Collapse and Fragmentation of Finite Magnetized Sheets

Details Collapse and Fragmentation of Finite Magnetized Sheets Collapse and Fragmentation of Finite Magnetized Sheets

Jan 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010aas...21541419h&link_type=abstract

American Astronomical Society, AAS Meeting #215, #414.19; Bulletin of the American Astronomical Society, Vol. 42, p.257

Astronomy and Astrophysics

Astronomy

Scientific paper

We revisit the collapse of finite uniform sheets, including magnetic fields and ambipolar drift. As demonstrated by Burkert & Hartmann (2004, BH04), the collapse of such sheets leads to a strong pile-up of mass at the edge of the sheet, due to the locally strongly varying gravitational accelerations. These edges are prone to rapid fragmentation and collapse before local perturbations inside the cloud have time to grow. This separates these models fundamentally from "periodic-box"-models, where linear perturbations have infinite time to grow. The introduction of magnetic fields under perfect flux freezing will suppress the strong gravitational focusing observed by BH04, unless the fields are extremely weak. Relaxing the flux-freezing assumption by introducing ambipolar drift leads to two effects. First, on a global scale, the collapse is slowed with respect to the hydrodynamical case, while it proceeds faster than in the perfect flux-freezing case. Second, due to the strong accelerations at the edges, sharp density and magnetic field gradients develop, which in turn lead to a local acceleration of ambipolar drift and thus allowing the rapid fragmentation of the dense ring. Thus, the combination of (sub- or super-critical) magnetic fields and ambipolar drift seems to enhance the results of the gravitational focusing effects of BH04. It should also be noted that a realistic field strength distribution will inevitably lead to an expansion of a sub-critical uniform sheet, raising the question about the nature of the required confining pressures.
F.H. acknowledges support by NSF grant AST-0807305.

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