Astro FOTD: Angular Momentum

The clouds of dust and gas from which stars form usually have a specific angular momentum several orders of magnitude larger than typical stellar and planetary systems. Consequently, proto-planetary systems need a mechanism for ridding themselves of excess angular momentum. Turbulent mechanisms involving magnetic or gravitational interactions in the proto-planetary disk are believed to transport angular momentum outward.

Schematically, imagine a disk rotating with Keplerian velocities (gravity just providing the required centrepetal force), if an inner portion is linked by magnetic flux lines to an outer portion, magnetic tension will supply a braking torque to the inner portion, thus transporting angular momentum outward.

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