National University of Ireland

Dwarf flash.
Artist's rendering of a radio flash above the magnetic pole of a brown dwarf.

Failed Stars Still Have a Pulse

PRESTON, U.K.--If someone discovered that turtles could sing like larks, biologists would take notice. Likewise, astronomers are intrigued by today's announcement that brown dwarfs--failed stars a couple dozen times more massive than Jupiter--apparently can behave like pulsars, the rapidly spinning, super-dense and highly magnetized remains of exploded giant stars. The finding, presented at the Royal Astronomical Society National Astronomy Meeting here, may ultimately help astronomers better understand the physics of these exotic objects.

Compared to pulsars, brown dwarfs come off as dull and boring. Too small and lightweight to ignite nuclear fusion reactions in their cores, these failed stars appear cool and faint in the night sky. Gregg Hallinan of the National University of Ireland in Galway and colleagues were therefore surprised when the Very Large Array radio observatory near Socorro, New Mexico, caught three brown dwarfs in our solar neighborhood emitting powerful radio pulses usually seen coming from much more energetic pulsars.

The radio waves are produced above the star's magnetic poles, the team reports. The pulses result from the star's rotation, which brings the poles in and out of view. Astronomers had thought that a pulsar's very strong magnetic fields were needed to produce such waves, but brown dwarf fields are a billion times weaker than those of pulsars.

"Apparently, the same universal mechanism of radio pulse production works over a very wide range of magnetic field strengths," says pulsar astronomer Joeri van Leeuwen of the University of California, Berkeley. Because brown dwarfs rotate much more slowly than pulsars, they may be easier to study, notes Hallinan, adding that "brown dwarfs may be the key to unlocking the long-standing mystery of how pulsars produce radio emissions." Hallinan's team now plans to survey a large number of nearby brown dwarfs in search of more pulsarlike behavior.

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