A Clumping Independent Diagnostic of Stellar Mass-loss Rates: Rapid Clump Destruction in Adiabatic Colliding Winds

Clumping in hot star winds can significantly affect estimates of mass-loss rates, the inferred evolution of the star and the environmental impact of the wind. A hydrodynamical simulation of a colliding winds binary (CWB) with clumpy winds reveals that the clumps are rapidly destroyed after passing through the confining shocks of the wind-wind collision region (WCR) for reasonable parameters of the clumps if the flow in the WCR is adiabatic. Despite large density and temperature fluctuations in the post-shock gas, the overall effect of the interaction is to smooth the existing structure in the winds. Averaged over the entire interaction region, the resulting X-ray emission is very similar to that from the collision of smooth winds. The insensitivity of the X-ray emission to clumping suggests it is an excellent diagnostic of the stellar mass-loss rates in wide CWBs, and may prove to be a useful addition to existing techniques for deriving mass-loss rates, many of which are extremely sensitive to clumping. Clumpy winds also have implications for a variety of phenomena at the WCR: particle acceleration may occur throughout the WCR due to supersonic MHD turbulence, re-acceleration at multiple shocks, and re-connection; a statistical description of the properties of the WCR may be required for studies of non-equilibrium ionization and the rate of electron heating; and the physical mixing of the two winds will be enhanced, as seems necessary to trigger dust formation.Comment: 4 pages, 3 figures, accepted for publication in ApJ

Non-thermal X-ray and Gamma-ray Emission from the Colliding Wind Binary WR140

WR140 is the archetype long-period colliding wind binary (CWB) system, and is well known for dramatic variations in its synchrotron emission during its 7.9-yr, highly eccentric orbit. This emission is thought to arise from relativistic electrons accelerated at the global shocks bounding the wind-collision region (WCR). The presence of non-thermal electrons and ions should also give rise to X-ray and gamma-ray emission from several separate mechanisms, including inverse-Compton cooling, relativistic bremsstrahlung, and pion decay. We describe new calculations of this emission and make some preliminary predictions for the new generation of gamma-ray observatories. We determine that WR140 will likely require several Megaseconds of observation before detection with INTEGRAL, but should be a reasonably strong source for GLAST.Comment: 4 pages, 1 figure, contribution to "Massive Stars and High-Energy Emission in OB Associations"; JENAM 2005, held in Liege (Belgium

Winds in Collision: high-energy particles in massive binary systems

High-resolution radio observations have revealed that non-thermal radio emission in WR stars arises where the stellar wind of the WR star collides with that of a binary companion. These colliding-wind binary (CWB) systems offer an important laboratory for investigating the underlying physics of particle acceleration. Hydrodynamic models of the binary stellar winds and the wind-collision region (WCR) that account for the evolution of the electron energy spectrum, largely due to inverse Compton cooling, are now available. Radiometry and imaging obtained with the VLA, MERLIN, EVN and VLBA provide essential constraints to these models. Models of the radio emission from WR146 and WR147 are shown, though these very wide systems do not have defined orbits and hence lack a number of important model parameters. Multi-epoch VLBI imaging of the archetype WR+O star binary WR140 through a part of its 7.9-year orbit has been used to define the orbit inclination, distance and the luminosity of the companion star to enable the best constraints for any radio emitting CWB system. Models of the spatial distribution of relativistic electrons and ions, and the magnetic energy density are used to model the radio emission, and also to predict the high energy emission at X-ray and gamma-ray energies. It is clear that high-energy facilities e.g. GLAST and VERITAS, will be important for constraining particle acceleration parameters such as the spectral index of the energy spectrum and the acceleration efficiency of both ions and electrons, and in turn, identify unique models for the radio spectra. This will be especially important in future attempts to model the spectra of WR140 throughout its complete orbit. A WCR origin for the synchrotron emission in O-stars, the progenitors of WR stars, is illustrated by observations of Cyg OB2 No. 9.Comment: Invited review at the 8th EVN Symposium, Torun September 26-29, 2006. 11 pages, 12 figure

3D Models of Radiatively Driven Colliding Winds In Massive O+O Star Binaries: I. Hydrodynamics

The dynamics of the wind-wind collision in massive stellar binaries is investigated using three-dimensional hydrodynamical models which incorporate gravity, the driving of the winds, the orbital motion of the stars, and radiative cooling of the shocked plasma. In this first paper we restrict our study to main-sequence O+O binaries. The nature of the wind-wind collision region is highly dependent on the degree of cooling of the shocked plasma, and the ratio of the flow timescale of the shocked plasma to the orbital timescale. The pre-shock wind speeds are lower in close systems as the winds collide prior to their acceleration to terminal speeds. Radiative inhibition may also reduce the pre-shock wind speeds. Together, these effects can lead to rapid cooling of the post-shock gas. Radiative inhibition is less important in wider systems, where the winds are accelerated to higher speeds before they collide, and the resulting collision region can be largely adiabatic. In systems with eccentric orbits, cold gas formed during periastron passage can persist even at apastron, before being ablated and mixed into its surroundings and/or accelerated out of the system.Comment: 21 pages, 15 figures, accepted for publication in MNRA

The dominant X-ray wind in massive star binaries

We investigate which shocked wind is responsible for the majority of the X-ray emission in colliding wind binaries, an issue where there is some confusion in the literature, and which we show is more complicated than has been assumed. We find that where both winds rapidly cool (typically close binaries), the ratio of the wind speeds is often more important than the momentum ratio, because it controls the energy flux ratio, and the faster wind is generally the dominant emitter. When both winds are largely adiabatic (typically long-period binaries), the slower and denser wind will cool faster and the stronger wind generally dominates the X-ray luminosity.Comment: 4 pages, 1 figure, accepted by A&A Letter