Institute for Advanced Study Astrophysics Seminar

ABSTRACT: Magnetic forces in the diffuse interstellar medium are much greater than the forces due to self-gravitation, thereby precluding star formation there. Historically, it has been conjectured that ambipolar diffusion, in which neutral molecules contract relative to the magnetized ions, is essential for allowing gravitational forces to exceed magnetic forces in star-forming clouds. However, observations of magnetic field strengths in molecular cloud cores have failed to find evidence of cores that are magnetically dominated. I shall report on ideal magnetohydrodynamic (MHD) simulations of a region in a turbulent molecular cloud and compare them directly with Zeeman observations of the line-of-sight magnetic field in molecular clouds. The simulations provide the total field strength and show how tangling of the field lines reduces the field measured by the Zeeman effect. Magnetic fields are so effective at extracting angular momentum from the gas accreting onto a protostar that many simulations have found that protostellar (and therefore protoplanetary) disks cannot form in the presence of observed interstellar magnetic fields. In a high-resolution simulation of the formation of a massive star in a turbulent, magnetized medium, we find that rotating protostellar disks can indeed form in the presence of magnetic fields. I conclude that ideal MHD in the presence of turbulence is not ideal.