USco J1606-1935: An Unusually Wide Low-Mass Triple System?

Abstract

We present photometric, astrometric, and spectroscopic observations of USco J160611.9-193532 AB, a candidate ultrawide (~1600 AU), low-mass (M_(tot) ~ 0.4 M_⊙) multiple system in the nearby OB association Upper Scorpius. We conclude that both components are young, comoving members of the association; we also present high-resolution observations that show that the primary is itself a close binary system. If the Aab and B components are gravitationally bound, the system would fall into the small class of young multiple systems that have unusually wide separations as compared to field systems of similar mass. However, we demonstrate that physical association cannot be assumed purely on probabilistic grounds for any individual candidate system in this separation range. Analysis of the association's two-point correlation function shows that there is a significant probability (25%) that at least one pair of low-mass association members will be separated in projection by ≾ 15", so analysis of the wide binary population in Upper Sco will require a systematic search for all wide systems; the detection of another such pair would represent an excess at the 98% confidence level.

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