What Sets the Initial Rotation Rates of Massive Stars?

TL;DR

This paper investigates the mechanisms determining initial rotation rates of massive stars, concluding magnetic torques are insufficient for spin-down during formation, implying other factors influence their initial spin.

Contribution

The study develops a comprehensive model of angular momentum evolution in stars, highlighting the limited role of magnetic torques in massive star spin-down during formation.

Findings

Magnetic torques cannot significantly spin down massive stars during formation.

Magnetic torques are also ineffective during the end stage of formation.

Longer disk lifetimes or stronger magnetic fields are needed for spin-down.

Abstract

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