# An abrupt change in the stellar spin-down law at the fully convective boundary

**Authors:** Yuxi Lu, Victor See, Louis Amard, Ruth Angus, Sean P. Matt  
**arXiv:** [2306.09119](https://arxiv.org/abs/2306.09119)  
**Date:** 2023-06-15  
**Summary page:** https://www.summarizepaper.com/en/arxiv-id/2306.09119v1/

## Key points

- Debate on the existence of a radiative core in generating a solar-like magnetic dynamo in stars
- Previous studies suggest that the thin layer between a star's radiative core and convective zone can produce shearing for a solar-like dynamo
- Recent research indicates similar period-activity relations for fully and partially convective stars, suggesting similar properties of dynamos
- Study used kinematic ages to investigate stellar spin-down law across fully convective boundary
- Found an abrupt change in angular momentum loss rate for fully convective stars compared to partially convective stars around this boundary
- Fully convective stars exhibited higher angular momentum loss rate, approximately 2.25 times higher torque than partially convective stars
- Significant difference suggests fundamental differences in dynamos of partially and fully convective stars
- Change implies need for larger dipole field strength or mass loss rate in fully convective stars compared to partially convective stars
- Stellar-wind torques depend on large-scale magnetic fields and mass loss rates derived from magnetic activity
- Abrupt change provides evidence for distinct dynamos in different types of stars
- Findings contribute to understanding stellar magnetic activity and mechanisms for generating magnetic fields in different types of stars

## Summary

The existence of a radiative core in generating a solar-like magnetic dynamo in stars has been a topic of debate. Previous studies using analytic models and magnetohydrodynamic simulations have suggested that the thin layer between a star's radiative core and its convective zone can produce shearing that reproduces key characteristics of a solar-like dynamo. However, recent research has indicated that fully and partially convective stars exhibit similar period-activity relations, implying that dynamos generated by stars with and without radiative cores may have similar properties. In this study, the authors used kinematic ages to investigate the stellar spin-down law across the fully convective boundary. They found an abrupt change in the angular momentum loss rate for fully convective stars compared to partially convective stars around this boundary. Specifically, fully convective stars exhibited a higher angular momentum loss rate, corresponding to a torque that was approximately 2.25 times higher for a given angular velocity than partially convective stars. This significant difference in spin-down behavior suggests that there may be fundamental differences in the dynamos of partially and fully convective stars. The observed change in the stellar spin-down law implies that fully convective stars require either a dipole field strength larger by a factor of approximately 2.5 or a mass loss rate about 4.2 times larger than those of partially convective stars, or some combination of both factors. Stellar-wind torques primarily depend on large-scale magnetic fields and mass loss rates which are derived from magnetic activity; thus, this abrupt change in the spin-down law provides evidence for distinct dynamos in partially and fully convective stars. These findings contribute to our understanding of stellar magnetic activity and provide insights into the mechanisms responsible for generating magnetic fields in different types of stars. Further research is needed to explore these differences more comprehensively and determine their implications for stellar evolution and other astrophysical processes.