Astronomers discover odd dwarf star with a magnetic personality

December 6th, 2007 - 1:05 pm ICT by admin  

Washington, Dec 6 (ANI): Astronomers have discovered a dwarf star that has an unusually active and complex magnetic field, stronger than our own Sun’s, and a huge hot spot that covers half of its surface.
The team of astronomers, led by Dr. Edo Berger, a Carnegie-Princeton postdoctoral fellow at Princeton University, looked at the star TVLM513-46546 in the constellation Bootes about 35 light-years away by simultaneously using four different telescopes.
The observations by the different telescopes would help to explain the flamboyant activity of this M-type dwarf star, which are assumed to be cold, quiet and dim.
Like other ultracool dwarf stars, TVLM513-46546 is an M-type star with surface temperatures below about 2400K (2127 Celsius) and a mass of only 8 to 10% that of our Sun. By contrast, the Sun is a G-type star with an average surface temperature of 6000K (5727 Celsius).
The star’s steady radio emission is interrupted with spectacular fireworks displays of minute-long flares. These flares come from the catastrophic collisions and merging of the magnetic fields in the corona of the star; these actions drive the annihilation of magnetic energy like a giant short-circuits in the fields. The team also observed soft x-ray emission and an x-ray flare.
According to reports from the Gemini Observatory, in Hilo, Hawaii, the ultracool M-type star has magnetic properties because it is convective. That is, the zone that transports heat to the surface of the star extends all the way from the stellar surface into the center, like the bubble of a huge boiling pot.
Such a simple structure has been predicted to generate a very basic magnetic field structure, perhaps more like the Earth’s than the complex fields we see on the Sun.
But objects like TVLM513-46546 were once thought to have little or no magnetic field activity.
“Theory has always said that as we look at cooler and cooler stars, the coolest will be essentially dead,” said Berger. “It turns out that stars like TVLM513-46546 have very complex magnetic activity around them, activity more like our Sun than that of a star that is barely functional,” he added.
The research group also charted an optical hydrogen-alpha emission that is periodic and comes from a large hot spot. The two-hour emission period matches the two-hour rotation period of the star. “The periodic signal comes from a hot spot that covers half of the surface of the star like a giant light bulb that rotates in and out of our field of view,” said Berger.
This star’s complicated magnetic field environment and possible hot spot may indicate some unusual activity beneath the star’s surface (in its dynamo) or possibly even the existence of a still-hidden companion.
“Though the idea of an unseen companion as an explanation for the star’s excitable magnetic disposition is an intriguing one, but no such object has yet been detected,” said Berger. “The main idea to consider here is an analogy to other systems where the presence of a companion directly or indirectly excites magnetic activity,” he added.
In order to find out if this star is just a stellar oddity, or if it might turn out be a typical prototype of ultracool dwarfs, the research team plans to continue with observations of other such stars. The team expects the larger sample to show how other candidate low-mass stars and brown dwarfs (objects too hot to be planets and too cool to be stars) generate magnetic fields. (ANI)

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