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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 M☉pc–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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