Molecular Cloud Fragmentation Driven by Self-Gravity, Magnetic Fields, Ambipolar Diffusion, and Nonlinear Flows

Details Molecular Cloud Fragmentation Driven by Self-Gravity, Magnetic Fields, Ambipolar Diffusion, and Nonlinear Flows Molecular Cloud Fragmentation Driven by Self-Gravity, Magnetic Fields, Ambipolar Diffusion, and Nonlinear Flows

May 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009aas...21431503b&link_type=abstract

American Astronomical Society, AAS Meeting #214, #315.03; Bulletin of the American Astronomical Society, Vol. 41, p.760

Astronomy and Astrophysics

Astronomy

Scientific paper

To understand the formation and collapse of dense protostellar cores within interstellar clouds, we have generated a collection of theoretical and observationally-testable results from a wide-ranging study of fragmentation models. They include the major effects of self-gravity, magnetic fields, magnetic flux loss through ambipolar diffusion, and "turbulent'' nonlinear flows. We have carried out an extensive parameter survey of the relative strength of each of these effects, in models that utilize the thin-sheet approximation.
A preferred fragmentation scale and mass is predicted from linear theory, and the nonlinear evolution results in scales that are very similar. A very sensitive dependence of the preferred scale on initial mass-to-magnetic-flux ratio means that even a narrow range of initial mass-to-flux ratios can lead to a broad distribution of core masses. Supercritical (gravity-dominated) regions result in cores with large-scale supersonic infall motions at their periphery or beyond, while subcritical (magnetic-field-dominated) regions are characterized by subsonic infall. Super-Alfvenic initial flows lead to prompt collapse and highly supersonic infall motions onto cores, and we are skeptical that such initial conditions can agree with observations. All of these results are verified by additional fully three-dimensional simulations.
Our models also reveal that long-lived oscillations can survive in a cloud that has good magnetic coupling, due to the restoring forces associated with the magnetic field anchored in the interstellar medium outside the molecular cloud. Finally, we propose that magnetic field curvature, as traced by polarization of dust emission, can measure a cloud's ambient mass-to-flux ratio.

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