  Using the NANTEN2 Observatory, we carried out a molecular line study of
high-mass star forming regions with reflection nebulae, NGC 2068 and NGC 2071,
in Orion in the 13CO(J=2-1) transition. The 13CO distribution shows that there
are two velocity components at 9.0 and 10.5 km/s . The blue-shifted component
is in the northeast associated with NGC 2071, whereas the red-shifted component
is in the southwest associated with NGC 2068. The total intensity distribution
of the two clouds shows a gap of ~1 pc, suggesting that they are detached at
present. A detailed spatial comparison indicates that the two show
complementary distributions. The blue-shifted component lies toward an
intensity depression to the northwest of the red-shifted component, where we
find that a displacement of 0.8 pc makes the two clouds fit well with each
other. Furthermore, a new simulation of non-frontal collisions shows that
observations from 60 degrees off the collisional axis agreed well with the
velocity structure in this region. On the basis of these results, we
hypothesize that the two components collided with each other at a projected
relative velocity 3.0 km/s estimated to be 0.3 Myr for an assumed axis of the
relative motion 60 degrees off the line of sight. We assume that the two most
massive early B-type stars in the cloud, illuminating stars of the two
reflection nebulae, were formed by collisional triggering at the interfaces
between the two clouds. Given the other young high-mass star forming regions,
namely, M42, M43, and NGC 2024 (Fukui et al. 2018b; Ohama et al. 2017a), it
seems possible that collisional triggering has been independently working to
form O-type and early B-type stars in Orion in the last Myr over a projected
distance of ~80 pc.
