Close companions around young stars
  Multiplicity is a fundamental property that is set early during stellar
lifetimes, and it is a stringent probe of the physics of star formation. The
distribution of close companions around young stars is still poorly constrained
by observations. We present an analysis of stellar multiplicity derived from
APOGEE-2 spectra obtained in targeted observations of nearby star-forming
regions. This is the largest homogeneously observed sample of high-resolution
spectra of young stars. We developed an autonomous method to identify double
lined spectroscopic binaries (SB2s). Out of 5007 sources spanning the mass
range of $\sim$0.05--1.5 \msun, we find 399 binaries, including both RV
variables and SB2s. The mass ratio distribution of SB2s is consistent with a
uniform for $q<0.95$ with an excess of twins with $q>0.95$. The period
distribution is consistent with what has been observed in close binaries ($<10$
AU) in the evolved populations. Three systems are found to have $q\sim$0.1,
with a companion located within the brown dwarf desert. There are not any
strong trends in the multiplicity fraction (MF) as a function of cluster age
from 1 to 100 Myr. There is a weak dependence on stellar density, with
companions being most numerous at $\Sigma_*\sim30$ stars/pc$^{-2}$, and
decreasing in more diffuse regions. Finally, disk-bearing sources are deficient
in SB2s (but not RV variables) by a factor of $\sim$2; this deficit is
recovered by the systems without disks. This may indicate a quick dispersal of
disk material in short-period equal mass systems that is less effective in
binaries with lower $q$.
