Sudden extreme obscuration of a Sun-like main-sequence star: evolution of the circumstellar dust around ASASSN-21qj

TL;DR

This paper reports the discovery of a Sun-like star experiencing sudden deep dimming events caused by a circumstellar dust cloud, likely from exocomet breakup, with new infrared excess evidence indicating ongoing dust production.

Contribution

It provides the first evidence of near-infrared excess in ASASSN-21qj and models the dust composition, size, and origin, linking the dimming to exocometary activity.

Findings

Deep occultations caused by a dust cloud of amorphous pyroxene.

Detection of near-infrared excess indicating circumstellar dust.

Estimated dust mass and grain size consistent with exocomet breakup.

Abstract

ASASSN-21qj is a distant Sun-like star that recently began an episode of deep dimming events after no prior recorded variability. Here we examine archival and newly obtained optical and near-infrared data of this star. The deep aperiodic dimming and absence of previous infrared excess are reminiscent of KIC 8462852 (``Boyajian's Star''). The observed occultations are consistent with a circumstellar cloud of sub-micron-sized dust grains composed of amorphous pyroxene, with a minimum mass of $1.50~\pm~0.04\times10^{-9}~M_{\oplus}$ derived from the deepest occultations, and a minimum grain size of $0.29^{+0.01}_{-0.18}~\mu$m assuming a power law size distribution. We further identify the first evidence of near-infrared excess in this system from NEOWISE 3.4 and 4.6~$\mu$m observations. The excess emission implies a total circumstellar dust mass of around $10^{-6} M_{\oplus}$, comparable to the extreme, variable discs associated with terrestrial planet formation around young stars. The quasiperiodic recurrence of deep dips and the inferred dust temperature (ranging from 1800 to 700~K across the span of observations) independently point to an orbital distance of $\simeq$0.2~au for the dust, supporting the occulting material and excess emission being causally linked. The origin of this extended, opaque cloud is surmised to be the breakup of one or more exocometary bodies.