An HST Survey for 100-1000 AU Companions around Young Stellar Objects in
  the Orion Molecular Clouds: Evidence for Environmentally Dependent
  Multiplicity
  We present a near-IR survey for the visual multiples in the Orion molecular
clouds region at separations between 100 and 1000 AU. These data were acquired
at 1.6~$\mu$m with the NICMOS and WFC3 cameras on the Hubble Space Telescope.
Additional photometry was obtained for some of the sources at 2.05~$\mu$m with
NICMOS and in the $L'$-band with NSFCAM2 on the IRTF. Towards 129 protostars
and 197 pre-main sequence stars with disks observed with WFC3, we detect 21 and
28 candidate companions between the projected separations of 100---1000 AU, of
which less than 5 and 8, respectively, are chance line of sight coincidences.
The resulting companion fraction ($CF$) after the correction for the line of
sight contamination is 14.4$^{+1.1}_{-1.3}$% for protostars and
12.5$^{+1.2}_{-0.8}$% for the pre-main sequence stars. These values are similar
to those found for main sequence stars, suggesting that there is little
variation in the $CF$ with evolution, although several observational biases may
mask a decrease in the $CF$ from protostars to the main sequence stars. After
segregating the sample into two populations based on the surrounding surface
density of YSOs, we find that the $CF$ in the high stellar density regions
($\Sigma_{YSO} > 45$~pc$^{-2}$) is approximately 50% higher than that found in
the low stellar density regions ($\Sigma_{YSO} < 45$~pc$^{-2}$). We interpret
this as evidence for the elevated formation of companions at 100 to 1000 AU in
the denser environments of Orion. We discuss possible reasons for this elevated
formation.
