Julian Pittard

Colliding stellar winds occur in systems where two massive stars orbit their mutual center of mass. The most massive stars have winds which are a billion times more powerful than the solar wind which we are used to. However, their precise power is difficult to measure, in part because the winds are highly structured, or clumpy. Many observational techniques are sensitive to the (unknown) degree of clumping, but the conditions in some colliding wind binaries are such that the clumps are rapidly destroyed within the interaction region (click on the image to the right for a movie of this interaction). In a recent ApJ letter I demonstrate that studies of the X-ray emission from the shocked gas have the potential to yield reliable estimates of the strength of the winds.

The nature of the wind-wind interaction can be very complex, allowing study into the nature of collisionless shocks, hydrodynamic instabilites, and the acceleration of cosmic rays (see below). I have recently calculated 3-dimensional models of this interaction which include the orbital motion of the stars, the radiative acceleration of their winds, and the cooling in the postshock region (which can be severe). Models of systems with eccentric orbits probe the response of the plasma to changing values of the ratio of the cooling to flow timescale (click on the movie to the right for a higher resolution movie of this interaction).

One of the most important individual massive stars is the enigmatic object Eta Carinae, which probably contains the most massive star known in our own galaxy. This object has recently been found to contain not one but two massive stars and is famous as the survivor of the most energetic non-supernova explosion ever recorded. This occurred in 1843, and produced the Homunculus Nebula. Eta Carinae is a 'test-bed' object in which it is possible to study both non-terminal stellar eruptions and the collision between the stellar winds from two individual massive stars. I have written a review article on Eta Carinae for Astronomy & Geophysics.

Observations also reveal that particles (electrons and ions) are accelerated to within a fraction of the speed of light at the region of wind-wind interaction. Recent models of the resulting synchrotron emission in the radio (an observation of WR140 is shown to the right) indicate that the non-thermal electron spectrum may be harder than expected. However, turbulent motions induced by clump destruction may naturally create a hard spectrum. Forthcoming observations at GeV energies with GLAST will provide a definitive value for the spectral index.