Revisiting the Rigidly Rotating Magnetosphere model for $\sigma$ Ori E - II. Magnetic Doppler imaging, arbitrary field RRM, and light variability

TL;DR

This study refines the Rigidly Rotating Magnetosphere (RRM) model for sigma Ori E by incorporating magnetic Doppler imaging to better understand magnetic field configurations and light variability, revealing the need for additional physics in the model.

Contribution

The paper introduces an arbitrary surface magnetic field configuration into the RRM model using magnetic Doppler imaging results, improving the understanding of magnetic topology and surface abundance effects.

Findings

Magnetic Doppler imaging reveals a predominantly dipolar magnetic field with a quadrupolar component.

Synthetic H-alpha emission and photometry generally match observations, but some features are poorly fit.

Photospheric abundance inhomogeneities do not account for all photometric variability discrepancies.

Abstract

The initial success of the Rigidly Rotating Magnetosphere (RRM) model application to the B2Vp star sigma OriE by Townsend, Owocki & Groote (2005) triggered a renewed era of observational monitoring of this archetypal object. We utilize high-resolution spectropolarimetry and the magnetic Doppler imaging (MDI) technique to simultaneously determine the magnetic configuration, which is predominately dipolar, with a polar strength Bd = 7.3-7.8 kG and a smaller non-axisymmetric quadrupolar contribution, as well as the surface distribution of abundance of He, Fe, C, and Si. We describe a revised RRM model that now accepts an arbitrary surface magnetic field configuration, with the field topology from the MDI models used as input. The resulting synthetic Ha emission and broadband photometric observations generally agree with observations, however, several features are poorly fit. To explore the possibility of a photospheric contribution to the observed photometric variability, the MDI abundance maps were used to compute a synthetic photospheric light curve to determine the effect of the surface inhomogeneities. Including the computed photospheric brightness modulation fails to improve the agreement between the observed and computed photometry. We conclude that the discrepancies cannot be explained as an effect of inhomogeneous surface abundance. Analysis of the UV light variability shows good agreement between observed variability and computed light curves, supporting the accuracy of the photospheric light variation calculation. We thus conclude that significant additional physics is necessary for the RRM model to acceptably reproduce observations of not only sigma Ori E, but also other similar stars with significant stellar wind-magnetic field interactions.