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 (17)

• Control of star formation by supersonic turbulence
• The Millennium Arecibo 21-CM Absorption Line Survey. II. Properties of the Warm and Cold Neutral Media
• Molecular Cloud Evolution II. From cloud formation to the early stages of star formation in decaying conditions
• Thermal condensation in a turbulent atomic hydrogen flow
• Turbulent Flow-Driven Molecular Cloud Formation: A Solution to the Post-T Tauri Problem?
• The Millennium Arecibo 21-cm Absorption Line Survey. IV. Statistics of Magnetic Field, Column Density, and Turbulence
• Clouds as Turbulent Density Fluctuations. Implications for pressure confinement and spectral line data interpretation
• 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
• Formation of Structure in Molecular Clouds: A Case Study
• Disk-Halo Interaction - I. Three-Dimensional Evolution of the Galactic Disk
• On the Structure of the Orion A Cloud and the Formation of the Orion Nebula Cluster
• A Turbulent Model for the Interstellar Medium. II. Magnetic Fields and Rotation
• Can WNM survive inside Molecular Clouds ?
• The Shocking Properties of Supersonic Flows: Dependence of the Thermal Overstability on M, alpha, and T_c/T_0
• Molecular Cloud Turbulence and Star Formation
• Compressible MHD Turbulence: Implications for Molecular Cloud and Star Formation

Cited by in corpus (9)

• Gravity or turbulence? II. Evolving column density PDFs in molecular clouds
• Molecular Cloud Evolution IV: Magnetic Fields, Ambipolar Diffusion, and the Star Formation Efficiency
• The sonic scale revealed by the world's largest supersonic turbulence simulation
• Molecular Cloud Evolution III. Accretion vs. stellar feedback
• Early turbulent mixing as the origin of chemical homogeneity in open star clusters
• Formation of Cold Filamentary Structure from Wind Blown Superbubbles
• The Effects of Inhomogeneities within Colliding Flows on the Formation and Evolution of Molecular Clouds
• Instrumental performance and results from testing of the BLAST-TNG receiver, submillimeter optics, and MKID arrays
• Supersonically Turbulent, Shock Bound Interaction Zones