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Radioactive Brown Dwarfs are a New Class of Pulsar

April 18, 2007

A study of brown dwarfs has revealed that these “failed stars” can possess powerful magnetic fields and emit lighthouse beams of radio waves thousands of times brighter than any detected from the Sun. The brown dwarfs are behaving like pulsars, one of the most exotic types of object in our Universe.

Gregg Hallinan of the National University of Ireland, Galway, who is presenting the discovery at the RAS National Astronomy Meeting in Preston on 18th April, said, “Brown dwarfs tend to be seen as a bit boring – the cinders of the galaxy.  Our research shows that these objects can be fascinating and dynamic systems, and may be the key to unlocking this long-standing mystery of how pulsars produce radio emissions.”

Since the discovery of pulsars forty years ago, astronomers have been trying to understand how the rotating neutron stars produce their flashing radio signals.  Although there have been many attempts to describe how they produce the extremely bright radio emissions, the vast magnetic field strengths of pulsars and the relativistic speeds involved make it extremely difficult to model. 

Brown dwarfs are now the second class of stellar object observed to produce this kind of powerful, amplified (coherent) radio signal at a persistent level. The emissions from the brown dwarfs appear to be very similar to those observed from pulsars, but the whole system is on a much slower and smaller scale, so it is much easier to decipher exactly what is going on.  Importantly, the mechanisms for producing the radio emissions in brown dwarfs are well understood, as they are almost identical to the processes that produce radio emissions from planets.

Hallinan said, “It looks like brown dwarfs are the missing step between the radio emissions we see generated at Jupiter and those we observe from pulsars”.

Jupiter’s volcanic moon, Io, is a source of electrically charged gas that is accelerated by the planet’s magnetic field and causes powerful radio laser, or maser, emissions.  The radiation can be so intense that Jupiter frequently outshines the Sun as a source of energy at radio wavelengths.

For some time, scientists have thought that there may be similarities between this type of maser emission and pulsars’ lighthouse-like beams of radio waves.  Observations of the brown dwarf, TVLM 513, using the Very Large Array (VLA) radio telescope, may provide the first direct evidence for that link.

The group observed the brown dwarf over a period of 10 hours at two different frequencies.  In both cases, a bright flash was observed every 1.96 hours.

As yet, the processes controlling the radio flashes from TVLM 513 are still unclear. There is no evidence of a binary system, so interaction of the magnetosphere with a stellar wind from a nearby star seems an unlikely cause, nor is there any sign of an orbiting planet that could produce a scenario like that of Jupiter and Io. However, rapid rotation is also thought to be a source of electron acceleration for a component of Jupiter’s maser emission and this may also be the main source of TVLM 513′s radio flashes.

The group is now planning a large survey of all the known brown dwarfs in the solar neighbourhood to find out how many are radio sources and how many of those are pulsing. If a large fraction of brown dwarfs are found to pulse, it could prove a key method of detection for these elusive objects.

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