The internal rotational dynamics of massive stars are poorly understood .
If angular momentum ( AM ) transport between the core and the envelope is inefficient , the large core AM upon core-collapse will produce rapidly rotating neutron stars ( NSs ) .
However , observations of low-mass stars suggest an efficient AM transport mechanism is at work , which could drastically reduce NS spin rates .
Here we study the effects of the baroclinic instability and the magnetic Tayler instability in differentially rotating radiative zones .
Although the baroclinic instability may occur , the Tayler instability is likely to be more effective for AM transport .
We implement Tayler torques as prescribed by into models of massive stars , finding they remove the vast majority of the core ’ s AM as it contracts between the main sequence and helium-burning phases of evolution .
If core AM is conserved during core-collapse , we predict natal NS rotation periods of P _ { NS } \approx 50 - 200 { ms } , suggesting these torques help explain the relatively slow rotation rates of most young NSs , and the rarity of rapidly rotating engine-driven supernovae .
Stochastic spin-up via waves just before core-collapse , asymmetric explosions , and various binary evolution scenarios may increase the initial rotation rates of many NSs .