Radio polarization from runaway star bowshocks-I. The general case

TL;DR

This paper models the radio polarization from bowshocks of runaway stars, revealing the importance of Faraday rotation, the influence of thermal emission, and challenges in magnetic field inference from polarization data.

Contribution

It provides the first detailed magnetohydrodynamic simulations of synchrotron polarization in stellar bowshocks, including depolarization effects and thermal emission considerations.

Findings

Faraday rotation significantly affects polarization at low frequencies

Thermal emission can dominate over polarized synchrotron emission

Inferring magnetic field direction solely from polarization can be misleading

Abstract

High velocity stars move through the interstellar medium with V > 30 km/s. When the star has powerful winds, under the appropriate conditions, the interaction of the wind with the interstellar material produces a system of shocks. The outer shock, called the bowshock, perturbs the ambient medium, heating and compressing the gas. The dust in the compressed bowshock cools, producing infrared radiation. This emission appears as extended coma-shape structures. The discovery of radio nonthermal emission from two stellar bowshock nebulae indicates that these sources might be accelerating electrons up to relativistic energies. The produced nonthermal radio emission is most probably synchrotron which has a high degree of polarization. In this work we model the synchrotron emission of runaway massive star bowshocks aiming to produce synthetic radio emission and polarization maps for two frequencies: 1.40 and 4.86 GHz. We model the interacting plasmas in a steady-state regime by means of magnetohydrodynamics simulations and we compute the injection and transport of the relativistic electrons in the diffusion approximation. We include in the model the most important depolarization effects. Our main conclusions are i) the effects of Faraday rotation within the source are important at the lowest frequency considered, ii) inferring the local magnetic field direction from polarization measurements only can be misleading, iii) thermal radio emission produced by ionized plasma within the bowshock structure and surroundings can surpass the polarized one for the considered frequencies, and iv) the contribution from the background electrons is minor.