ASASSN-24fw: An 8-month long, 4.1 mag, optically achromatic and polarized dimming event

TL;DR

This paper reports on the discovery and analysis of a long-duration, achromatic, and polarized dimming event in the star ASASSN-24fw, suggesting the presence of a circumbinary disk with large dust grains, and surveys similar occultation events.

Contribution

It presents the first detailed analysis of an 8-month dimming event in ASASSN-24fw, proposing a circumbinary disk with large grains as the cause, and provides a survey of similar occultation systems.

Findings

The dimming lasted about 8 months with a 4.1 mag decrease.

The event was nearly achromatic and showed polarization up to 4%.

A survey found 46 similar systems, many with IR excess.

Abstract

We discuss ASASSN-24fw, a 13th-magnitude star that optically faded by $\Delta g = 4.12 \pm 0.02$ mag starting in September 2024 after over a decade of quiescence in ASAS-SN. The dimmimg lasted $\sim$8 months before returning to quiescence in late May 2025. The spectral energy distribution (SED) before the event is that of a pre-main sequence or a modestly evolved F star with some warm dust emission. The shape of the optical SED during the dim phase is unchanged and the optical and near-infrared spectra are those of an F star. The SED and the dilution of some of the F star infrared absorption features near minimum suggest the presence of a $\sim$0.25$M_\odot$ M dwarf binary companion. The 43.8 year period proposed by Nair & Denisenko (2024) appears correct and is probably half the precession period of a circumbinary disk. The optical eclipse is nearly achromatic, although slightly deeper in bluer filters, $\Delta (g-z)=0.31\pm0.15$ mag, and the $V$ band emission is polarized by up to 4%. The materials most able to produce such small optical color changes and a high polarization are big ($\sim$20 $\mu$m) carbonaceous or water ice grains. Particle distributions dominated by big grains are seen in protoplanetary disks, Saturn-like ring systems and evolved debris disks. We also carry out a survey of occultation events, finding 46 additional systems, of which only 7 (4) closely match $\varepsilon$ Aurigae (KH 15D), the two archetypes of stars with long and deep eclipses. The full sample is widely distributed in an optical color-magnitude diagram, but roughly half show a mid-IR excess. It is likely many of the others have cooler dust since it seems essential to produce the events.