The MiMeS survey of Magnetism in Massive Stars: Magnetic properties of the O-type star population

TL;DR

The MiMeS survey analyzed magnetic fields in O-type stars, finding no evidence for universal strong fields and suggesting a bimodal distribution of magnetic strengths, but could not confirm magnetic flux decay.

Contribution

This study provides the first comprehensive Bayesian analysis of magnetic field strengths in a large sample of O-type stars, revealing a likely bimodal magnetic field distribution.

Findings

A 100 G dipolar field is ruled out for all stars in the sample.

The magnetic field distribution appears bimodal, with stars having either weak or strong fields.

The survey cannot confirm magnetic flux decay in O-type stars.

Abstract

In this paper, we describe an analysis of the MiMeS Survey of O-type stars to explore the range of dipolar field strengths permitted by the polarisation spectra that do not yield a magnetic detection. We directly model the Stokes V profiles with a dipolar topology model using Bayesian inference. The noise statistics of the Stokes V profiles are in excellent agreement with those of the null profiles. Using a Monte-Carlo approach we conclude that a model in which all the stars in our sample were to host a 100\,G, dipolar magnetic field can be ruled out by the MiMeS data. Furthermore, if all the stars with no detection were to host a magnetic field just below their detection limit, the inferred distribution in strength of these undetected fields would be distinct from the known distribution in strength of the known magnetic O-type stars. This indicates that the Initial magnetic (B-)field Function (IBF) is likely bimodal -- young O-type stars are expected to either have weak/absent magnetic fields, or strong magnetic fields. We also find that better upper limits, by at least a factor of 10, would have been necessary to rule out a detection bias as an explanation for the apparent lack of evolved main-sequence magnetic O-type stars reported in the literature, and we conclude that the MiMeS survey cannot confirm or refute a magnetic flux decay in O-type stars.