- The unexpected observation of a bright flare on a nearby
brown dwarf has shown astronomers that failed and fading stars
like this still have some life left in them.
surprised astronomers who expected to see little or no activity
on the brown dwarf during a planned 12-hour observation by
the Chandra X-Ray Observatory.
after nine hours of seeing nothing, a flare flashed brightly,
then faded out over the next two hours. The flare, very similar
to the flares on our Sun, is the first ever observed on a
expected to see nothing - I hoped to see nothing, to prove
that there was no hot corona surrounding the brown dwarf,"
said principal investigator Gibor Basri, a professor of astronomy
at the University of California, Berkeley.
his collaborators originally planned the long observation
by Chandra to eliminate the possibility that older brown dwarfs
like this have hot coronae. A stellar corona - the upper atmosphere
that stretches far into space and can reach a couple of million
degrees Celsius - should emit copious X-rays. Work by Basri
and his graduate student, Subu Mohanty, has suggested that
brown dwarfs lose their hot coronae as they cool below about
2,200 degrees Celsius.
that the X-ray satellite detected nothing for most of its
12-hour observation of the brown dwarf proves this hypothesis.
was a bonus," Basri said. "We've shown that older brown dwarfs
don't have coronae, but the flare tells us they still have
magnetic fields and also that subsurface flares occasionally
punch through into the atmosphere."
the strongest evidence yet that brown dwarfs and possibly
young giant planets have magnetic fields, and that a large
amount of energy can be released in a flare," said team member
Eduardo L. Martin of the California Institute of Technology
describing the observations has been accepted by Astrophysical
Journal Letters and will be published July 21. The paper is
available on-line at http://xxx.lanl.gov/abs/astro-ph/0005559.
are failed stars somewhere in mass between a large planet
and a small star. This makes them large enough to collapse
and heat up, but not big enough to ignite the steady nuclear
fires necessary to keep stars burning for billions of years.
Instead, after a brief hot flash, brown dwarfs cool off until
they become dead cinders.
extremely dim and went undetected until the recent construction
of larger, more sensitive telescopes. Basri confirmed the
first lithium brown dwarf in 1995 using Hawaii's Keck I Telescope,
and since then several dozen have been found in nearby clusters
or floating freely in the solar neighborhood.
dwarf known as LP 944-20 is one of the nearest, only16 light
years from Earth. In fact, it was first detected more than
25 years ago but was thought to be a very dim red star called
a red dwarf. The recent observation of lithium in its atmosphere
marks it as a brown dwarf.
located in the constellation Fornax in the southern skies,
is about 500 million years old and has a mass that is at most
60 times that of Jupiter, or six percent of the sun's mass.
Its diameter is about one-tenth that of the sun, and it has
a rotation period of less than five hours.
observation of LP 944-50 by the Advanced CCD Imaging Spectrometer
of the Chandra X-Ray Observatory, a satellite operated by
the National Aeronautics and Space Administration, took place
on December 15, 1999.
who studies the evolution of brown dwarfs, teamed up several
years ago with colleague Lars Bildsten, a theoretical astrophysicist
formerly at UC Berkeley but now at the Institute for Theoretical
Physics at UC Santa Barbara. Their coauthors on the paper
reporting the flare are two former UC Berkeley post-doctoral
researchers now working at Caltech: Robert Rutledge, who used
to work with Bildsten; and Martin, who used to work with Basri.
brown dwarfs are observed to have coronae, Basri said. They
are hot enough to have sufficiently ionized atmospheres that
can tangle with their magnetic fields. As they get entangled,
the magnetic fields twist and occasionally cross, arcing like
an electrical short and creating a flare. These flares are
thought to inject high energy particles into the upper atmosphere
or corona, producing temperatures up to several million degrees
dwarfs cool, however, the atmosphere should cool and the gases
de-ionize into a neutral gas. Magnetic fields do not interact
with a neutral atmosphere, and thus atmospheric activity dies
down and the corona disappears.
with the Chandra satellite were designed to test this hypothesis
on a relatively old brown dwarf, and put an upper limit on
the amount of X-ray emission from the corona. The observations
show no quiescent X-ray emission at all. The sensitivity of
NASA's latest "Great Observatory" allows this limit to be
pushed to new lows, Basri said.
an important confirmation of the trend that hot gas in the
atmospheres of lower mass stars is produced only in flares,"
said Bildsten, and not by any quiescent emission from the
emitted in the brown dwarf flare was comparable to a small
solar flare, and was a billion times greater than observed
X-ray flares from Jupiter. The flaring energy is believed
to come from a twisted magnetic field.
could have its origin in the turbulent magnetized hot material
beneath the surface of the brown dwarf," Basri said. "A sub-surface
flare could heat the atmosphere, allowing currents to flow
and give rise to the X-ray flare - like a stroke of lightning."
plans future X-ray observations of other brown dwarfs to further
explore flare activity.
was funded by NASA and the National Science Foundation.
images available for download
Images associated with this release, including high-resolution
digital versions of the X-ray image, are available on the
World Wide Web at http://chandra.harvard.edu