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Luminous Fast Blue Optical Transients as very massive star core-collapse events

This paper evaluates the viability of very massive star core-collapse events as the origin of Luminous Fast Blue Optical Transients (LFBOTs), finding that while the formation rates and low-metallicity progenitors of massive black holes align with observations, the model faces challenges regarding host galaxy metallicities and emission mechanisms, ultimately suggesting LFBOTs as promising targets for detecting super-kilonovae and studying r-process enrichment.

Original authors: A. A. Chrimes, P. G. Jonker, A. J. Levan, A. Mummery

Published 2026-02-18

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CC BY 4.0

Original authors: A. A. Chrimes, P. G. Jonker, A. J. Levan, A. Mummery

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

The Mystery of the "Cosmic Cow"

Imagine the universe is a giant, dark ocean. Usually, when a massive star dies, it goes out with a massive bang—a Supernova. It's like a firework that lights up the whole sky.

But every now and then, astronomers spot something weird: a Luminous Fast Blue Optical Transient (LFBOT). The most famous one is nicknamed "The Cow" (AT 2018cow).

Fast: It flashes on and off much quicker than a normal supernova.
Blue: It's incredibly hot and bright blue.
Quiet: It doesn't seem to have the usual "fireworks" (hydrogen and helium lines) that normal dying stars leave behind.
Mysterious: We don't know what causes it. Is it a black hole eating a white dwarf? Is it two stars crashing? Or is it something else entirely?

This paper asks a specific question: Could these "Cows" be massive stars that tried to explode but failed, only to collapse directly into a black hole?

The Investigation: A Cosmic Detective Story

The authors, led by Ashley Chrimes, decided to play detective. They used a giant computer simulation (like a "Universe Simulator" video game called BPASS) to see if this "Failed Supernova" theory holds up.

Here is how they broke it down:

1. The Rate Check: Are there enough "Cows"?

The Analogy: Imagine you are running a bakery. You want to know if you can bake enough "special cookies" (LFBOTs) to match the number of people ordering them.

The Theory: If massive stars collapse into black holes without exploding, how often does that happen?
The Result: The computer simulation showed that stars massive enough to become very heavy black holes (about 30 to 40 times the mass of our Sun) collapse at a rate that matches the number of "Cows" we see in the sky.
Verdict: ✅ Pass. The numbers add up. There are enough of these events happening to explain the "Cows."

2. The Location Check: Where do they live?

The Analogy: Think of stars as plants. Some plants only grow in rich, muddy soil (high metallicity), while others need poor, sandy soil (low metallicity).

The Theory: The simulation predicted that these massive black holes only form in "sandy soil" galaxies—places with very low metal content (less than 30% of our Sun's metal content).
The Reality: When astronomers looked at the galaxies where the "Cows" appeared, most were indeed in "sandy soil." However, a few "Cows" were found in "muddy soil" galaxies (higher metallicity).
The Problem: The simulation says they shouldn't be in the muddy soil.
The Fix: The authors suggest that maybe the "Cows" are actually hiding in small, sandy pockets inside those muddy galaxies, or our measurements of the soil are slightly off.
Verdict: ⚠️ Mostly Pass, but with a glitch. It fits mostly, but the "muddy soil" cases are a bit of a headache for the theory.

3. The Clues: What does the "corpse" look like?

The Theory: If a star collapses into a black hole without a big explosion, it should leave behind a very specific type of "corpse" (the material left over).

The Clue 1 (No H or He): Normal stars are mostly Hydrogen and Helium. But "Cows" don't show these elements. The simulation showed that the massive stars capable of this collapse are "stripped" of their outer layers, leaving them Hydrogen and Helium poor.
- Verdict: ✅ Perfect Match.
The Clue 2 (The Wind): Before they die, these stars should be blowing a dense wind (circumstellar medium). "Cows" are surrounded by very dense gas. The simulation showed these stars do have strong winds, but maybe not quite dense enough to explain the "Cows."
- Verdict: ⚠️ Close, but maybe missing something. The models might be missing a final, violent burst of wind right before the star dies.

