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The physical mechanisms that set the initial rotation rates in massive starsare a crucial unknown in current star formation theory. Observations of young,massive stars provide evidence that they form in a similar fashion to theirlow-mass counterparts. The magnetic coupling between a star and its accretiondisk may be sufficient to spin down low-mass pre-main sequence (PMS) stars towell below breakup at the end stage of their formation when the accretion rateis low. However, we show that these magnetic torques are insufficient to spindown massive PMS stars due to their short formation times and high accretionrates. We develop a model for the angular momentum evolution of stars over awide range in mass, considering both magnetic and gravitational torques. Wefind that magnetic torques are unable to spin down either low or high massstars during the main accretion phase, and that massive stars cannot be spundown significantly by magnetic torques during the end stage of their formationeither. Spin-down occurs only if massive stars' disk lifetimes aresubstantially longer or their magnetic fields are much stronger than currentobservations suggest.

What Sets the Initial Rotation Rates of Massive Stars?