# Angular momentum transport in massive stars and natal neutron star rotation rates

## TL;DR

This paper investigates how magnetic and rotational instabilities in massive stars influence angular momentum transport, predicting neutron star spin rates that align with observations and explaining the rarity of rapidly rotating young neutron stars.

## Contribution

It demonstrates that Tayler instability-driven angular momentum transport significantly reduces core spin, leading to more accurate predictions of neutron star rotation periods.

## Key findings

- Tayler instability effectively transports angular momentum in massive stars.
- Predicted neutron star periods are 50-200 ms, consistent with observations.
- Additional processes may increase initial neutron star spin rates.

## Abstract

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 Fuller et al. 2019 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 $P_{\rm NS} \approx 50-200 \, {\rm ms}$, 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.