The Nearby, Young, χ ¹ Fornacis Cluster: Membership, Age, and an Extraordinary Ensemble of Dusty Debris Disks

Abstract

Only four star clusters are known within ∼100 pc of Earth. Of these, the χ ¹ For cluster has barely been studied. We use the Gaia DR2 catalog and other published data to establish the cluster membership, structure, and age. The age of and distance to the cluster are ∼40 Myr and 104 pc, respectively. A remarkable, unprecedented aspect of the cluster is the large percentage of M-type stars with warm excess infrared emission due to orbiting dust grains—these stars lie in an annulus that straddles the tidal radius of the cluster. The χ ¹ For cluster appears to be closely related to two extensive, previously known, groups of comoving, coeval stars (the Tucana-Horologium and Columba Associations) that are spread over much of the southern sky. While Tuc-Hor and χ ¹ For are comoving and coeval, the difference in the frequency of their warm dusty debris disks at M-type stars could hardly be more dramatic.