Odd Little Star has Magnetic
(Additional comments below)
A dwarf star with a surprisingly
magnetic personality and a huge hot spot covering half its surface
area is showing astronomers that life as a cool dwarf is not
necessarily as simple and quiet as they once assumed.
Simultaneous observations made by four of the most powerful Earth-
and space-based telescopes revealed an unusually active magnetic
field on the ultracool low-mass star TVLM513-46546. A team of
astronomers, led by Dr. Edo Berger, a Carnegie-Princeton
postdoctoral fellow at Princeton University, is using these
observations to explain the flamboyant activity of this M-type dwarf
that lies about 35 light-years away in the constellation Booetes.
The team's observations of TVLM513-46546 combine radio data from the
Very Large Array, optical spectra from the Gemini North 8-meter
telescope, ultraviolet images from the orbiting Swift observatory
and x-ray data from NASA's Chandra X-ray Observatory. This is the
first time that such a powerful set of telescopes has been trained
on one of the smallest known stars. The study is part of a program
that looks at the origins of magnetic fields in ultracool dwarfs,
stars that astronomers always assumed were simple, quiet, and more
tranquil than their hotter and more massive siblings.
"With such a unique set of observations you always expect to find
the unexpected," said Berger, "but we were shocked at the level of
complexity that this object exhibits."
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.
Also for the first time, the group charted optical hydrogen-alpha
emission with a period of two hours that matches the two-hour
rotation period of the star. "We find a hot spot that covers half of
the surface of the star like a giant lighthouse that rotates in and
out of our field of view," said Berger. "We still do not know why
only half of the star is lit up in hydrogen and if this situation
remains unchanged over days, weeks, years, or centuries."
Berger describes the dwarf star's magnetic field as probably being a
simple dipole (north-south orientation, like the Earth's much weaker
magnetic field) that extends out at least one stellar radius above
the surface. There is also a smaller-scale field that has loops
similar to those seen on the Sun, but smaller. "Those loops and arcs
occur on random places on the surface of the star, "said Berger.
"That's where the flares originate that last only a few minutes,
whereas the overall field doesn't get disturbed."
Objects like TVLM513-46546 were once thought to be models of stellar
quiescence and simplicity, with little to 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."
This one'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. The idea of an unseen companion as an explanation for the
star's excitable magnetic disposition is an intriguing one, says
Berger, but no such object has yet been detected. "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
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
Imagine the interior of the Sun layered like an onion. Its internal
convection is the process by which heat from the nuclear fusion at
the core is transported by large spinning currents that move through
the Sun's outer layers. Differential rotation is simply the term for
the different spin rates of different layers. Together these motions
of electrically charged gas spin up the magnetic field structures we
see at the Sun.
By contrast, an ultracool M-type star like TVLM513-46546 is fully
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. Why TVLM513-46546 has such a complex field and
activity remains to be studied.
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. Berger
also notes that he'd like to get more observations to try and spot
any possible companions to such stars. "The issue of a possible
companion is really pure speculation at this point," he said.
"However, I am trying to get observations that will assess this
These results are being published in the February 10, 2008 issue of
the Astrophysical Journal. A preprint of the paper can be found
Partial studies of magnetic activity on these types of stars have
been performed previously, but this is the first time that such a
powerful set of telescopes has been simultaneously pointed at the
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Magnetars–A Computer's Dream World