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

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.