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

TL;DR

This study models a long-lasting dimming event of ASASSN-24fw as caused by optically thick rings around a sub-stellar companion, revealing complex dust and gas structures and suggesting a possible brown dwarf or gas giant with circumplanetary material.

Contribution

It introduces a novel interpretation of the dimming as occultation by rings around a sub-stellar object, supported by detailed light curve and spectral analysis.

Findings

Dimming lasted about 275 days with a flat-bottomed profile.

Spectral features indicate a late-type companion, around M8.

Variable H-alpha emission suggests evolving circumplanetary material.

Abstract

ASASSN-24fw is a main-sequence F-type star that experienced a rapid and long-lasting dimming event beginning in late 2024 and continuing until mid 2025. Its pre-dimming spectral energy distribution shows a persistent infrared excess with a fractional luminosity of approximately 0.5 percent. We model this excess using a two-component blackbody fit and find dust components with temperatures of about 1070 K and 390 K. Archival light curves indicate that ASASSN-24fw was photometrically stable prior to the event, suggesting that the dimming is caused by an external occulting body rather than intrinsic stellar variability. The event lasted about 275 days and exhibits a distinctive flat-bottomed profile of nearly 200 days, unlike most long-duration occultation events reported in the last decade. We analyze the light curve and spectra obtained during dimming to study the properties of both the star and the occulting material. A parametric light-curve model reveals multiple ingress phases, consistent with variations in the density and structure of the obscuring material. A second transit model favors an occulting body consistent with a gas giant or brown dwarf with a minimum mass of about 3.4 Jupiter masses and surrounded by an extended circumplanetary disk or rings of radius roughly 0.17 au. Near-infrared spectra taken during dimming show enhanced infrared excess and spectral features consistent with a late-type companion, approximately M8. We also detect variable H-alpha emission, suggesting evolving gas and dust in the occulting structure. Imaging from LCOGT identifies a nearby object within 3 arcsec, likely a bound companion at a projected separation of about 3000 au. Systems like ASASSN-24fw appear rare, and continued follow-up will help constrain the nature of the occulting body and the circumstellar environment.