Origin of the initial spin of stars

Determine what sets the initial spin of a star at the time of its formation.

Background

Observations indicate that many fast-rotating stars are in binary systems with companions at separations of tens of astronomical units, suggesting a possible link between multiplicity and stellar spin. While disc-locking tends to spin down young stars, shortened disc lifetimes—common in binary systems due to disc truncation—can reduce spin-down, but this alone does not explain the initial distribution of stellar rotation rates.

Existing models of rotational evolution over up to 1 Gyr reproduce fast rotators by assuming shorter disc lifetimes, yet they do not explain how stars acquire their initial spins. This motivates the explicit open question of identifying the physical processes that determine the initial stellar spin at birth.

References

What sets the initial spin of a star is still an open question.

— Protostellar spin-up and fast rotator formation through binary star formation  (2404.14210 - Kuruwita et al., 2024) in Introduction

The nature of this slow rotation in Ap stars has long remained an open question (Stepien 1998).

— Search for magnetic fields in seven slowly rotating A stars  (2608.28258 - Rull et al., 28 Aug 2026) in Section 1, Introduction, page 2