4. The Engine: How does it keep glowing?

The Mystery: The "Cow" didn't just flash once; it kept glowing in ultraviolet light for years. This suggests a central engine is still running, like a generator.

The Theory: When the star collapses, it doesn't swallow everything instantly. Some "leftover" material falls back onto the new black hole, forming a spinning accretion disk (a hot, swirling pizza of gas). This disk powers the long glow.
The Catch: For the simulation to produce a glow as bright and long-lasting as the "Cow," the disk needs to be huge and spinning very fast. It requires a "perfect storm" of conditions.
Verdict: ⚠️ Possible, but difficult. It can happen, but only if everything lines up perfectly.

The Big Surprise: The "Super-Kilonova" Connection

Here is the most exciting part of the paper.

Super-Kilonovae are theoretical explosions that create heavy elements like gold and platinum (the "r-process"). Scientists thought these only happened when two neutron stars crashed.
The authors realized: The "Cows" might actually be Super-Kilonovae in disguise!
If the "Failed Supernova" theory is true, these events are churning out massive black holes and accretion disks that are perfect for making gold.
The Takeaway: Every time we see a "Cow," we might be witnessing the universe's factory for heavy metals.

The Final Verdict

The authors conclude that massive stars collapsing directly into black holes is a very plausible explanation for "Cows."

Pros: The math works, the "no hydrogen" clue fits, and it explains the heavy metal production.
Cons: It struggles a bit with the metal content of some host galaxies and the density of the gas around them.

In simple terms: The "Failed Supernova" theory is a strong contender for solving the "Cow" mystery. It's like finding a suspect who fits the height, weight, and shoe size, even if their alibi for one specific location is a little shaky. If they are right, it means the universe is making gold in these quiet, failed explosions much more often than we thought.

1. Problem Statement

Luminous Fast Blue Optical Transients (LFBOTs) are rare, rapidly evolving extragalactic events (e.g., AT 2018cow) characterized by:

Peak optical-UV luminosities rivaling superluminous supernovae (SLSNe).
Featureless, hot blackbody spectra ( $T > 20,000$ K) initially lacking Hydrogen (H) and Helium (He) lines.
Rapid decay rates ( $\gtrsim 0.3$ mag/day), ruling out significant $^{56}$ Ni powering.
Bright, long-lasting X-ray and radio emission indicating a central engine and dense circumstellar medium (CSM).

While proposed origins include tidal disruption events (TDEs) of white dwarfs by intermediate-mass black holes (IMBHs) or stellar mergers, the failed supernova (failed SN) hypothesis suggests LFBOTs result from the core-collapse of very massive stars directly into black holes (BHs), followed by the accretion of fallback material. This paper investigates the viability of this specific channel by comparing population synthesis predictions with observational constraints.

2. Methodology

The authors employed Binary Population and Spectral Synthesis (BPASS) models (v2.2.1) coupled with a metallicity-dependent cosmic star formation history (CSFH) to simulate the formation of massive black holes.

Population Synthesis: They utilized a grid of binary stellar evolution models with a Kroupa initial mass function (IMF) (0.1–300 $M_\odot$ ) across 13 metallicities.
Remnant Formation: Core-collapse was triggered based on CO/ONe core masses. Remnant masses were calculated by injecting $10^{51}$ erg of energy to determine unbound mass; the remainder was the BH mass. Pair-instability supernovae (PISNe) and pulsational PISNe were accounted for.
Rate Calculation: The volumetric formation rate of BHs with mass $> M_{BH,min}$ was calculated as a function of redshift ( $z < 0.4$ ) and compared to the observed LFBOT rate ( $\sim 0.1\%$ of the core-collapse supernova rate).
Metallicity Analysis: The metallicity distribution of progenitors for massive BHs was compared against observed metallicities of LFBOT host galaxies (derived from SED fitting, IFU spectroscopy, or mass-metallicity relations).
Emission Modeling:
- Spectra: Checked H/He mass fractions in the "leftover" (ejected/fallback) material.
- CSM: Calculated mass-loss rates ( $\dot{M}$ ) and wind speeds ( $v_w$ ) to estimate CSM density ( $\dot{M}/v_w$ ).
- UV Plateaus: Modeled long-lived UV emission from accretion discs formed by fallback material using the Mummery & Balbus (2020) framework, varying angular momentum and mass transfer fractions.

