Richard M. Crutcher
Role of Magnetic Fields in Star Formation
Depending on their strength, interstellar magnetic fields can affect the flow of the interstellar medium to form self-gravitating clouds and the subsequent contraction of those clouds to form stars. This talk will discuss primarily the observations of magnetic fields in order to test theoretical models. The quantity that governs the importance of the magnetic field in star formation is the ratio of gravitational-to-magnetic energy (M/Φ, the mass-to-flux ratio, which is proportional to the ratio of column density to magnetic field strength). The magnetic field strength can only be measured directly in neutral gas via the Zeeman effect, although only the line-of-sight component can be measured. This talk will therefore focus on Zeeman observations, and on the five major compilations and surveys. These Zeeman data are used to test explicitly the theory that star formation is driven by ambipolar diffusion, the process by which neutrals contract gravitationally through the magnetic field and the ions frozen to the field. Several observational tests will be described. (1) A plot of the line-of-sight magnetic field strength (BLOS) vs. the hydrogen column density (NH) will be examined to look for evidence of lower density subcritical (magnetically supported) clouds that may evolve to supercritical molecular clouds collapsing to form stars. (2) Results of a Bayesian analysis of the BLOS to infer the probability distribution function (PDF) of the total magnetic field strength Btotal will be presented and compared with the predictions of theory. (3) An experiment to directly measure the increase in M/Φ from cloud envelope to cloud core that is predicted by ambipolar diffusion will be described and the results discussed. (4) Ambipolar diffusion models for two specific clouds will be compared with observations. The conclusion to be presented is that there is no definitive observational evidence in support of ambipolar-diffusion driven star formation, while the observations are generally consistent with models that have magnetic fields important with respect to kinetic energy (thermal plus turbulent). The data do not, however, rule out ambipolar-diffusion driven star formation being important in some cases.