Statistics of wide pre-main sequence binaries in the Orion OB1
  association
  Statistics of low-mass pre-main sequence binaries in the Orion OB1
association with separations ranging from 0.6" to 20" (220 to 7400 au at 370
pc) are studied using images from the VISTA Orion mini-survey and astrometry
from Gaia. The input sample based on the CVSO catalog contains 1137 stars of K
and M spectral types (masses between 0.3 and 0.9 Msun), 1021 of which are
considered to be association members. There are 135 physical binary companions
to these stars with mass ratios above ~0.13. The average companion fraction is
0.09+-0.01 over 1.2 decades in separation, slightly less than, but still
consistent with, the field. We found a difference between the Ori OB1a and OB1b
groups, the latter being richer in binaries by a factor 1.6+-0.3. No overall
dependence of the wide-binary frequency on the observed underlying stellar
density is found, although in the Ori OB1a off-cloud population these binaries
seem to avoid dense clusters. The multiplicity rates in Ori OB1 and in sparse
regions like Taurus differ significantly, hinting that binaries in the field
may originate from a mixture of diverse populations.
