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R. M. Crutcher (Univ. Illinois)
Current theoretical models for what drives star formation (especially low-mass star formation) are: (1) magnetic support of self-gravitating clouds with ambipolar diffusion removing support in cores and triggering collapse, and (2) compressible turbulence forming self-gravitating clumps that collapse as soon as the turbulent cascade produces insufficient turbulent support. A crucial observational difference between the two models is the mass to magnetic flux ratio, M/\Phi. The magnetic support model requires that M/\Phi be subcritial initially, becoming mildly supercritical in the core once ambipolar diffusion drives the star formation process. Although compressible turbulence makes no specific prediction for M/\Phi, in general the diffuse ISM should be magnetically supercritical if turbulence is to form clouds, and M/\Phi must be supercritical in molecular clouds if magnetic support does not dominate. We review the current state of observations of M/\Phi in molecular clouds. The observations of molecular cloud cores suggest that they are approximately critical; that is, there is no clear evidence that sub- or supercritical clouds dominate. Hence, the observations remain inconclusive in deciding between the two extreme-case models of what drives star formation. What is needed to further advance our understanding of the role of magnetic fields in the star formation process are additional high sensitivity surveys of magnetic field strengths and other cloud properties in order to further refine the assessment of the importance of magnetic fields in molecular cores and envelopes.
This work was partially supported by NSF grant AST 02-05810.
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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.