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Paper: Molecular Cloud Evolution
Volume: 438, The Dynamic Interstellar Medium: A Celebration of the Canadian Galactic Plane Survey
Page: 83
Authors: Vázquez-Semadeni, E.
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 ∼ 1.5×1021 cm–2 ≈ 10 Mpc–2. At this point, the cloud begins to contract gravitationally. However, before its global collapse is completed (∼107 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. More work is necessary to clarify the details and characteristic scales of this process.
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