass and low kinetic energy are inconsistent with the core collapse of a very massive single WR star. Instead, they align with a scenario where a lower-mass star was stripped of its hydrogen envelope via mass transfer to a companion. The dense, shell-like CSM likely originated from an eruptive mass-loss event or unstable mass transfer shortly before the explosion.
• Residual Hydrogen: The detection of H$\alpha$ suggests the progenitor was not completely stripped of hydrogen, or that the hydrogen was confined to the outer layers of the CSM, possibly transferred from the companion star.
• Alternative Scenarios:
  • Massive WR with Fallback: A late-type WR star with fallback accretion is not entirely ruled out but is less favored due to the high ejecta velocity and lack of predicted oxygen-rich chemical signatures.
  • Merger: A merger-driven destruction of a WR star is a possible but less constrained alternative.
  • Type IIb in He-rich CSM: While similar to SN 2018gjx, the weak early H$\alpha$ and strong interaction signatures make a standard Type IIb classification unlikely.

Conclusion

SN 2024acyl represents a distinct subclass of SNe Ibn characterized by a fast, linear decline, low explosion energy, and a low-mass ejecta interacting with a dense, shell-like, helium-rich CSM containing residual hydrogen. The findings strongly support a progenitor channel involving a low-mass helium star in a binary system, challenging the assumption that all SNe Ibn originate from very massive single stars. This work highlights the diversity of progenitor systems for interacting supernovae and underscores the importance of early flash-ionization features and precise light-curve modeling in constraining explosion physics.