Cool dwarfs in wide multiple systems: Paper 2: A distant M8.5V companion to HD 212168 AB

TL;DR

This paper reports the discovery and confirmation of a wide, physically bound triple system called Koenigstuhl 5, including a very-low-mass M8.5V star as a distant companion to a G0 V star, based on long-term astrometric and kinematic data.

Contribution

It provides the first confirmation of a wide, triple system with an ultracool dwarf as a distant companion, demonstrating the physical binding through long-term measurements.

Findings

Confirmed the physical binding of the triple system Koenigstuhl 5.

Identified the ultracool dwarf as one of the least-bound in multiple systems.

Measured long-term proper motions and radial velocities to establish companionship.

Abstract

DENIS J222644.3-750342 is a nearby, very-low-mass, M8.5 V-type star that has been repeatedly targeted by kinematic and activity studies. Thought to be an isolated star, it is actually a wide common-proper-companion at 265 arcsec from the bright G0 V Hipparcos star HD 212168. The third member in the trio is CD-75 1242, a poorly investigated K dwarf. We confirm the physical binding of the triple system, to which we call Koenigstuhl 5, by compiling common radial velocities and proper-motions and measuring constant angular separations and position angles between components A, B and C over very long time baselines (A-B: 114 yr; A-C: 22 yr). With about 0.095 Msol, the M8.5 V star at 6090 AU to the G0 V primary is one of the least-bound, ultracool dwarfs in multiple systems.