Triggered O star formation in M20 via cloud-cloud collision: Comparisons
  between high-resolution CO observations and simulations
  High-mass star formation is one of the top-priority issues in astrophysics.
Recent observational studies are revealing that cloud-cloud collisions may play
a role in high-mass star formation in several places in the Milky Way and the
Large Magellanic Cloud. The Trifid Nebula M20 is a well known galactic HII
region ionized by a single O7.5 star. In 2011, based on the CO observations
with NANTEN2 we reported that the O star was formed by the collision between
two molecular clouds ~0.3,Myr ago. Those observations identified two molecular
clouds towards M20, traveling at a relative velocity of 7.5 km/s. This velocity
separation implies that the clouds cannot be gravitationally bound to M20, but
since the clouds show signs of heating by the stars there they must be
spatially coincident with it. A collision is therefore highly possible. In this
paper we present the new CO J=1-0 and J=3-2 observations of the colliding
clouds in M20 performed with the Mopra and ASTE telescopes. The high resolution
observations revealed the two molecular clouds have peculiar spatial and
velocity structures, i.e., the spatially complementary distribution between the
two clouds and the bridge feature which connects the two clouds in velocity
space. Based on a new comparison with numerical models, we find that this
complementary distribution is an expected outcome of cloud-cloud collisions,
and that the bridge feature can be interpreted as the turbulent gas excited at
the interface of the collision. Our results reinforce the cloud-cloud collision
scenario in M20.
