The Dynamical Structure and Evolution of Giant Molecular Clouds

McKee, Christopher F.

doi:10.1007/978-94-011-4509-1_2

“…If MHD turbulence decays quickly, then serious problems face the researchers attempting to explain important observational facts, e.g. turbulent motions seen within molecular clouds without star formation [119] and rates of star formation [106]. Earlier studies attributed the rapid decay of turbulence to compressibility effects [99].…”

Section: Imbalanced Cascadementioning

confidence: 99%

“…In a recent review [106] McKee pointed out that the fast damping of MHD turbulence observed in numerical simulations is difficult to reconcile with the fact that "a GMC such as G216, which has no visible star formation, can have a level of turbulence that exceeds that in the Rosette molecular cloud, which has an embedded OB association". He pointed out that the conclusions obtained on the basis of numerics should be treated with caution as they do not resolve the microscales.…”

Section: Support and Compression Of Molecular Cloudsmentioning

confidence: 99%

MHD Turbulence: Scaling Laws and Astrophysical Implications

Cho

¹

,

Lazarían

²

,

Vishniac

³

2003

Turbulence and Magnetic Fields in Astrophysics

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Abstract. Turbulence is the most common state of astrophysical flows. In typical astrophysical fluids, turbulence is accompanied by strong magnetic fields, which has a large impact on the dynamics of the turbulent cascade. Recently, there has been a significant breakthrough on the theory of magnetohydrodynamic (MHD) turbulence. For the first time we have a scaling model that is supported by both observations and numerical simulations. We review recent progress in studies of both incompressible and compressible turbulence. We compare Iroshnikov-Kraichnan and Goldreich-Sridhar models, and discuss scalings of Alfvén, slow, and fast waves. We also discuss the completely new regime of MHD turbulence that happens below the scale at which hydrodynamic turbulent motions are damped by viscosity. In the case of the partially ionized diffuse interstellar gas the viscosity is due to neutrals and truncates the turbulent cascade at ∼parsec scales. We show that below this scale magnetic fluctuations with a shallow spectrum persist and discuss the possibility of a resumption of the MHD cascade after ions and neutrals decouple. We discuss the implications of this new insight into MHD turbulence for cosmic ray transport, grain dynamics, etc., and how to test theoretical predictions against observations.

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“…Both statements seem highly implausible in a dynamical, nonlinear ISM. McKee (1999) has suggested two possibilities, in order to assume VE for MC: One is that, for a single cloud, Ï = 0 if the considered averaging time is much larger than the dynamical timescale of the cloud, t avg ≫ t dyn . The other is that for an ensemble of clouds, some of them may have positive values ofÏ, and some others will have negative values, so that VE holds for the ensemble.…”

Section: Virial Theorem: Are Clouds Actually In Virialmentioning

confidence: 99%

“…ballesteros.tex; 26/06/2018; 18:55; p.4 star-forming cores (Taylor et al, Lee & Myers, 1996, 1999; see also Evans, 1999 and references therein).…”

Section: Virial Theorem: Are Clouds Actually In Virialmentioning

confidence: 99%

Molecular Clouds: Formation and Disruption

Ballesteros-Paredes¹

2004

Astrophysics and Space Science

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Abstract.Molecular clouds (MC) are the densest and coldest component of the interstellar gas, and the sites of star formation. They are also turbulent and fractal and their masses and sizes span several orders of magnitude. It is also generally believed that they are close to Virial equilibrium (VE). Since this statement has been questioned by a number of authors, with important implications on molecular clouds' lifetimes, we will review this subject within the context of a turbulent ISM. In this framework, there is significant numerical evidence that MCs are not in VE, that there is a strong exchange of mass, momentum and energy between clouds and their surrounding medium, and that it is difficult (if not impossible) to form quasistatic cores inside MCs, suggesting that they must be transient, short-lived phenomena. Thus, their formation and disruption must be primarily dynamical, and probably not due to just a single mechanism, but rather to the combination of several processes. This picture seems consistent with recent estimates of ages of stars in the solar neighborhood.

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“…ballesteros.tex; 22/03/2018; 1:37; p.4 star-forming cores (Taylor et al, Lee & Myers, 1996, 1999; see also Evans, 1999 and references therein).…”

Section: Virial Theorem: Are Clouds Actually In Virialmentioning

confidence: 99%

Molecular Clouds: Formation and Disruption

Ballesteros-Paredes¹

2004

From Observations to Self-Consistent Modelling of the ISM in Galaxies

1

0

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Abstract.Molecular clouds (MC) are the densest and coldest component of the interstellar gas, and the sites of star formation. They are also turbulent and fractal and their masses and sizes span several orders of magnitude. It is also generally believed that they are close to Virial equilibrium (VE). Since this statement has been questioned by a number of authors, with important implications on molecular clouds' lifetimes, we will review this subject within the context of a turbulent ISM. In this framework, there is significant numerical evidence that MCs are not in VE, that there is a strong exchange of mass, momentum and energy between clouds and their surrounding medium, and that it is difficult (if not impossible) to form quasistatic cores inside MCs, suggesting that they must be transient, short-lived phenomena. Thus, their formation and disruption must be primarily dynamical, and probably not due to just a single mechanism, but rather to the combination of several processes. This picture seems consistent with recent estimates of ages of stars in the solar neighborhood.

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The Dynamical Structure and Evolution of Giant Molecular Clouds

Cited by 83 publications

References 91 publications

MHD Turbulence: Scaling Laws and Astrophysical Implications

MHD Turbulence: Scaling Laws and Astrophysical Implications

Molecular Clouds: Formation and Disruption

Molecular Clouds: Formation and Disruption

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