Determine the optically thick thermal radio spectral index of Wolf–Rayet winds

Determine the actual optically thick thermal spectral index of Wolf–Rayet stellar winds to improve extrapolation of high-frequency thermal reference fluxes to 1.3 GHz.

Background

The empirical extrapolation method estimates the 1.3-GHz thermal emission of detected Wolf–Rayet stars by rescaling radio measurements of reference stars of the same spectral subtype. Because the reference measurements are available at 4.8 or 8.6 GHz, they must be extrapolated to the MeerKAT survey frequency using an assumed thermal spectral index.

The paper states that the actual optically thick thermal spectral index is not known and adopts a median value of 0.7 with an uncertainty of 0.1. Establishing this spectral index would reduce a significant source of uncertainty in distinguishing thermal wind emission from possible synchrotron radio excesses.

References

However, the actual optically thick thermal spectral index is not known and should lie within a given range, with a lower value of 0.6.

— Meerkat measurement of the radio emission from massive stars in the Galactic plane I. Wolf-Rayet stars  (2609.19950 - Tasseroul et al., 17 Sep 2026) in Section 3.2.2, “Empirical extrapolation method”