Radio Spectral Energy Distributions for Single Massive Star Winds with Free-Free and Synchrotron Emission

TL;DR

This paper models the combined free-free and synchrotron emission in the radio spectra of single massive star winds, exploring conditions for non-thermal spectral energy distributions and their observational signatures.

Contribution

It introduces a simplified model for the radio SEDs of single massive stars considering both free-free and synchrotron processes, including Razin suppression effects.

Findings

Synchrotron emission can alter the spectral slope, making it steeper or shallower.

Non-thermal SEDs are generally not pure power-laws but can appear approximately so over limited wavelengths.

Synchrotron effects are subtle in single stars, often requiring fine wavelength sampling for detection.

Abstract

The mass-loss rates from single massive stars are high enough to form radio photospheres at large distances from the stellar surface where the wind is optically thick to (thermal) free-free opacity. Here we calculate the far-infrared, millimeter, and radio band spectral energy distributions (SEDs) that can result from the combination of free-free processes and synchrotron emission, to explore the conditions for non-thermal SEDs. Simplifying assumptions are adopted in terms of scaling relations for the magnetic field strength and the spatial distribution of relativistic electrons. The wind is assumed to be spherically symmetric, and we consider the effect of Razin suppression on the synchrotron emission. Under these conditions, long-wavelength SEDs with synchrotron emission can be either more steep or more shallow than the canonical asymptotic power-law SED from a non-magnetic wind. When non-thermal emission is present, the resultant SED shape is generally not a power-law; however, the variation in slope can change slowly with wavelength. Consequently, over a limited range of wavelengths, the SED can masquerade as approximately a power law. While most observed non-thermal long-wavelength spectra are associated with binarity, synchroton emission can have only a mild influence on single-star SEDs, requiring finer levels of wavelength sampling for detection of the effect.