A model for the thermal radio-continuum emission from radiative shocks in colliding stellar winds

TL;DR

This paper presents a semi-analytical model to predict thermal radio emission from colliding stellar winds in binary systems, accounting for radiative shocks and comparing results with observations of specific Wolf-Rayet binaries.

Contribution

The study introduces a new semi-analytical model for calculating thermal emission from radiative shocks in colliding stellar winds, incorporating effects of binary separation and wind momentum ratio.

Findings

Emission scales as D^{4/5} with binary separation.

Predicted flux densities increase with frequency, showing higher spectral indices.

Model results agree well with observed spectra of WR 98 and WR 113.

Abstract

Aims. The interaction of two isotropic stellar winds is studied in order to calculate the free-free emission from the wind collision region. The effects of the binary separation and the wind momentum ratio on the emission from the wind-wind interaction region are investigated. Methods. We developed a semi-analytical model for calculating the thermal emission from colliding stellar winds. Assuming radiative shocks for the compressed layer, which are expected in close binaries, we obtained the emission measure of the thin shell. Then, we computed the total optical depth along each line of sight to obtain the emission from the whole configuration. Results. Here, we present predictions of the free-free emission at radio frequencies from analytic, radiative shock models in colliding wind binaries. It is shown that the emission from the wind collision region mainly arises from the optically thick region of the compressed layer and scales as ~ D^{4/5}, where D is the binary separation. The predicted flux density from the wind collision region becomes more important as the frequency increases, showing higher spectral indices than the expected 0.6 value from the unshocked winds. We also investigate the emission from short-period WR+O systems calculated with our analytic formulation. In particular, we apply the model to the binary systems WR 98 and WR 113 and compare our results with the observations. Our theoretical results are in good agreement with the observed thermal spectra from these sources.