SN 2024hpj: A perspective on SN 2009ip-like events

This paper analyzes the spectrophotometric evolution of SN 2024hpj and a broader sample of SN 2009ip-like events to characterize their star-forming origins, propose four sub-classes based on light curve diversity linked to circumstellar medium variations, and estimate progenitor masses between 25 and 31 solar masses.

The Gist

Imagine the life of a massive star as a dramatic, high-stakes play. Usually, when these stars reach the end of their lives, they put on a spectacular finale: a supernova. But sometimes, the script gets weird.

This paper is about a specific type of cosmic drama called "SN 2009ip-like events." Think of these as stars that have a "false alarm" before the real explosion.

Here is the breakdown of the story, told in simple terms:

1. The "False Alarm" (Event A)

Imagine a firework that fizzes and sparks a little bit, then goes dark. That's Event A.

• What happens: A massive star starts acting up. It has a small outburst, glowing brighter than usual but not too bright. It's like the star is coughing up a bit of its outer skin (gas and dust).
• The confusion: Astronomers often think this is just a normal star having a bad day, not a death knell.

2. The "Grand Finale" (Event B)

Then, a few weeks or months later, the real show starts.

• What happens: The star actually explodes. This is Event B. It's much brighter and more violent. The shockwave from the explosion slams into the gas the star coughed up earlier (the "coughed-up skin").
• The result: This collision creates a massive, glowing fireball that we can see from millions of light-years away.

3. The New Star: SN 2024hpj

The main character of this paper is a new star called SN 2024hpj, discovered in 2024.

• The Triple Threat: Most of these "false alarm" stars have two peaks (the cough, then the explosion). SN 2024hpj is special because it has three. It had the small cough (Event A), the big explosion (Event B), and then a second smaller bump later on.
• The Analogy: Imagine a drummer hitting a snare drum once (Event A), then smashing the whole kit (Event B), and then hitting the cymbal one last time for a dramatic finish (the third peak).
• The Sound: When astronomers looked at the "sound" of the light (the spectrum), they saw a mix of fast-moving gas and slow-moving gas. It's like hearing a jet engine roar (fast) mixed with a gentle breeze (slow) at the same time. This confirms the explosion is hitting a dense cloud of gas left behind by the star.

4. The Neighborhood (Host Galaxies)

The authors looked at where these stars live.

• The Trend: These stars prefer to live in small, messy, star-forming neighborhoods (dwarf galaxies) rather than big, quiet cities (massive galaxies).
• Why it matters: These neighborhoods are like construction sites full of new, young, blue stars. It suggests these exploding stars are relatively young and massive, likely living in a chaotic environment.

5. The Mystery of the "Who" (Progenitors)

Who is the actor playing the star?

• The Debate: Is it a super-massive star that is unstable (like a volatile celebrity)? Or is it a star in a binary system (a star with a partner) that got into a fight?
• The Evidence: By counting how often these events happen and looking at how bright they are, the authors estimate the star was likely 25 to 31 times heavier than our Sun.
• The Binary Theory: The paper leans toward the idea that these stars might be in a "dance" with a partner. The partner might strip away the star's skin, or they might crash into each other, causing the "false alarm" before the final explosion.

6. The Big Picture

The authors grouped 24 of these weird supernovae into four "families" based on how their light curves (brightness over time) looked:

1. The Twins: Very similar to SN 2024hpj.
2. The Speedsters: Brighter and fading faster.
3. The Slowpokes: Fading very slowly.
4. The Plateau: Those that stay bright for a long time.

The Takeaway:
This paper tells us that the universe is full of stars that don't just "go boom." They have complex, multi-stage finales. By studying SN 2024hpj and its cousins, we are learning that these explosions are likely caused by massive stars (25+ Suns) that are either unstable or interacting with a partner, shedding layers of gas before finally dying in a spectacular, multi-peaked display.

It's like realizing that some fireworks don't just go off once; they have a pre-show, a main act, and an encore, all because of how the star was living its final days.

Technical Summary

Here is a detailed technical summary of the paper "SN 2024hpj: A perspective on SN 2009ip-like events" by Salmaso et al.

1. Problem Statement

Supernovae (SNe) IIn are massive star explosions interacting with dense circumstellar media (CSM). A specific subset, known as "SN 2009ip-like" events, exhibits a peculiar light curve characterized by an initial, fainter peak (Event A) attributed to pre-explosion stellar variability, followed by a brighter peak (Event B) interpreted as the terminal explosion. The nature of these events remains debated:

• Progenitor Identity: Are they Luminous Blue Variables (LBVs), Yellow Supergiants (YSGs), or products of binary mergers?
• Explosion Mechanism: Is Event B a true core-collapse supernova or a massive eruption?
• Diversity: How do variations in CSM mass, distribution, and progenitor mass influence the observed spectrophotometric properties?

The paper aims to address these questions by presenting a comprehensive analysis of the new transient SN 2024hpj and comparing it with a sample of similar objects to constrain progenitor masses and formation environments.

