Spatially resolved centrifugal magnetosphere caught in motion around the secondary component of ρ Oph A

European Organisation for Astronomical Research in the Southern Hemisphere (ESO), Casilla 19001, Santiago 19, Chile

^★ Corresponding author: robertklement@gmail.com

Received: 21 March 2025
Accepted: 7 May 2025

Abstract

Aims. The recently discovered spectroscopic binary ρ Oph A is a rare magnetic hot binary system composed of two early B-type stars, comprising a nonmagnetic primary (Aa) and a slightly less massive magnetic secondary (Ab). Using near-IR interferometry, we aim to resolve the system’s astrometric orbit. The high-spectral-resolution VLTI/GRAVITY data also enable further information to be obtained about the two stellar components, and about the centrifugal magnetosphere orbiting the magnetic star.

Methods. We obtained a time-series of K-band interferometric data from VLTI/GRAVITY and analyzed them with the geometrical model-fitting tool PMOIRED. The continuum was used to derive the relative astrometry and relative fluxes of the two components, leading to the astrometric orbital solution. Spectro-interferometry covering the Brγ line was then used to obtain high-angular-resolution information about the dominant magnetospheric cloud in corotation with the magnetic Ab component.

Results. The combination of the astrometric orbital solution and the published radial velocities for both components enabled us to obtain a three-dimensional orbital solution, dynamical masses, and a dynamical parallax. These results are in a good agreement with previous estimates and confirm the previously inferred alignment of the orbital axis and the rotation axis of the Ab component at a much higher precision. A detailed analysis of the Brγ spectro-interferometry then revealed the changing position of the magnetospheric cloud relative to the Ab component. Comparison to the location of the cloud predicted from ρ Oph Ab ’s oblique rotator model and from the Hα emission properties resulted in excellent agreement. We are additionally able to demonstrate that the magnetic star exhibits prograde rotation. This is the first time that the motion of a centrifugal magnetosphere has been detected in angularly resolved observations.