Search for associations containing young stars (SACY). VI. Is multiplicity universal? Stellar multiplicity in the range 3-1000 au from adaptive-optics observations

Context. Young loose nearby associations are unique samples of close (<150 pc), young (≈5-100 Myr) pre-main-sequence (PMS) stars. A significant number of members of these associations have been identified in the SACY (search for associations containing young stars) collaboration. We can use the proximity and youth of these members to investigate key ingredients in star formation processes, such as multiplicity.
Aims: With the final goal of better understanding multiplicity properties at different evolutionary stages of PMS stars, we present the statistics of identified multiple systems from 113 confirmed SACY members. We derive multiplicity frequencies, mass-ratio, and physical separation distributions in a consistent parameter space, and compare our results to other PMS populations and the field.
Methods: We have obtained adaptive-optics assisted near-infrared observations with the Nasmyth Adaptive Optics System and Near-Infrared Imager and Spectrograph (NACO), ESO/VLT, and the Infrared Camera for Adaptive optics at Lick observatory (IRCAL), Lick Observatory, for at least one epoch of all 113 SACY members. We have identified multiple systems using co-moving proper-motion analysis for targets with multi-epoch data, and using contamination estimates in terms of mass-ratio and physical separation for targets with single-epoch data. We have explored ranges in projected separation and mass-ratio of a [3-1000 au], and q [0.1-1], respectively.
Results: We have identified 31 multiple systems (28 binaries and 3 triples). We derive a multiplicity frequency (MF) of MF_{3-1000 au=28.4^+4.7_{-3.9}}% and a triple frequency (TF) of TF_{3-1000 au} =2.8 ^+2.5_-0.8% in the separation range of 3-1000 au. We do not find any evidence for an increase in the MF with primary mass. The estimated mass-ratio of our statistical sample (with power-law index γ = -0.04 ± 0.14) is consistent with a flat distribution (γ = 0).
Conclusions: Analysis from previous work using tight binaries indicated that the underlying multiple system distribution of the SACY dataset and the young star-forming region Taurus are statistically similar, supporting the idea that these two populations formed in a similar way. In this work, we show further similarities (but also hints of discrepancies) between the two populations: flat mass-ratio distributions and statistically similar MF and TF values. We also compared the SACY sample to the field (in the separation range of 19-100 au), finding that the two distributions are indistinguishable, suggesting a similar formation mechanism.

Based on observations obtained using the instruments NACO at the VLT (077.C-0483, 081.C-0825, 088.C-0506, 089.C-0207) and the adaptive optics facilities at the Lick Observatory (UCO 3 m).Appendices are available in electronic form at http://www.aanda.orgTable B.1 is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/580/A88