Angular momentum loss during pre-main sequence contraction

Abstract

Current ideas on the early stages of star formation suggest strongly that proto-stars enter the pre-main sequence phase rotating rapidly. The subsequent rotational evolution of both single and binary stars is largely determined by angular momentum transport by a magnetically-controlled stellar wind. A massive star with a cool corona but with a modest relic magnetic field may be prevented from fragmenting into a binary system through the braking action of a centrifugally-driven wind. A late-type pre-main sequence star will generate by dynamo action a magnetic field of surface strength B_o that depends on the angular velocity Ω. The consequent braking can be powerful enough to cause spin-down in spite of the contraction. The predicted Ω(t) law depends on the B_o (Ω) relation, on whether the wind is thermal, thermo-centrifugal, or magneto-centrifugal, and on the fraction of the magnetosphere with open field-line. Angular momentum loss from a tidally locked binary pair can lead to the formation of late-type contact binaries with rapidly rotating members, and ultimately to their coalescence into one rapid rotator.