Polarimetric observations of $sigma$ Orionis E

Alex C. Carciofi, Daniel M. Faes, Richard H. D. Townsend, Jon E. Bjorkman

Instituto de Astronomia, Geof'isica e Ci^encias Atmosf'ericas, Universidade de S~ao Paulo, Rua do Mat~ao 1226, Cidade Universit'aria, 05508-900, S~ao Paulo, SP, BRAZIL
Department of Astronomy, University of Wisconsin-Madison, Sterling Hall, 475 N. Charter Street, Madison, WI 53706, USA
Ritter Observatory, Department of Physics & Astronomy, University of Toledo,Toledo, OH 43606, USA

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 $150fdg0$ 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.

Reference: Accepted by The Astrophysical Journal Letters
Status: Manuscript has been accepted

Weblink: http://arxiv.org/abs/1302.4684


Email: carciofi@usp.br