Non-thermal processes in bowshocks of runaway stars

Application to ζ Ophiuchi

M. V. del Valle^1,2⋆ and G. E. Romero^1,2⋆⋆

¹ Instituto Argentino de Radioastronomía, C.C.5, (1894) Villa Elisa, Buenos Aires, Argentina
e-mail: maria@iar-conicet.gov.ar
² Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque, 1900 La Plata, Argentina

Received: 1 February 2012
Accepted: 13 April 2012

Abstract

Context. 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.

Aims. We aim to model the non-thermal radiation produced in these sources.

Methods. 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 ζ 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.

Results. Spectral energy distributions of massive runaways are predicted for the whole electromagnetic spectrum.

Conclusions. 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 ζ Oph.