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The nature of ASASSN-24fw's occultation: modelling the event as dimming by optically thick rings around a sub-stellar companion

The paper characterizes the 275-day dimming event of the F-type star ASASSN-24fw as an occultation by a massive sub-stellar companion (approximately 3.4 Jupiter masses) surrounded by optically thick rings or a circumplanetary disk, supported by multi-wavelength observations revealing distinct dust temperatures, a flat-bottomed light curve, and spectral features indicative of a late-type companion.

Original authors: Sarang Shah, Jonathan P. Marshall, Carlos del Burgo, Gergely Hajdu, Isabel Rebollido, Bogumił Pilecki, Ashish Mahabal, Mansi M. Kasliwal, Viraj Karambelkar, Matthew J. Graham, Stanislav G. Djorgovski

Published 2026-02-18

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Original authors: Sarang Shah, Jonathan P. Marshall, Carlos del Burgo, Gergely Hajdu, Isabel Rebollido, Bogumił Pilecki, Ashish Mahabal, Mansi M. Kasliwal, Viraj Karambelkar, Matthew J. Graham, Stanislav G. Djorgovski, Daniel Stern, Sascha T. Zeegers, Bacham Eswar Reddy, Ciska Kemper

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 a star named ASASSN-24fw as a bright lighthouse in a vast, dark ocean. For decades, this lighthouse shone steadily, never flickering. But then, in late 2024, something strange happened: the light didn't just flicker; it was slowly, then suddenly, smothered by a giant, invisible blanket, plunging the lighthouse into near-darkness for nearly 200 days.

This paper is the detective story of how astronomers tried to figure out what covered the light.

The Mystery: A Star That Went Dark

Usually, when a star dims, it's because a planet passes in front of it (like a tiny moth flying past a streetlamp). But this wasn't a moth. This was a giant, 200-day blackout. The star's brightness dropped so low it almost disappeared from our telescopes.

The astronomers knew it wasn't the star itself having a bad day (like a sunspot or a flare) because the star had been perfectly stable for years before this event. Something external had to be blocking the light.

The Suspect: A "Brown Dwarf" with a Giant Ring System

The team built two different models to solve the mystery, like trying to reconstruct a crime scene from a blurry photo.

The "Shape" Model: They looked at the curve of the dimming. It wasn't a smooth dip; it had distinct steps, like a staircase going down. This suggested the object blocking the light wasn't a solid ball, but something with layers of varying density.
The "Ring" Model: This was the winning theory. They proposed that a sub-stellar companion (a "failed star" called a Brown Dwarf, or a super-sized planet) was passing in front of the main star. But here's the kicker: this companion wasn't just a ball; it was wearing a gigantic, opaque ring system.

The Analogy: Imagine a tiny, dark marble (the Brown Dwarf) wearing a pair of giant, dusty sunglasses (the rings). The rings are so wide they stretch out about 0.17 astronomical units (that's roughly 16 times the size of our Sun!). As this "marble with sunglasses" drifted across the face of the main star, the rings blocked the light, creating a long, flat-bottomed shadow.

The Evidence: What the "Fingerprints" Said

The astronomers didn't just guess; they looked at the "fingerprints" left behind by the event:

The Color of the Dust: By looking at how the star's light changed colors as it dimmed, they figured out the rings were made of astronomical silicate dust (basically, space sand) about the size of fine grains of sugar.
The "Ghost" in the Machine: During the dimming, the star's spectrum (its chemical fingerprint) showed a weird "wiggly" line in the hydrogen gas (H-alpha). It looked like the gas was glowing slightly. This suggests the rings aren't just dead dust; they might have some gas in them that is re-emitting the star's light.
The Age Problem: The star itself is likely an older, middle-aged star (about 2 billion years old), not a baby star. This is a huge puzzle. Usually, giant rings like this only exist around very young, baby stars. If this system is old, how did the rings survive for so long without falling apart?
- The Theory: Maybe the rings are being constantly "refilled" by the Brown Dwarf crashing through a dense cloud of gas in space, or perhaps a recent collision created a fresh cloud of debris. It's like a snowball that keeps getting bigger because it's rolling through a fresh snowstorm.

