Journal Article

Gravity or turbulence? Velocity dispersion–size relation

Javier Ballesteros-Paredes, Lee W. Hartmann, Enrique Vázquez-Semadeni, Fabian Heitsch and Manuel A. Zamora-Avilés

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 411, issue 1, pages 65-70
Published in print February 2011 | ISSN: 0035-8711
Published online January 2011 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2010.17657.x
Gravity or turbulence? Velocity dispersion–size relation

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We discuss the nature of the velocity dispersion versus size relation for molecular clouds. In particular, we add to previous observational results showing that the velocity dispersions in molecular clouds and cores are not purely functions of the spatial scale but involve surface gas densities as well. We emphasize that hydrodynamic turbulence is required to produce the first condensations in the progenitor medium. However, as the cloud is forming, it also becomes bound, and gravitational accelerations dominate the motions. Energy conservation in this case implies |Eg| ∼Ek, in agreement with observational data, and providing an interpretation for two recent observational results: the scatter in the δvR plane, and the dependence of the velocity dispersion on the surface density δv2/R∝Σ. We argue that the observational data are consistent with molecular clouds in a state of hierarchical and chaotic gravitational collapse, i.e. developing local centres of collapse throughout the whole cloud while the cloud itself is collapsing, and making equilibrium unnecessary at all stages prior to the formation of actual stars. Finally, we discuss how this mechanism need not be in conflict with the observed star formation rate.

Keywords: turbulence; stars: formation; ISM: clouds; ISM: general; ISM: kinematics and dynamics

Journal Article.  4203 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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