Nonthermal Emission from Stellar Bow Shocks

Abstract

Abstract Since the detection of nonthermal radio emission from the bow shock of the massive runaway star BD +43°3654, simple models have predicted high-energy emission, at X-rays 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 nonthermal emission from massive runaway star bow shocks. The main goal is to establish whether these systems are efficient nonthermal emitters, even if they are not strong enough yet to be 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, as well as the nonthermal emission they produce. For this purpose we solve a 3D (two 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 nonthermal emission, mostly produced by inverse Compton scattering of dust-emitted photons by relativistic electrons, and second by synchrotron radiation. This represents a fraction of ∼10−5 to 10−4 of the wind kinetic power. Given the better sensibility of current instruments at radio wavelengths, these systems are more prone to be detected at radio through the synchrotron emission they produce rather than at gamma energies.