Non-thermal processes in bowshocks of runaway stars. Application to Zeta Oph

TL;DR

This paper models the non-thermal and thermal radiation from bowshocks of runaway stars, predicting detectable high-energy gamma-ray emission, especially from stars like Zeta Oph, due to relativistic particles accelerated in the shock regions.

Contribution

The study introduces a detailed model of non-thermal emission in bowshocks of runaway stars, applying it specifically to Zeta Oph, and predicts potential gamma-ray detectability.

Findings

Non-thermal radiation could be detectable at high energies for nearby runaway stars.

Inverse Compton scattering dominates the high-energy gamma-ray spectrum.

Predicted spectra approach Fermi telescope sensitivities for Zeta Oph.

Abstract

Runaway massive stars are O- and B-type stars with high spatial velocities with respect to the interstellar medium. These stars can produce bowshocks in the surrounding gas. Bowshocks develop as arc-shaped structures, with bows pointing to the same direction as the stellar velocity, while the star moves supersonically through the interstellar gas. The piled-up shocked matter emits thermal radiation and a population of locally accelerated relativistic particles is expected to produce non-thermal emission over a wide range of energies. We aim to model the non-thermal radiation produced in these sources. Under some assumptions, we computed the non-thermal emission produced by the relativistic particles and the thermal radiation caused by free-free interactions, for O4I and O9I stars. We applied our model to Zeta Oph (HD 149757), an intensively studied massive star seen from the northern hemisphere. This star has spectral type O9.5V and is a well-known runaway. Spectral energy distributions of massive runaways are predicted for the whole electromagnetic spectrum. We conclude that the non-thermal radiation might be detectable at various energy bands for relatively nearby runaway stars, especially at high-energy gamma rays. Inverse Compton scattering with photons from the heated dust gives the most important contribution to the high-energy spectrum. This emission approaches Fermi sensitivities in the case of Zeta Oph.