ASASSN-24fw: Candidate circumplanetary disk occultation of a main-sequence star

TL;DR

This study reports a long-duration occultation event of a main-sequence star by a gas-rich circumsecondary disk, providing insights into circumplanetary disks and their potential origins in an older star system.

Contribution

First detection of a candidate circumplanetary disk causing a long-lasting occultation around a main-sequence star, with spectroscopic evidence of gas-rich material and disk wind or rotation.

Findings

Detected low-ionization metal emission lines indicating gas-rich occulter

Confirmed historic occultations, estimating the occulter's orbit at 14 AU

Suggested the disk may result from planetary collision in an old star system

Abstract

Dusty disks around planetary and substellar companions in outer reaches of exo-planetary systems can be detected as long-lasting occultations, provided the observer is close to the secondary's orbital plane. Here we report optical spectroscopy with KOSMOS (APO), MagE (Magellan) and GHOST (Gemini-S) of ASASSN-24fw (Gaia 07:05:18.97+06:12:19.4), a 4-magnitude dimming event of a main-sequence star which lasted 8.5 months. We discover multiple low-ionization metal emission lines with velocity dispersion $\lesssim 10$ km/s blue-shifted by 27 km/s with respect to the star, as well as kinematically complex Na D absorption. If associated with the occulter, these detections suggest that the occulter is gas-rich. Further, we detect blue-shifted and broad ($\sim 200$ km/s) H$\alpha$ line, which likely originates in the inner circumstellar disk. We confirm the previously reported occultations in 1981 and 1937 seen in historic data, yielding a semi-major axis of the occulter's orbital motion around the star of 14 AU. If the occulter is a circumsecondary disk filling 30-100% of the Hill radius, we estimate the minimum mass of the secondary to be a few Jupiter masses and a disk mass of 1% of the mass of the Moon. Given the age of the star ($>2$ Gyr), the disk is unlikely to be a survivor of the planet formation stage and may be a result of a planetary collision. If Na D absorption and/or metal emission lines originate in the disk, the observations presented here are the first discovery of a circumsecondary disk wind or rotation.