Non-thermal processes in bowshocks of runaway stars

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.

Catalogue of particle-accelerating colliding-wind binaries

“…As a result, this study does not concern massive X-ray and/or γ-ray binaries or microquasars that include a compact object (Fender & Maccarone 2004;Khangulyan & Aharonian 2005;Bosch-Ramon 2007;Bosch-Ramon & Rieger 2011) and is neither related to supernova remnants, known to be efficient particle accelerators (Romero 2004;Reynolds 2011;Vink 2013). Moreover, the class of objects debated does not include rapidlymoving runaway massive stars whose stellar wind interacts with the interstellar medium, producing strong shocks likely to accelerate particles (Benaglia et al 2010b;Romero et al 2011;Peri et al 2011;del Valle & Romero 2012).…”

Modeling nonthermal emission from stellar bow shocks

“…Benaglia et al (2010) detected radio emission from the bow shock produced by the runaway star BD+43 3654, consistent with synchrotron radiation produced by the interaction of the relativistic electrons with the magnetic field of the acceleration region. Later, del Valle & Romero (2012) presented a nonthermal emission model that predicted the production of high-energy photons in these acceleration regions in sufficiently large numbers to be detected in X-rays. López-Santiago et al (2012) published the first detection of X-ray emission from a bow shock produced by a runaway star (AE Aurigae), while Terada et al (2012) reported the non-detection of X-ray emission in the bow shock region formed by BD+43 3654 from a long-duration X-ray observation with Suzaku.…”

“…For this purpose, we applied the model derived by del Valle & Romero (2012), where different nonthermal processes are proposed to explain the origin of the emission in these shocks. Further details on the nonthermal processes are also found in Adams (1980), Kelner et al (2006), Longair (2011) or Rybicki & Lightman (1979).…”

Search for new stellar sources of gamma-rays

“…Instruments such as the Fermi Gamma Ray Space Telescope 1 , or the ground-based atmospheric imaging Cherenkov telescopes MAGIC 2 , H.E.S.S. 3 , and VERITAS 4 , among others, have been key to unveil new kinds of energetic sources. Outstanding among them there are the so called 'gamma-ray binaries' (see e.g.…”

“…In parallel, other stellar gammaray sources have been predicted to exist based on theoretical work, although their firm detection remains yet to be accomplished. These include T-Tauri stars [2], bipolar outflows from massive protostars [3] as well as bowshocks from runaway stars [4].…”