Sub-luminous Type IIP SN 2024abfl as a result of a significantly low energy Fe-core collapse

This paper presents multiwavelength observations and hydrodynamical modeling of the exceptionally faint Type IIP supernova SN 2024abfl in NGC 2146, revealing it to be the result of a low-energy core-collapse explosion from a compact, low-mass progenitor that provides new constraints on the mechanisms of such events.

The Gist

Imagine the universe as a giant, bustling construction site where massive stars are the skyscrapers. Most of these skyscrapers, when they reach the end of their lives, don't just crumble quietly; they explode in spectacular, blinding fireworks known as supernovae. Usually, these explosions are like massive fireworks displays, bright enough to outshine entire cities (galaxies) for weeks.

But astronomers recently spotted a very different kind of explosion: SN 2024abfl.

Think of this event not as a massive firework, but as a very quiet, dim sparkler compared to the usual pyrotechnics. Here is the story of what the scientists found, explained simply:

1. The "Faintest Sparkler" in the Neighborhood

The team discovered a supernova named SN 2024abfl. It is a "Type IIP" supernova, which is a specific family of stellar explosions that usually have a long, flat "plateau" phase where they stay bright for a while before fading.

However, SN 2024abfl is the faintest member of this family ever recorded.

• The Analogy: If a normal supernova is like a bright stadium floodlight, SN 2024abfl is like a single, dim nightlight.
• The "Flat" Plateau: Usually, these stars fade away gradually. But this one was incredibly stubborn. Its brightness stayed almost exactly the same for a long time, dropping so slowly it was like watching a candle that refuses to burn down. It held its dim glow for about 126 days with almost no change.

2. A Mystery of Distance

The star exploded in a galaxy called NGC 2146. For a long time, astronomers argued about how far away this galaxy is. Some said it was far (like 15 million miles away), others said it was closer.

• The Detective Work: The team used a method called the "Expanding Photosphere Method." Imagine watching a balloon inflate. If you know how fast the air is pushing the rubber out, and you measure how big the balloon looks from your window, you can calculate exactly how far away you are.
• The Result: They determined the galaxy is actually quite close (about 7 to 9 million light-years away). This distance is crucial because it confirmed that the star wasn't just "far away and dim"; it was genuinely intrinsically faint. It was a weak explosion to begin with.

3. The "Slow-Motion" Explosion

When stars explode, they shoot out gas at incredible speeds.

• The Analogy: A normal supernova is like a cannon firing a cannonball at 10,000 miles per hour. SN 2024abfl was more like a gentle breeze pushing a feather.
• The Evidence: By looking at the light (spectra), the team saw that the gas from this explosion was moving very slowly. The lines in the light were very narrow, indicating the material wasn't rushing outward with much force.

4. The "Weak Engine" and the Tiny Progenitor

Why was it so weak? The team used powerful computer simulations (like a virtual crash-test for stars) to figure out what kind of star caused this.

• The Progenitor: They found the star that exploded was likely a Red Supergiant (a huge, cool star) but on the smaller end of the "giant" scale, weighing about 9 to 10 times the mass of our Sun.
• The Energy: The explosion was incredibly low-energy.
  • The Analogy: A typical supernova releases energy equivalent to a billion nuclear bombs. This one released energy equivalent to a very small firecracker in comparison (about 0.05 "foe," a unit astronomers use for supernova energy).
• The Nickel: These explosions usually create a heavy metal called Nickel-56, which acts like a radioactive battery to keep the star glowing. SN 2024abfl had almost no battery left; it produced a tiny amount of Nickel (about 0.003 times the mass of our Sun). This explains why it was so dim.

5. The Neighborly Connection

Interestingly, this faint star exploded in the same galaxy as another famous supernova, SN 2018zd, which happened just a few years ago. They are neighbors in the cosmic sense. While SN 2018zd was a different type of event, having two such distinct explosions in the same galaxy helps astronomers understand the different ways stars can die in that specific environment.

The Bottom Line

SN 2024abfl is a cosmic anomaly. It proves that not all massive stars go out with a bang. Some, specifically those that are on the lower end of the mass scale, can have a "low-energy" collapse. They don't need a massive explosion to die; they can simply fizzle out with a gentle, dim, and very slow fade.

This discovery helps astronomers understand the full range of how stars end their lives, showing us that even the "weak" explosions have a story to tell about the physics of the universe.

