Detectability of small-scale magnetic fields in early-type stars

TL;DR

This study uses detailed simulations to interpret the null results of magnetic surveys in early-type stars, constraining the strength and scale of small-scale magnetic fields that could be present but undetected.

Contribution

It provides the first comprehensive theoretical analysis linking spectropolarimetric signatures to small-scale magnetic fields in massive stars, setting upper limits based on observational data.

Findings

Small-scale magnetic fields stronger than 50-250 G are ruled out in sharp-lined stars.

Fields above ~1 kG are unlikely in broad-lined stars.

Simulations help interpret non-detections in magnetic surveys.

Abstract

Strong, globally-organized magnetic fields are found for a small fraction of O, B, and A stars. At the same time, many theoretical and indirect observational studies suggested ubiquitous presence of weak localized magnetic fields at the surfaces of massive stars. However, no direct detections of such fields have been reported yet. We have carried out the first comprehensive theoretical investigation of the spectropolarimetric observational signatures of the structured magnetic fields. These calculations are applied to interpret null results of the recent magnetic surveys of massive stars. The intensity and circular polarization spectra of early-type stars are simulated using detailed polarized radiative transfer calculations with LTE model atmospheres. Similar to observational analyses, the mean Stokes I and V line profiles are obtained by applying a multi-line averaging technique. Different spectropolarimetric observables are examined for multiple realizations of randomly distributed radial magnetic field spots with different spatial scales. We characterize the amplitude of the circular polarization profiles and the mean longitudinal magnetic field as a function of magnetic spot sizes. The dependence of these observables on the effective temperature, projected rotational velocity, and inclination angle is also investigated. Using results of the recently completed Magnetism in Massive Stars (MiMeS) survey, we derive upper limits on the small-scale magnetic fields compatible with the MiMeS non-detections. According to our simulations, existing spectropolarimetric observations of sharp-lined massive stars rule out the presence of the small-scale fields stronger than 50-250 G, depending on the typical spot sizes. For broad-lined stars, the observations constrain such fields to be below approximately 1 kG.