Non-thermal emission from stellar bow shocks

Abstract

Since the detection of non-thermal radio emission from the bow shock of the massive runaway star BD +43$^{\circ}$3654 simple models have predicted high-energy emission, at X and gamma-rays, from these Galactic sources. Observational searches for this emission so far give no conclusive evidence but a few candidates at gamma rays. In this work we aim at developing a more sophisticated model for the non-thermal emission from massive runaway star bow shocks. The main goal is to establish whether these systems are efficient non-thermal emitters, even if they are not strong enough to be yet detected. For modeling the collision between the stellar wind and the interstellar medium we use 2D hydrodynamic simulations. We then adopt the flow profile of the wind and the ambient medium obtained with the simulation as the plasma state for solving the transport of energetic particles injected in the system, and the non-thermal emission they produce. For this purpose we solve a 3D (2 spatial + energy) advection-diffusion equation in the test-particle approximation. We find that a massive runaway star with a powerful wind converts 0.16-0.4% of the power injected in electrons into non-thermal emission, mostly produced by inverse Compton scattering of dust-emitted photons by relativistic electrons, and secondly by synchrotron radiation. This represents a fraction of $\sim$ $10^{-5}-10^{-4}$ of the wind kinetic power. Given the better sensibility of current instruments at radio wavelengths theses systems are more prone to be detected at radio through the synchrotron emission they produce rather than at gamma energies.