Polarimetric observations of $\sigma$ Orionis E

TL;DR

This study reports the first detection of a magnetosphere in continuum linear polarization around $\sigma$ Orionis E, revealing asymmetric plasma structures and providing new insights into the star's magnetic environment through polarimetric monitoring.

Contribution

It presents the first polarimetric detection of a magnetosphere in a massive star and introduces a toy model with co-rotating blobs to interpret polarization data.

Findings

Detection of intrinsic polarization indicating asymmetry.

Polarization modulation with half the rotation period.

Mass ratio of blobs to disk approximately 1.2.

Abstract

Some massive stars possess strong magnetic fields that confine plasma in the circumstellar environment. These \textit{magnetospheres} have been studied spectroscopically, photometrically and, more recently, interferometrically. Here we report on the first firm detection of a magnetosphere in continuum linear polarization, as a result of monitoring of $\sigma$\,Ori\,E at the Pico dos Dias Observatory. {The non-zero intrinsic polarization indicates an asymmetric structure, whose minor elongation axis is oriented $150\fdg0$ east of the celestial north.} A modulation of the polarization was observed, with a period of half of the rotation period, which supports the theoretical prediction of the presence of two diametrally opposed, co-rotating blobs of gas. A phase lag of -0.085 was detected between the polarization minimum and the primary minimum of the light curve, suggestive of a complex shape of the plasma clouds. We present a preliminary analysis of the data with the Rigidly Rotating Magnetosphere model, which could not reproduce simultaneously the photometric and polarimetric data. A toy model comprising two spherical co-rotating blobs {joined by a thin disk} proved more successful in reproducing the polarization modulation. {With this model we were able to determine that the total scattering mass of the thin disk is similar to the mass of the blobs ($2M_{\rm b}/M_{\rm d}=1.2$) and that the blobs are rotating counterclockwise on the plane of the sky.} This result shows that polarimetry can provide a diagnostic of the geometry of clouds, which will serve as an important constraint for {improving} the Rigidly Rotating Magnetosphere model.