Why This Matters

This event is rare and weird. It challenges our understanding of how planetary systems work.

It's not a normal planet: The object blocking the light is likely a Brown Dwarf (a "failed star" that is too heavy to be a planet but too light to be a real star).
It's not a normal ring: The rings are massive and have survived for billions of years, which shouldn't be possible according to standard physics.

The Conclusion

ASASSN-24fw is a cosmic mystery box. It's an old star being temporarily eclipsed by a "failed star" wearing a pair of giant, dusty, space-rings.

The astronomers conclude that this is a unique system that forces us to rethink how long planetary rings can last and how massive objects form around older stars. To solve the rest of the puzzle, they need to wait for the rings to move away and study the star again, hoping to catch a glimpse of the "failed star" and its giant rings in the clear.

In short: A star went dark because a "failed star" with a massive, dusty ring system walked in front of it. It's a cosmic traffic jam that has kept astronomers guessing for months.

1. Problem Statement

The paper investigates the nature of ASASSN-24fw, an F-type main-sequence star that underwent a rare, deep, and long-duration dimming event beginning in late 2024 and lasting until June 2025.

The Phenomenon: The star's brightness dropped from $g' \approx 13.20$ mag to $\approx 16.70$ mag (near the detection limit of ASAS-SN) within two weeks, remained stable at this low level for nearly 200 days, and then mirrored the ingress during egress.
The Mystery: While long-duration dimming events are increasingly observed, ASASSN-24fw is distinguished by its pronounced flat-bottomed lightcurve. The star also exhibits a persistent mid-infrared (IR) excess ( $\sim 10\%$ fractional luminosity), raising questions about whether the dimming is caused by the circumstellar material (debris disc) or an external occulter (e.g., a planet or brown dwarf with rings).
Historical Context: Historical data suggests similar dimming events occurred in 1937 and 1982, hinting at a periodicity of $\sim 43.8$ years ( $\sim 16,000$ days).

2. Methodology

The authors employed a multi-faceted approach combining photometric, spectroscopic, and modeling techniques:

Data Acquisition:
- Photometry: Utilized data from ASAS-SN, LCOGT (multi-band $g', r', i', z_s$ ), AAVSO, NEOWISE (mid-IR), and TESS (pre-dimming).
- Spectroscopy: Obtained low-resolution spectra (Keck LRIS, Palomar TripleSpec) and high-resolution spectra (Keck HiRES, Magellan MIKE) during and after the dimming event.
- Astrometry: Analyzed Gaia DR3 data for parallax, proper motion, and 3D velocity.
Stellar Characterization:
- Modeled the pre-dimming Spectral Energy Distribution (SED) using Castelli-Kurucz atmospheric models and Bayesian inference (del Burgo & Allende Prieto code) against PARSEC evolutionary tracks to determine stellar mass, age, and evolutionary status (Pre-Main Sequence vs. Main Sequence).
- Analyzed kinematics to determine if the star belongs to a young stellar population or the Galactic thick disc.
Lightcurve Modeling:
- Transit-Profile Model: Applied a parametric Generalised Gaussian Distribution combined with Gaussian Process Regression (GPR) to characterize the ingress/egress structure and handle noise.
- Occulting Disc Model: Used the pyPplusS package to model the event as a point-like body (sub-stellar companion) with massive, optically thick circumplanetary rings transiting the star. Parameters included ring inner/outer radii, inclination, azimuth, and opacity.
Extinction & Composition Analysis:
- Calculated reddening vectors using LCOGT multi-band data.
- Used optool to simulate dust opacities for various grain compositions (silicates, carbon, iron) and sizes to match observed color changes.

