We carried out a molecular line study toward the high-mass star forming regions with reflection nebulae , NGC2068 and NGC2071 , in Orion with NANTEN2 in the ^ { 13 } CO ( \it J = 2 - 1 ) transition .
The ^ { 13 } CO distribution shows that there are two velocity components at 8.25 km s ^ { -1 } and 11.5 km s ^ { -1 } .
The blue-shifted component is in the northeast associated with NGC2071 , and the red-shifted component is in the southwest associated with NGC2068 .
The two clouds have a gap of \sim 1 pc in total intensity distribution , suggesting that they are detached at present .
A detailed spatial comparison between them indicates that the two show complementary distribution ; the blue-shifted component lies toward an intensity depression in the northwest of the red-shifted component , where we find that a displacement of 0.6 pc makes the two clouds fit well with each other .
Based on these results we hypothesize that the two components collided with each other at a projected relative velocity 2.5 km s ^ { -1 } .
The timescale of the collision is estimated to be 2 \times 10 ^ { 5 } yrs for an assumed angle 45 deg of the relative motion to 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 the collisional triggering at the interfaces between the two clouds .
Along with the other young high-mass star forming regions , M42 , M43 , and NGC2024 ( Fukui et al .
( 2017b ) ; Ohama et al .
2018 ) , it seems possible that collisional triggering is independently working to form O–type and early B–type stars in Orion in the last Myr over a projected distance of \sim 80 pc .