Detection of a centrifugal magnetosphere in one of the most massive stars in the $\rho$ Oph star-forming cloud

TL;DR

This study confirms the presence of a magnetic field in the massive star rho Oph A, revealing a centrifugally supported magnetosphere through spectropolarimetry and X-ray variability analysis.

Contribution

First detection of a centrifugal magnetosphere in a very massive star within the rho Oph star-forming cloud using spectropolarimetry and X-ray data.

Findings

Magnetic field detected with longitudinal components of -419 G and 538 G.

Estimated dipole magnetic strength of approximately 1.9 kG.

Presence of a centrifugally supported, magnetically confined plasma around the star.

Abstract

Recent XMM-Newton observations of the B2 type star rho Oph A indicated a periodicity of 1.205d, which was ascribed to rotational modulation. Since variability of X-ray emission in massive stars is frequently the signature of a magnetic field, we investigated whether the presence of a magnetic field can indeed be invoked to explain the observed X-ray peculiarity. Two FORS2 spectropolarimetric observations in different rotation phases revealed the presence of a negative (<B_z>_all=-419+-101G) and positive (<B_z>_all=538+-69G) longitudinal magnetic field, respectively. We estimate a lower limit for the dipole strength as B_d = 1.9+-0.2kG. Our calculations of the Kepler and Alfven radii imply the presence of a centrifugally supported, magnetically confined plasma around rho Oph A. The study of the spectral variability indicates a behaviour similar to that observed in typical magnetic early-type Bp stars.