The Origin of the Bimodal Distribution of Magnetic Fields in Early-type Stars

TL;DR

This paper investigates the origin of the bimodal magnetic field distribution in early-type stars, proposing that strong fields are fossil remnants from star formation or mergers, while weak fields result from dynamo processes.

Contribution

It introduces a calculation of the critical magnetic field strength that determines whether a fossil field can survive surface convection, explaining the bimodal distribution.

Findings

Critical field strength matches observed lower limits (~300 G) in magnetic stars.

Critical field strength slightly increases during main sequence evolution.

Supports two distinct origins for magnetic fields: fossil and dynamo.

Abstract

In early-type stars a fossil magnetic field may be generated during the star formation process or be the result of a stellar merger event. Surface magnetic fields are thought to be erased by (sub)surface convection layers, which typically leave behind weak disordered fields. However, if the fossil field is strong enough it can prevent the onset of (sub)surface convection and so be preserved onto the main sequence. We calculate the critical field strength at which this occurs, and find that it corresponds well with the lower limit amplitude of observed fields in strongly magnetised Ap/Bp stars ($\approx$ 300 G). The critical field strength is predicted to increase slightly during the main sequence evolution, which could also explain the observed decline in the fraction of magnetic stars. This supports the conclusion that the bimodal distribution of observed magnetic fields in early-type stars reflects two different field origin stories: strongly magnetic fields are fossils fields inherited from star formation or a merger event, and weak fields are the product of on-going dynamo action.