Investigating the origin of cyclical wind variability in hot, massive stars - I. On the dipolar magnetic field hypothesis

TL;DR

This study investigates whether dipolar magnetic fields cause wind variability in OB stars with DACs, finding no evidence for such fields and suggesting alternative mechanisms are responsible.

Contribution

The paper provides the first systematic spectropolarimetric analysis of magnetic fields in OB stars with DACs, challenging the magnetic hypothesis for wind variability.

Findings

No surface dipolar magnetic fields detected in the sample.

Magnetic wind confinement and brightness enhancements unlikely to cause DACs.

Alternative mechanisms beyond dipolar fields are needed to explain wind variability.

Abstract

OB stars exhibit various types of spectral variability associated with wind structures, including the apparently ubiquitous discrete absorption components (DACs). These are proposed to be caused by either magnetic fields or non-radial pulsations (NRPs). In this paper, we evaluate the possible relation between large-scale, dipolar magnetic fields and the DAC phenomenon by investigating the magnetic properties of a sample of 13 OB stars exhibiting well-documented DAC behaviour. Using high-precision spectropolarimetric data acquired in part in the context of the Magnetism in Massive Stars (MiMeS) project, we find no evidence for surface dipolar magnetic fields in any of these stars. Using Bayesian inference, we compute upper limits on the strengths of the fields and use these limits to assess two potential mechanisms by which the field may influence wind outflow: magnetic wind confinement and local photospheric brightness enhancements. Within the limits we derive, both mechanisms fail to provide a systematic process capable of producing DACs in all of the stars of our sample. Therefore, this implies that dipolar fields are highly unlikely to be responsible for these structures in all massive stars, meaning that some other mechanism must come into play.