THE long thin structures cutting across the centre of our Galaxy have been
puzzling astronomers ever since they were discovered 17 years ago. Now, two
Australian researchers suggest that the longest of these radio-emitting
filaments, the Snake, is in fact a magnetic tube 200 light years long and
twisted into two loops. Such twisted filaments may help explain how stars form
in some molecular clouds.
In a paper that will be published in Astrophysical Journal Letters,
Geoff Bicknell and Jianke Li of the Australian National University in Canberra
describe how the two kinks in the Snake, where radio emission is especially
bright, were a crucial clue to its nature.
According to the researchers’ theoretical model, the Snake is a giant
magnetic flux tube, a bundle of magnetic field lines drawn out into a thin
strand, and the kinks are loops in the tube. They say the loops formed because
the Snake’s ends are anchored in rotating molecular clouds, which twist the tube
until it doubles back on itself, like the knots that appear in a twisted rubber
band.
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Where the loop crosses back on itself, the magnetic fields short-circuit,
annihilating part of the field and releasing energy that accelerates nearby
electrons close to the speed of light. As the electrons spiral round the
magnetic field they emit radio waves, just as particles accelerated in a
synchrotron here on Earth emit radiation. This explains why the Snake’s kinks
are so bright.
The flux tube model could explain some of the finer features of the Snake’s
radio emission. For example, the proportion of high-frequency radio emissions
increases the further you get from the kinks. That makes sense, says Bicknell.
High-speed electrons cause high-frequency emissions, and the faster ones diffuse
more quickly away from the kink.
“It’s a pretty detailed model. It’s a big step forward,” says Miller Goss,
director of the Very Large Array radio telescope in New Mexico, and one of the
co-discoverers of the Snake.
But not everyone is totally convinced by the new model. At least one end of
the Snake is associated with a molecular cloud, says Cornelia Lang of the
University of Massachusetts in Amherst, “but more detailed images of the Snake
are necessary to confirm that it is twisted”. Lang is working on just such a map
of the Snake’s magnetic field, and expects to finish it by April.
Filaments have so far only been found at the centre of the Galaxy, and
although their role is still far from clear, Bicknell speculates that they may
play a key role in star formation. To make a star, a molecular cloud must
contract and shed angular momentum. A twisted flux tube might draw off angular
momentum very effectively, says Bicknell.
