Molecular Cloud Evolution

Abstract

I describe the scenario of molecular cloud (MC) evolution that has emerged over the past decade or so. MCs can start out as cold atomic clouds formed by compressive motions in the warm neutral medium (WNM) of galaxies. Such motions can be driven by large-scale instabilities, or by local turbulence. The compressions induce a phase transition to the cold neutral medium (CNM) to form growing cold atomic clouds, which in their early stages may constitute thin CNM sheets. Several dynamical instabilities soon destabilize a cloud, rendering it turbulent. For solar neighborhood conditions, a cloud is coincidentally expected to become molecular, magnetically supercritical, and gravitationally dominated at roughly the same column density, $N \sim 1.5 \times 10^21 \psc \approx 10 \Msun$ pc${}^{-2}$. At this point, the cloud begins to contract gravitationally. However, before its global collapse is completed ($\sim 10^7$ yr later), the nonlinear density fluctuations within the cloud, which have shorter local free-fall times, collapse first and begin forming stars, a few Myr after the global contraction started. Large-scale fluctuations of lower mean densities collapse later, so the formation of massive star-forming regions is expected to occur late in the evolution of a large cloud complex, while scattered low-mass regions are expected to form earlier. Eventually, the local star formation episodes are terminated by stellar feedback, which disperses the local dense gas, although more work is necessary to clarify the details and characteristic scales of this process.

15 pages, 7 figure files. Invited review for "The Dynamic ISM: A celebration of the Canadian Galactic Plane Survey," ASP Conference Series, from conference held in Naramata BC, Canada June 6-10, 2010

References in corpus (5)

• Molecular Cloud Evolution II. From cloud formation to the early stages of star formation in decaying conditions
• The Birth of Molecular Clouds: Formation of Atomic Precursors in Colliding Flows
• The Global Evolution of Giant Molecular Clouds. I: Model Formulation and Quasi-Equilibrium Behavior
• On the Structure of the Orion A Cloud and the Formation of the Orion Nebula Cluster
• Molecular Cloud Turbulence and Star Formation