Technical Summary

Technical Summary: Sub-luminous Type IIP SN 2024abfl as a result of a significantly low energy Fe-core collapse

Problem and Motivation
The nature of low-luminosity Type IIP supernovae (LLIIP) remains a subject of debate, particularly regarding their progenitor masses and explosion mechanisms. While direct imaging often suggests low-mass red supergiant (RSG) progenitors ($9-12 M_\odot$), hydrodynamical models and analytical constraints have historically yielded a wide range of masses, including moderate-to-high mass stars with significant fallback. Furthermore, the distance to the host galaxy NGC 2146, where the nearby SN 2024abfl and the well-studied SN 2018zd reside, has been uncertain (ranging from $\sim13$ to $22$ Mpc in literature), complicating the derivation of intrinsic explosion parameters. This paper addresses these uncertainties by presenting extensive multiwavelength observations of SN 2024abfl to constrain its distance, explosion energy, nickel mass, and progenitor properties.

Methodology
The authors conducted extensive photometric and spectroscopic monitoring of SN 2024abfl from discovery (JD 2460630.08) through the nebular phase ($\sim160$ days).

• Observations: High-cadence photometry was obtained using the GROWTH-India Telescope (GIT) and the 2.0-m Himalayan Chandra Telescope (HCT) in SDSS filters, supplemented by ZTF, ATLAS, and Swift/UVOT data. Optical spectra were secured using the HCT's HFOSC instrument, covering 4000–9000 Å.
• Distance Determination: The authors performed an independent distance analysis using the Standardized Candle Method (SCM) and the Expanding Photosphere Method (EPM). They utilized Fe II $\lambda$5169 velocities and color temperatures, applying dilution factors from Hamuy et al. (2001), Dessart & Hillier (2005), and Vogl et al. (2019).
• Physical Parameter Estimation: The $^{56}$Ni mass was estimated by comparing the pseudo-bolometric light curve to SN 1987A and by fitting radioactive decay models.
• Hydrodynamical Modeling: The authors employed the MESA (Modules for Experiments in Stellar Astrophysics) code to evolve progenitor models and the STELLA code to simulate the explosion and synthetic observables (light curves and velocities). They tested a range of Zero-Age Main Sequence (ZAMS) masses (9–15 $M_\odot$) and explosion energies.

Key Results

• Distance: The EPM and SCM analyses yield a distance to NGC 2146 of approximately $7-9$ Mpc (specifically $9.6 \pm 1.0$ Mpc is adopted for analysis), significantly lower than some previous estimates ($\sim15.6$ Mpc).
• Light Curve Properties: SN 2024abfl is identified as one of the faintest Type IIP supernovae discovered, with a mid-plateau absolute magnitude of $M_V \approx -13.8$ mag. It exhibits an unprecedentedly flat plateau evolution with a decline rate of only $0.10 \pm 0.04$ mag/100 days. The plateau duration is estimated at $126 \pm 1$ days.
• Explosion Parameters: The event is characterized by extremely low explosion energy ($E_{exp} \lesssim 0.05$ foe) and a very low synthesized nickel mass ($M_{Ni} \approx 0.0035 \pm 0.0006 M_\odot$).
• Spectral Evolution: Spectra show narrow P-Cygni profiles indicating low expansion velocities ($\sim1500-2000$ km s$^{-1}$ for Fe II). The spectral evolution is similar to other LLIIP events but with distinctively narrower lines. A peculiar double-peaked H$\alpha$ profile emerged around mid-plateau, potentially linked to cold dense shells or specific metal lines, though not definitively attributed to CSM interaction in this work.
• Progenitor Constraints: Hydrodynamical modeling indicates that higher-mass progenitors ($\gtrsim 12-15 M_\odot$) fail to reproduce the observed faint luminosity and low velocities. The observations are best matched by the explosion of a compact, low-mass RSG progenitor with a ZAMS mass of $9-10 M_\odot$, an ejecta mass of $7-8 M_\odot$, and a radius $< 500 R_\odot$.
• Explosion Mechanism: Color evolution analysis at the plateau-to-tail transition suggests the event originated from an Iron-core collapse rather than an Electron-Capture Supernova (ECSN), despite the low energy.

Significance and Claims
The paper claims that SN 2024abfl represents an extreme case of a low-energy Fe-core collapse explosion. By combining independent distance measurements with detailed hydrodynamical modeling, the authors provide strong constraints that favor a low-mass ($9-10 M_\odot$) progenitor over the higher-mass scenarios often proposed for LLIIP events. The study highlights that such low-energy explosions can occur in Fe-core collapse scenarios without requiring significant fallback from massive progenitors, thereby offering important constraints on the limits of stellar evolution and the diversity of core-collapse mechanisms. The authors note that while their models satisfactorily reproduce the general behavior, a perfect one-to-one match for the plateau luminosity might require even more compact progenitors or different mixing of $^{56}$Ni, which were not fully explored.