Magnetic Archaeology of Early-Type Stellar Dynamos

TL;DR

This paper explores how magnetic fields in early-type stars evolve, suggesting that surface magnetism reflects fossil fields or dynamo activity, with evolution timescales influenced by mass loss and magnetic diffusion.

Contribution

It extends the theory of magnetic field processing in early-type stars, analyzing the timescales of magnetic evolution and the impact of mass loss on surface magnetism.

Findings

Mass loss strips near-surface regions faster than magnetic diffusion.

Surface magnetism reflects frozen-in dynamo remnants.

Magnetic fields in stars with weak fields evolve on mass-loss timescales.

Abstract

Early-type stars show a bimodal distribution of magnetic field strengths, with some showing very strong fields ($\gtrsim 1\,\mathrm{kG}$) and others very weak fields ($\lesssim 10\,\mathrm{G}$). Recently, we proposed that this reflects the processing or lackthereof of fossil fields by subsurface convection zones. Stars with weak fossil fields process these at the surface into even weaker dynamo-generated fields, while in stars with stronger fossil fields magnetism inhibits convection, allowing the fossil field to remain as-is. We now expand on this theory and explore the time-scales involved in the evolution of near-surface magnetic fields. We find that mass loss strips near-surface regions faster than magnetic fields can diffuse through them. As a result, observations of surface magnetism directly probe the frozen-in remains of the convective dynamo. This explains the slow evolution of magnetism in stars with very weak fields: these dynamo-\emph{generated} magnetic fields evolve on the time-scale of the mass loss, not that of the dynamo.