The role of mass and environment in multiple-star formation: A 2mass survey of wide multiplicity in three young associations

We present the results of a search for wide binary systems among 783 members of three nearby young associations: Taurus-Auriga, Chamaeleon I, and two subgroups of Upper Scorpius. Near-infrared (JHK) imagery from 2MASS was analyzed to search for wide (1 ''-30 ''; similar to 150-4500 AU) companions to known association members, using color-magnitude cuts to reject likely background stars. We identify a total of 131 candidate binary companions with colors consistent with physical association, of which 39 have not been identified previously in the literature. Our results suggest that the wide binary frequency is a function of both mass and environment, with significantly higher frequencies among high-mass stars than lower mass stars and in the T associations than in the OB association. We discuss the implications for wide binary formation and conclude that the environmental dependence is not a direct result of stellar density or total association mass, but instead might depend on another environmental parameter like the gas temperature. The binary populations in these associations generally follow the empirical mass-maximum separation relation observed for field binaries, but we have found one candidate low-mass system (USco 160611.9-193532; M-tot similar to 0.4 M-circle dot) that has a projected separation (10.8 ''; 1550 AU) much larger than the suggested limit for its mass. Finally, we find that the binary frequency in the USco-B subgroup is significantly higher than in the USco-A subgroup and is consistent with the measured values in Taurus and ChamI. This discrepancy, the absence of high-mass stars in USco-B, and its marginally distinct kinematics suggest that it might not be directly associated with the OB associations of Sco-Cen but instead represents an older analog of the younger rho Oph or Lupus associations.