2. Methodology

The authors employed a multi-faceted approach combining deep observational follow-up with statistical analysis of a broader sample:

• Observations of SN 2024hpj:
  • Photometry: Multi-band light curves ($u, g, r, i, z, J, H, K$, and ATLAS bands) were obtained from various telescopes (NOT, TNG, GTC, CAHA, Asiago) and public surveys (ZTF, ATLAS, Pan-STARRS).
  • Spectroscopy: Optical spectra were acquired using ALFOSC, DOLORES, CAFOS, and OSIRIS+ from discovery through the nebular phase.
  • Data Reduction: Standard IRAF and Python-based pipelines (including ecsnoopy and hotpants) were used for bias correction, flat-fielding, cosmic ray rejection, and template subtraction to isolate the SN flux.
• Sample Compilation:
  • A sample of 24 SN 2009ip-like candidates was assembled, including 19 from literature and 5 unpublished objects (SNe 2019mry, 2022ytx, 2022mop, 2024uzf, 2025csc).
  • Selection criteria included the presence of Event A (or spectral signatures of Event B resembling SN 2009ip) and specific light curve morphologies.
• Analysis Techniques:
  • Light Curve Modeling: Pseudo-bolometric light curves were constructed by integrating flux across available bands. Event A and B peaks were fitted to determine magnitudes, durations, and separation times.
  • Spectral Analysis: Velocity evolution of P-Cygni profiles (H$\alpha$, He I, Fe II) and narrow emission components were measured. Blackbody (BB) fits were applied to continuum spectra to derive temperature, radius, and luminosity.
  • Host Galaxy Characterization: Morphological classification was performed via visual inspection and literature cross-matching. Spectral Energy Distribution (SED) fitting using BAGPIPES (with exponential star-formation histories and Calzetti extinction) was used to derive host stellar masses, star formation rates (SFR), and metallicities.
  • Statistical Rate Analysis: The observed rate of SN 2009ip-like events within 50–100 Mpc was compared against theoretical Core-Collapse Supernova (CCSN) rates derived from the Salpeter Initial Mass Function (IMF) to estimate progenitor mass ranges.

3. Key Contributions

• Discovery and Characterization of SN 2024hpj: The paper provides the first detailed spectrophotometric analysis of SN 2024hpj, identifying it as a triple-peaked SN IIn.
• Definition of Sub-classes: Based on light curve evolution (rise/decay times and peak luminosity), the authors categorize SN 2009ip-like events into four distinct sub-classes:
  1. Group 1: Similar to SN 2024hpj (rise/decay times and peak luminosity).
  2. Group 2: Brighter peaks with faster declines (similar to SN 2009ip).
  3. Group 3: Slower declines than SN 2024hpj.
  4. Group 4: Exhibiting a plateau phase after Event B.
• Host Environment Analysis: The study systematically characterizes the host galaxies of these events, revealing a preference for low-mass, star-forming dwarf galaxies, often located in spiral arms or outskirts.
• Progenitor Mass Constraints: By combining the observed event rate with theoretical CCSN rates, the authors derive statistical constraints on the progenitor masses, moving beyond single-object case studies.

4. Key Results

• SN 2024hpj Properties:

  • Light Curve: Features Event A ($M \approx -14.5$ mag), a gap of 13 days, Event B ($M \approx -16.3$ mag), and a distinct secondary peak 67 days after Event B. The secondary peak is attributed to ejecta-CSM interaction with a thin shell ejected ~1.8 years prior.
  • Spectra: Shows broad P-Cygni features topped by narrow emissions (typical of SNe IIn). The narrow H$\alpha$ component has a FWHM of $\sim 600$ km s$^{-1}$, implying a progenitor wind velocity of $220 \pm 30$km s$^{-1}$, inconsistent with Red Supergiants (RSGs) but consistent with LBVs or YSGs.
  • Energy Budget: The pseudo-bolometric light curve suggests a $^{56}$Ni mass upper limit of $0.07 M_\odot$, but the interaction with the CSM likely dominates the energy output, masking the radioactive decay signature.

• Sample Statistics & Correlations:

  • Event A vs. B: A positive correlation exists between the peak magnitudes of Event A and Event B, suggesting that brighter events possess more massive CSM.
  • Spectral Evolution: Group 1 objects (including SN 2024hpj) show composite H$\alpha$ profiles with narrow P-Cygni absorption, while other groups show more symmetric, electron-scattering dominated profiles.
  • Host Galaxies: The sample preferentially explodes in dwarf, star-forming galaxies (SFRs often above the main sequence). This contrasts with older samples found in grand-design spirals, likely due to survey depth improvements. The environments suggest young stellar populations.

• Progenitor Mass Estimation:

  • Statistical analysis of the event rate (approx. 1–2% of all CCSNe within 50 Mpc) suggests progenitor masses in the range of 25 – 31 $M_\odot$.
  • This range is consistent with intermediate-mass stars (possibly YSGs or LBVs) and potentially binary merger scenarios.
  • The authors note that if the sample is incomplete (due to faint Event A detections), the true mass range could extend to lower masses.

5. Significance

• Clarifying the SN 2009ip Enigma: The paper reinforces the view that SN 2009ip-like events are a distinct class of transients, likely representing the terminal explosion of massive stars ($25-31 M_\odot$) that underwent significant pre-explosion mass loss.
• Binary vs. Single Star: The results support scenarios involving binary interactions (mergers or stripping) or pulsational pair-instability (though the latter is disfavored by high expansion velocities). The diversity in light curves (Groups 1–4) suggests a continuum of progenitor properties (mass, envelope retention) and CSM geometries rather than a single uniform mechanism.
• Future Outlook: The study highlights the difficulty in detecting faint Event A peaks with current surveys. It emphasizes the need for deeper, higher-cadence monitoring (e.g., with LSST and SOXS) to construct a complete sample and definitively constrain the progenitor population of these rare, interaction-powered explosions.

In conclusion, SN 2024hpj serves as a critical anchor point for understanding the SN 2009ip-like class, providing evidence for a progenitor mass range of $25-31 M_\odot$ and a strong preference for star-forming dwarf galaxy environments.