Spin equilibrium in strongly-magnetized accreting stars

TL;DR

This paper investigates how accretion outbursts influence the spin evolution and equilibrium period of strongly magnetized accreting stars, highlighting the importance of disc trapping in their spin dynamics.

Contribution

It introduces models for angular momentum loss during accretion outbursts, demonstrating that outflows extend spin equilibrium timescales and alter final spin periods, emphasizing the role of disc trapping.

Findings

Outflows increase spin equilibrium timescales by about ten times.

Outflows lead to shorter equilibrium spin periods than traditional models predict.

Disc trapping significantly influences the spin evolution of magnetic stars.

Abstract

Strongly magnetized accreting stars are often hypothesized to be in `spin equilibrium' with their surrounding accretion flows, which requires that the accretion rate changes more slowly than it takes the star to reach spin equilibrium. This is not true for most magnetically accreting stars, which have strongly variable accretion outbursts on time-scales much shorter than the time it would take to reach spin equilibrium. This paper examines how accretion outbursts affect the time a star takes to reach spin equilibrium and its final equilibrium spin period. I consider several different models for angular momentum loss -- either carried away in an outflow, lost to a stellar wind, or transferred back to the accretion disc (the `trapped disc'). For transient sources, the outflow scenario leads to significantly longer times to reach spin equilibrium ($\sim$10x), and shorter equilibrium spin periods than would be expected from spin equilibrium arguments, while the `trapped disc' does not. The results suggest that disc trapping plays a significant role in the spin evolution of strongly magnetic stars, with some caveats for young stellar objects.