3. Key Contributions & Results

A. Event Rates and BH Mass Thresholds

The formation rate of black holes with masses $> 30\text{--}40 M_\odot$ is found to be $\sim 0.1\%$ of the core-collapse supernova (CCSN) rate.
This matches the observed volumetric rate of LFBOTs.
The result is robust against variations in the assumed cosmic star formation history (CSFH), though the specific mass threshold shifts slightly (e.g., $>33 M_\odot$ for higher metallicity CSFH, $>41 M_\odot$ for lower).

B. Metallicity Constraints

Prediction: Progenitors of these massive BHs are strongly biased toward low metallicity ( $Z < 0.3 Z_\odot$ ).
Observation: LFBOT host galaxies generally show low metallicities ( $Z \lesssim 0.5 Z_\odot$ ), but some (e.g., AT 2018cow, AT 2024wpp) have measured metallicities higher than the model's strict cutoff.
Resolution: The authors suggest that while integrated host metallicities may be high, LFBOTs likely occur in localized, low-metallicity pockets within these galaxies that current measurements (often galaxy-integrated) miss.

C. Spectral and CSM Characteristics

H/He Poor Spectra: The "leftover" material from the selected progenitors is predominantly H and He-poor (median surface H fraction $X_{surf} \approx 0.003$ ). $\sim 69\%$ of models are Wolf-Rayet stars, consistent with the featureless early spectra of LFBOTs.
Dense CSM: The models predict mass-loss rates and wind speeds yielding $\dot{M}/v_w$ values lower than those typically inferred from LFBOT radio observations (which require $\dot{M}/v_w \sim 1\text{--}100$ in units of $10^{-4} M_\odot \text{yr}^{-1} / 1000 \text{km s}^{-1}$ ).
Explanation: The discrepancy may be resolved by eruptive mass loss in the final decades before collapse, a phase not fully captured by BPASS (which ends at core carbon burning).

D. Long-Lived UV Emission (AT 2018cow)

The paper models the long-lived UV plateau seen in AT 2018cow as emission from an accretion disc formed by fallback material.
Conditions: Reproducing the observed flux requires maximal conditions: a large fraction of the leftover mass and angular momentum must be transferred to the disc.
Timescales: The viscous timescales of the discs ( $t_{visc}$ ) span a wide range ( $0.5\text{--}10^6$ days). Only the shortest timescales can explain the rapid early rise, implying other mechanisms may drive the initial peak.

4. Significance and Implications

Progenitor Overlap with Super-Kilonovae: The failed supernova channel for LFBOTs involves the formation of massive BHs ( $>30\text{--}40 M_\odot$ ) with accretion discs. This overlaps significantly with the progenitor requirements for super-kilonovae (events producing r-process elements in high-accretion discs).
r-Process Nucleosynthesis: If LFBOTs are indeed massive star core-collapse events, they could contribute non-negligibly to the galactic r-process enrichment budget. The authors suggest LFBOTs are prime targets for searching for super-kilonovae signatures (e.g., near-infrared excesses from r-process elements).
Challenges: The model faces two main hurdles:
1. Metallicity: The strict low-metallicity requirement of the models vs. some higher-metallicity host measurements.
2. CSM Density: The difficulty in reproducing the extremely dense circumstellar media observed in radio without invoking late-stage eruptive mass loss not fully modeled.

5. Conclusion

The authors conclude that very massive star core-collapse (failed supernovae) is a plausible explanation for at least a subset of LFBOTs. The event rates and progenitor characteristics (H/He poor, massive BH formation) align well with observations, provided that:

Local metallicity pockets explain the host galaxy discrepancies.
Late-stage eruptive mass loss explains the dense CSM.
Extreme fallback conditions (high mass and angular momentum transfer) occur to power long-lived UV emission.

This work bridges the gap between stellar evolution theory and transient astronomy, proposing a unified framework where LFBOTs, massive BH formation, and potential r-process production are intrinsically linked.