Unusually Wide Binaries: Are They Wide or Unusual?

TL;DR

This study investigates the properties of wide binary systems in young star-forming regions, revealing their distribution, mass ratios, and the influence of dynamical evolution, highlighting that only low-mass systems are truly unusually wide.

Contribution

It provides the first comprehensive survey of wide binaries in Taurus and Upper Sco, analyzing their distribution, mass ratios, and formation limits, and compares these with field populations.

Findings

Higher-mass stars have more wide companions.

Wide binary separation distribution is log-flat.

Systems below 0.3 Msun are unusually wide.

Abstract

We describe an astrometric and spectroscopic campaign to confirm the youth and association of a complete sample of candidate wide companions in Taurus and Upper Sco. Our survey found fifteen new binary systems (3 in Taurus and 12 in Upper Sco) with separations of 3-30" (500-5000 AU) among all of the known members with masses of 2.5-0.012 Msun. The total sample of 49 wide systems in these two regions conforms to only some expectations from field multiplicity surveys. Higher-mass stars have a higher frequency of wide binary companions, and there is a marked paucity of wide binary systems near the substellar regime. However, the separation distribution appears to be log-flat, rather than declining as in the field, and the mass ratio distribution is more biased toward similar-mass companions than the IMF or the field G dwarf distribution. The maximum separation also shows no evidence of a limit at <5000 AU until the abrupt cessation of any wide binary formation at system masses of ~0.3 Msun. We attribute this result to the post-natal dynamical sculpting that occurs for most field systems; our binary systems will escape to the field intact, but most field stars are formed in denser clusters and do not. In summary, only wide binary systems with total masses <0.3 Msun appear to be "unusually wide".