3. Key Results

A. Stellar Properties

Nature: ASASSN-24fw is an evolved F-type main-sequence star (Mass $\approx 1.5-1.7 M_\odot$ $\approx 1.5 - 1.7 M_{⊙}$ , Age $\approx 2.75$ $\approx 2.75$ Gyr), not a Young Stellar Object (YSO).
- Evidence: Lack of intrinsic variability in TESS data, absence of strong Li absorption (an age indicator for YSOs), kinematic association with the Galactic thick disc, and lack of X-ray emission.
Infrared Excess: The star has a persistent IR excess ( $\sim 10\%$ ) modeled by a two-component blackbody ( $T \approx 1100$ K and $400$ K), indicating a debris disc. However, this disc is spatially extended and stable (non-variable in NEOWISE), ruling it out as the cause of the deep, rapid dimming.

B. The Occulter (The Dimming Cause)

Lightcurve Structure: The dimming occurred in three distinct phases, suggesting a complex, multi-layered occulter with varying optical depth.
Physical Model: The event is best explained by a sub-stellar companion (Brown Dwarf or Super-Jupiter) surrounded by a massive, optically thick ring system.
- Companion Mass: Minimum mass $\ge 3.42 M_J$ (Jupiter masses).
- Ring System:
  - Outer radius: $\approx 0.17$ au ( $\approx 16 R_*$ ).
  - Inner radius: $\approx 0.11 R_*$ .
  - Opacity ( $\tau_R$ ): $\approx 0.96$ (optically thick).
  - Geometry: Inclined $\approx 40^\circ$ to the orbital plane, rotated $\approx 50^\circ$ azimuthally.
- Velocity: Transverse velocity $\approx 17$ km/s, consistent with an orbital period of $\sim 16,000$ days.

C. Spectral Analysis & Composition

H $\alpha$ Variability: During the event, H $\alpha$ showed a "w-shaped" reversal (central emission within absorption). Post-dimming spectra returned to pure absorption. This suggests the emission feature originates from the occulting material (likely hydrogen-rich gas in the rings) rather than the host star's accretion.
Dust Composition: Color-color analysis indicates the occulting dust consists primarily of astronomical silicate grains with sizes between 0.4 and 0.6 $\mu$ m.
Companion Identification: The companion is likely a Brown Dwarf rather than a Super-Jupiter. While Super-Jupiters can host discs, a massive, optically thick ring system of this size ( $\sim 0.17$ au) is more stable around a more massive brown dwarf, which can retain surrounding material longer.

4. Key Contributions

Rare System Characterization: Provides one of the most detailed analyses of a long-duration, flat-bottomed stellar dimming event, distinguishing it from typical YSO "dippers" or R CrB variables.
Ring System Modeling: Successfully models the event as a transit by a ringed sub-stellar companion, deriving specific geometric parameters (inclination, radius, opacity) that explain the 200-day duration and depth.
Evolutionary Context: Demonstrates that such massive circumplanetary structures can exist around evolved (2 Gyr) stars, challenging standard theories of disc dispersal timescales.
Spectral Diagnostics: Identifies variable H $\alpha$ features associated specifically with the occulter, providing a method to distinguish between stellar activity and external occultation.

5. Significance and Future Implications

Disc Survival Challenge: The existence of a massive, optically thick ring system around a brown dwarf orbiting a 2.75 Gyr old star is theoretically challenging. Standard disc dispersal mechanisms (photo-evaporation, dynamical clearing) should have depleted such a disc long ago. The authors propose mechanisms like Bondi-Hoyle-Lyttleton accretion (passing through dense ISM) or recent collisional replenishment to explain its survival.
Formation Mechanisms: The system's properties (wide separation, massive companion) suggest a formation history involving both core accretion and gravitational instability, or dynamical interactions in a mature system.
Future Observations: The paper outlines a roadmap for future studies using JWST (MIRI/NIRSpec) to determine dust composition and high-angular resolution imaging (VLT/ELT) to resolve the faint companion, which is crucial for understanding the evolution of circumplanetary discs in mature systems.

In conclusion, ASASSN-24fw represents a unique laboratory for studying the long-term stability of circumplanetary rings and the dynamics of sub-stellar companions in evolved stellar systems.