Massive star formation in the Carina nebula complex and Gum 31 -- II. a
  cloud-cloud collision in Gum 31
  We present the results of analyses of the 12CO (J=1-0), 13CO (J=1-0), and
12CO (J=2-1) emission data toward Gum 31. Three molecular clouds separated in
velocity were detected at -25, -20, and -10 km/s . The velocity structure of
the molecular clouds in Gum 31 cannot be interpreted as expanding motion. Two
of them, the -25 km/s cloud and the -20 km/s cloud, are likely associated with
Gum 31, because their 12CO (J=2-1)/12CO (J=1-0) intensity ratios are high. We
found that these two clouds show the observational signatures of cloud-cloud
collisions (CCCs): a complementary spatial distribution and a V-shaped
structure (bridge features) in the position-velocity diagram. In addition,
their morphology and velocity structures are very similar to the numerical
simulations conducted by the previous studies. We propose a scenario that the
-25 km/s cloud and the -20 km/s cloud were collided and triggered the formation
of the massive star system HD 92206 in Gum 31. This scenario can explain the
offset of the stars from the center and the morphology of Gum 31
simultaneously. The timescale of the collision was estimated to be ~1 Myr by
using the ratio between the path length of the collision and the assumed
velocity separation. This is consistent with that of the CCCs in Carina Nebula
Complex in our previous study.
