Weak magnetic fields in early-type stars: failed fossils

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 are still evolving dynamically as we observe them. The time-scale of this evolution is given in terms of the Alfvén time-scale and the rotation frequency by τevol ∼ τ2A Ω, which is then comparable to the age of the star. According to this theory, all intermediate- and high-mass stars should contain fields of at least the strength found so far in Vega and Sirius. Faster rotators are expected to have stronger magnetic fields. Stars may experience an increase in surface 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. The field strength may be higher at the poles than at the equator.