Radio Variability from Co-Rotating Interaction Regions Threading Wolf-Rayet Winds

TL;DR

This study investigates radio variability caused by Co-rotating Interaction Regions (CIRs) in Wolf-Rayet star winds, demonstrating that CIRs can produce detectable radio signals with characteristic wavelength-dependent features.

Contribution

The paper presents a phenomenological model of CIR-induced radio variability, exploring its observational signatures and feasibility in dense stellar winds.

Findings

Radio variability at the 10% level can be produced by CIRs.

Waveform characteristics depend on curvature, density contrast, inclination, and wavelength.

Wavelength-dependent amplitude and phase shifts can help detect CIRs.

Abstract

The structured winds of single massive stars can be classified into two broad groups: stochastic structure and organized structure. While the former is typically identified with clumping, the latter is typically associated with rotational modulations, particularly the paradigm of Co-rotating Interaction Regions (CIRs). While CIRs have been explored extensively in the UV band, and moderately in the X-ray and optical, here we evaluate radio variability from CIR structures assuming free-free opacity in a dense wind. Our goal is to conduct a broad parameter study to assess the observational feasibility, and to this end, we adopt a phenomenological model for a CIR that threads an otherwise spherical wind. We find that under reasonable assumptions, it is possible to obtain radio variability at the 10% level. The detailed structure of the folded light curve depends not only on the curvature of the CIR, the density contrast of the CIR relative to the wind, and viewing inclination, but also on wavelength. Comparing light curves at different wavelengths, we find that the amplitude can change, that there can be phase shifts in the waveform, and the the entire waveform itself can change. These characterstics could be exploited to detect the presence of CIRs in dense, hot winds.