Weak magnetic fields in early-type stars: failed fossils

TL;DR

This paper investigates the origin and evolution of weak magnetic fields in early-type stars, proposing they are evolving remnants of inherited or created fields, with implications for their strength and structure over stellar lifetimes.

Contribution

It introduces a model linking magnetic field evolution to the Alfven timescale and rotation, suggesting all intermediate- and high-mass stars may host detectable magnetic fields.

Findings

Stars likely contain magnetic fields similar to Vega and Sirius.

Faster rotators tend to have stronger magnetic fields.

Field strength generally decreases over most of the star's life.

Abstract

Weak magnetic fields have recently been detected in Vega and Sirius. Here, we explore the possibility that these fields are the remnants of some field inherited or created during or shortly after star formation and, unlike true fossil fields, are still evolving as we observe them. The timescale of this evolution is given in terms of the Alfven timescale and the rotation frequency by tau_evol ~ tau_A^2 Omega, which would be comparable to the age of the star. It is shown that it is likely that all intermediate- and high-mass stars contain fields of at least the order of the strength found so far in Vega and Sirius. Faster rotators are expected to have stronger magnetic fields. Stars may experience an increase in field strength during their early main-sequence, but for most of their lives field strength will decrease slowly. The length scale of the magnetic structure on the surface may be small in very young stars but should quickly increase to at least very approximately a fifth of the stellar radius.