ASASSN-21js: A multi-year transit of a ringed disc

TL;DR

This paper models a multi-year eclipse of the star ASASSN-21js caused by a structured ring system around a substellar object, predicting the eclipse's duration and properties using light curve analysis.

Contribution

It introduces a detailed geometric model of a ringed disc transiting a star, applying MCMC to determine ring parameters and orbital characteristics.

Findings

The ring system is likely a two-ring structure at high inclination.

The orbit of the transiting object is estimated to be around 13,000 au from the star.

The eclipse is expected to conclude around May 2027.

Abstract

The early-type star ASASSN-21js started to fade in 2021, as was detected by the All Sky Automated Survey for Supernovae, undergoing a multi-year eclipse that is still underway. We interpret this event as being due to a structured disc of material transiting in front of the star. The disc is in orbit around a substellar object with the mass and luminosity of a brown dwarf or smaller. We want to determine the expected duration and ending date of the eclipse. We modelled a tilted and inclined azimuthally symmetric ring system around an unseen companion and calculated the resulting time-varying light curve as the object transited in front of the star. We made an initial estimate of the ring parameters and used these as inputs to an MCMC algorithm to determine the geometric properties of the rings with associated uncertainties. The model most consistent with the light curve to date is a two-ring system at high inclination with respect to the line of sight that has a semi-major axis of 71.6 stellar radii. With an estimate of the stellar radius, the transverse velocity is around 0.7 km/s, which if bound to the star is an orbit with a semi-major axis of around 13000 au, placing it in the Oort cloud of the parent star. The transit is ongoing and will finish around MJD 61526 (May 1 2027). We encourage the community to continue observing this object in order to understand its properties.