The auroral radio emission of the magnetic B-type star rho OphC

TL;DR

This study presents multi-frequency radio observations of the magnetic star rho OphC, revealing coherent auroral radio emission indicative of stellar magnetospheric processes and plasma conditions, with unique viewing geometry effects.

Contribution

It provides the first evidence of always-visible auroral radio emission in an early-type magnetic star due to stellar geometry and polarization characteristics.

Findings

Detection of 60% circular polarization at 1.6 GHz

Evidence of O-mode dominated maser amplification

Auroral radio emission always visible regardless of rotation phase

Abstract

The non-thermal radio emission of main-sequence early-type stars is a signature of stellar magnetism. We present multi-wavelength (1.6-16.7 GHz) ATCA measurements of the early-type magnetic star rho OphC, which is a flat-spectrum non-thermal radio source. The rho OphC radio emission is partially circularly polarized with a steep spectral dependence: the fraction of polarized emission is about 60% at the lowest frequency sub-band (1.6 GHz) while is undetected at 16.7 GHz. This is clear evidence of coherent Auroral Radio Emission (ARE) from the rho OphC magnetosphere. Interestingly, the detection of the rho OphC's ARE is not related to a peculiar rotational phase. This is a consequence of the stellar geometry, which makes the strongly anisotropic radiation beam of the amplified radiation always pointed towards Earth. The circular polarization sign evidences mainly amplification of the ordinary mode of the electromagnetic wave, consistent with a maser amplification occurring within dense regions. This is indirect evidence of the plasma evaporation from the polar caps, a phenomenon responsible for the thermal X-ray aurorae. rho OphC is not the first early-type magnetic star showing the O-mode dominated ARE but is the first star with the ARE always on view.