2025-05-27 –, Main Conference Room
Molecular clouds (MCs) are known to be supersonically turbulent, and this turbulence has long been associated with maintaining the clouds in near virial equilibrium with their self-gravity. However, recent observations and simulations indicate that MCs lie at the intersections of shells, suggesting that they are actually formed by the collision of the shells. But clearly this mechanism cannot apply to the first generation of stars that formed the shells. In this contribution, I will discuss the evolution of molecular clouds and their star formation activity in the galactic context from the standpoint of a continuous gas flow entering the gravitational potential of a stellar spiral arm. Warm atomic gas entering the arm suffers a phase transition to the cold phase, and continues to be compressed to become molecular and gravitationally contracting. As a consequence, the star formation rate increases and massive stars begin to form after a few Myr. The virial parameter is of order unity during the contraction stage. The feedback is capable of dispersing the remaining gas, pushing the virial parameter to large values, and reducing or stopping the local star formation episode, while collecting the dense gas somewhere else. This reinitiates the cycle somewhere else, possibly with weakening strength, until the gas leaves the spiral arm, leaving behind some molecular shreds that may survive into the interarm region.
