A T8.5 brown dwarf member of the ξ ursae majoris system

摘要

The Wide-field Infrared Survey Explorer has revealed a T8.5 brown dwarf (WISE J111838.70+312537.9) that exhibits common proper motion with a solar-neighborhood (8 pc) quadruple star system - ξ Ursae Majoris. The angular separation is 8.′5, and the projected physical separation is ≈4000 AU. The sub-solar metallicity and low chromospheric activity of ξ UMa A argue that the system has an age of at least 2 Gyr. The infrared luminosity and color of the brown dwarf suggests the mass of this companion ranges between 14 and 38 M_J for system ages of 2 and 8 Gyr, respectively.