Revealing a spiral-shaped molecular cloud in our galaxy - Cloud
  fragmentation under rotation and gravity
  The dynamical processes that control star formation in molecular clouds are
not well understood, and in particular, it is unclear if rotation plays a major
role in cloud evolution. We investigate the importance of rotation in cloud
evolution by studying the kinematic structure of a spiral-shaped Galactic
molecular cloud G052.24+00.74. The cloud belongs to a large filament, and is
stretching over ~ 100 pc above the Galactic disk midplane. The spiral-shaped
morphology of the cloud suggests that the cloud is rotating. We have analysed
the kinematic structure of the cloud, and study the fragmentation and star
formation. We find that the cloud exhibits a regular velocity pattern along
west-east direction - a velocity shift of ~ 10 km/s at a scale of ~ 30 pc. The
kinematic structure of the cloud can be reasonably explained by a model that
assumes rotational support. Similarly to our Galaxy, the cloud rotates with a
prograde motion. We use the formalism of Toomre (1964) to study the cloud's
stability, and find that it is unstable and should fragment. The separation of
clumps can be consistently reproduced assuming gravitational instability,
suggesting that fragmentation is determined by the interplay between rotation
and gravity. Star formation occurs in massive, gravitational bound clumps.
