ASASSN-24fw: Candidate circumplanetary disk occultation of a main-sequence star
Nadia L. Zakamska, Gautham Adamane Pallathadka, Dmitry Bizyaev, Jaroslav Merc, James E. Owen, Henrique Reggiani, Kevin C. Schlaufman, Karolina Bąkowska, Sławomir Bednarz, Krzysztof Bernacki, Agnieszka Gurgul, Kirsten R. Hall, Franz-Josef Hambsch, Barbara Joachimczyk, Krzysztof Kotysz, Sebastian Kurowski, Alexios Liakos, Przemysław J. Mikołajczyk, Erika Pakštienė, Grzegorz Pojmański, Adam Popowicz, Daniel E. Reichart, Łukasz Wyrzykowski, Justas Zdanavičius, Michał Żejmo, Paweł Zieliński, Staszek Zola
TL;DR
ASASSN-24fw reveals a rare, long-duration occultation of a main-sequence star by a likely gas-rich circumsecondary disk, supported by multi-epoch spectroscopy showing deep Na I D absorption, H$\alpha$ emission, and numerous metal lines, plus a historical 44-year orbital cadence. The data imply a tidally truncated disk around an unseen companion at $a_{\rm orb}\approx13.9$ AU, with the occulter extending to $R_{\rm CSD}\approx0.7$ AU; the companion mass is constrained from a few $M_{\rm Jup}$ up to $\lesssim 0.25\,M_\odot$ depending on the disk’s Hill-sphere filling factor. The extinction curve indicates unusually large grain sizes, consistent with a dusty debris environment potentially produced by a planetary collision or dynamical instability in an old system ($>2$ Gyr). The combination of infrared excess, H$\alpha$ emission, and gas signatures around a mature star presents a compelling case for an evolved, gas-rich circumsecondary disk and possibly a disk wind, offering a rare window into late-stage disk dynamics. Future high-resolution post-occultation spectroscopy and submillimeter observations could decisively map the gas/dust distributions and confirm the system’s architecture and evolution.
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$α$ 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.