Radio observations of candidate magnetic O stars

TL;DR

This study investigates radio emissions from candidate magnetic O stars to find evidence of magnetic fields through non-thermal synchrotron radiation, providing insights into stellar wind properties and magnetic activity.

Contribution

It presents new radio observations of five candidate magnetic O stars, detecting non-thermal emission in one, and discusses implications for stellar wind mass-loss rates and magnetic field presence.

Findings

Detected non-thermal radio spectrum in $\xi$ Per.

Observed thermal flux lower than expected, indicating wind clumping.

Supports overestimation of mass-loss rates from H$eta$ line measurements.

Abstract

Context: Some O stars are suspected to have to have (weak) magnetic fields because of the observed cyclical variability in their UV wind-lines. However, direct detections of these magnetic fields using optical spectropolarimetry have proven to be very difficult. Aims: Non-thermal radio emission in these objects would most likely be due to synchrotron radiation. As a magnetic field is required for the production of synchrotron radiation, this would be strong evidence for the presence of a magnetic field. Such non-thermal emission has already been observed from the strongly magnetic Ap/Bp stars. Methods: We have performed 6 & 21 cm observations using the WSRT and use these, in combination with archival VLA data at 3.6 cm and results from the literature, to study the radio emission of 5 selected candidate magnetic O stars. Results: Out of our five targets, we have detected three: $\xi$ Per, which shows a non-thermal radio spectrum, and $\alpha$ Cam and $\lambda$ Cep, which show no evidence of a non-thermal spectrum. In general we find that the observed free-free (thermal) flux of the stellar wind is lower than expected. This is in agreement with recent findings that the mass-loss rates from O stars as derived from the H$\alpha$ line are overestimated because of clumping in the inner part of the stellar wind.