Jenny Nicholls and Elisabeth Le Strange have modelled a puzzling long, thin radio source near the centre of our Galaxy in terms of a star trail and a supernova remnant.
Recent high-resolution radio images reveal a wealth of fascinating sources at the Centre of our Galaxy. Not least among these are filaments, incredibly long, narrow sources, about 300 light years long by one light year wide, most of which lie perpendicular to the plane of the Galaxy. One particular filament, called the `Snake', was discovered by Andrew Gray, a PhD student in the Astrophysics Department. The radio emission from the Snake is synchrotron radiation - a result which is apparent from the way the intensity of the emission varies with the frequency, together with the fact that the emission is polarized. The direction of the polarization indicates that the magnetic field lines lie along the filament. At its southern end the Snake appears to intersect a supernova remnant.
How the filaments were created is a puzzle - there is no obvious mechanism which produces long, thin astrophysical objects filled with relativistic particles. An explosion would produce something more spherical than the thin features we observe, and a shock between colliding molecular clouds would lead to a sheet of emission not a filament. Even a sheet seen edge on would not look like the filaments in the Galactic Centre.
Jenny Nicholls and a summer student, Elisabeth Le Strange, have produced a model to explain the origin of the Snake. The model involves a two stage process. In the first stage a very massive star, blowing a dense and fast wind, travels at high speed (at least 700 km per sec. perpendicular to the plane of the Galaxy. It could be falling through the galactic disk from the halo of stars that surrounds the disk of the Galaxy, or it could have been ejected from a cluster of such hot, bright stars. If the star were stationary its wind would blow a bubble in the interstellar medium surrounding it. A star which is moving very much faster than the local speed of sound blows a thin cylindrical trail instead.
: A sketch of the star trail model for the Snake.
The suggestion is that the Snake is such a star trail, but the particles that make up the stellar wind filling the trail are not energetic enough to produce the synchrotron emission picked up by radio telescopes. So why is it bright at radio wavelengths? The second stage of this model suggests how the electrons in the star trail are accelerated to ultrarelativistic energies, and hence emit synchrotron radiation. While the star was producing the trail another massive star came to the end of its life and exploded as a supernova. This explosion produced an expanding shock. The star trail and the expanding supernova remnant intersected at a fairly early stage in the supernova remnant's life, and part of the shock of the supernova travelled up the star trail accelerating electrons in the trail as it went. The supernova shock speed is partly determined by the density of the gas through which it propagates. As the star trail is less dense than the surrounding medium, the shock travelled faster up the star trail than in the surrounding interstellar medium. As a result of synchrotron emission from the relativistic electrons accelerated by the shock, radio emission is seen from the SNR and in the long filament protruding from the SNR.