Impact of orbital motion on the structure and stability of adiabatic shocks in colliding wind binaries

Abstract

Context. The collision of winds from massive stars in binaries results in the formation of a double-shock structure with observed signatures from radio to X-rays. Aims. We study the structure and stability of the colliding wind region as it turns into a spiral owing to the orbital motion. We focus on adiabatic winds, where mixing between the two winds is expected to be restricted to the Kelvin-Helmholtz instability. Mixing of the Wolf-Rayet wind with hydrogen-rich material is important for dust formation in pinwheel nebulae such as WR 104, where the spiral structure has been resolved in infrared. Methods. We use the hydrodynamical code RAMSES to solve the equations of hydrodynamics on an adaptive grid. A wide range of binary systems with different wind velocities and mass-loss rates are studied with two-dimensional simulations. A specific threedimensional simulation is performed to model WR 104. Results. Orbital motion leads to the formation of two distinct spiral arms where the Kelvin-Helmholtz instability develops differently. We find that the spiral structure is destroyed when there is a large velocity gradient between the winds, unless the collimated wind is much faster. We argue that the Kelvin-Helmholtz instability plays a major role in determining whether the structure is maintained. We discuss the consequences for various colliding-wind binaries. When their spiral structure is stable, there is no straightforward relationship between the spatial step of the spiral, the wind velocities, and the orbital period. Our 3D simulation of WR 104 indicates that the colder, well-mixed trailing arm has more favourable conditions for dust formation than the leading arm. The single-arm infrared spiral follows more closely the mixing map than the density map, suggesting that the dust-to-gas ratio may vary between the leading and trailing density spirals. However, the density is much lower than what dust formation models require. Including radiative cooling would lead to higher densities, and also to thin shell instabilities whose impact on the large-scale structure remains unknown.

Simulations of stellar/pulsar-wind interaction along one full orbit

“…This is smaller than the conventional value Γ ∼ 10 4 −10 6 (see Khangulyan et al 2012;Aharonian et al 2012, and references therein), but high enough to reproduce the wind velocity contrast in the bow-like shock region, where instabilities trigger, and to capture important relativistic effects. Similarities and differences between the Γ-value adopted here, a more realistic one, and the non-relativistic case, are discussed in Sect.…”

“…The processes underlying the non-thermal radiation in PSR B1259−63/LS2883 seem to be shocks that take place in the two-wind interaction region and accelerate electrons. These electrons most likely radiate from radio-to-X-rays through synchrotron and at GeV and TeV energies through inverse Compton emission (IC), although some gamma rays may come from IC in the unshocked pulsar wind (e.g., Tavani & Arons 1997;Ball & Kirk 2000;Khangulyan et al 2007Khangulyan et al , 2011Khangulyan et al , 2012. The details of the dynamics of the interacting flows are still poorly known, which affects the interpretation of the observed variability and spectra not only in PSR B1259−63/LS2883 but also in other gamma-ray emitting binaries that may host a non-accreting pulsar (LS 5039 and LS I +61 303, see discussions in, e.g., Chernyakova et al 2006;Dubus 2006;Romero et al 2007;Bosch-Ramon & Khangulyan 2009;Torres 2011;Bednarek 2011;Barkov & Khangulyan 2012; HESS J0632+057, see, e.g., Bongiorno et al 2011; 1FGL J1018.6−5856, see, e.g., Ackermann et al 2012;Abramowski et al 2012).…”

“…This will lead to isotropization of the flow mass, momentum, and energy fluxes, and to reexpansion, with the consequent loss of structure. In this context, nonrelativistic two-dimensional (2D) simulations done by Lamberts et al (2012) showed that the shocked flow may keep a spirallike shape up to large distances, although the adopted light-todense wind luminosity ratios, χ ≤ 1.25, and the non-relativistic nature of the simulations may prevent one from generalizing this conclusion.…”

The 2.35 year itch of Cygnus OB2 #9

“…The influence of orbital motion, namely the curvature to the shocks resulting from the Coriolis force (see also, for example, Parkin & Pittard 2008;Parkin et al 2009;Pittard 2009;van Marle et al 2011;Lamberts et al 2012;Madura et al 2013), is apparent in the arms of the CWR. However, the shape of the shocks within a binary separation of the apex remains largely the same as at apastron, indicating that the wind momentum ratio does not change considerably through the orbit.…”

Periodic morphological changes in the radio structure of the gamma-ray binary LS 5039

“…Also for PSR B1259−63, Okazaki et al (2011) and Takata et al (2012) presented 3D simulations of the tidal and wind interactions. Lamberts et al (2012) considered the general case of colliding wind binaries and described the outflow structure at larger distances. Finally, the flow structure for a case similar to LS 5039 was described in Bosch-Ramon et al (2012) for scales up to 100 times the orbit size.…”