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1996-01-12
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CONTACT: Ray Villard/STScI EMBARGOED UNTIL: 3:00 P.M. MST
410-338-4514 January 10, 1995
Dr. Jeffrey Linsky/JILA PRESS RELEASE NO.: STScI-PR95-03
303-492-7838
RED DWARF DYNAMO RAISES PUZZLE OVER
INTERIORS OF LOWEST-MASS STARS
NASA's Hubble Space Telescope has uncovered surprising evidence that
powerful magnetic fields might exist around the lowest mass stars in
the universe, which are near the threshold of stellar burning
processes.
"New theories will have to be developed to explain how these strong
fields are produced, since conventional models predict that these low
mass red dwarfs should have very weak or no magnetic fields," says Dr.
Jeffrey Linsky of the Joint Institute for Laboratory Astrophysics
(JILA) in Boulder, Colorado. "The Hubble observations provide clear
evidence that very low mass red dwarf stars must have some form of
dynamo to amplify their magnetic fields."
His conclusions are based upon Hubble's detection of a high-temperature
outburst, called a flare, on the surface of the extremely small, cool
red dwarf star Van Biesbroeck 10 (VB10) also known as Gliese 752B.
Stellar flares are caused by intense, twisted magnetic fields that
accelerate and contain gasses which are much hotter than a star's
surface.
Explosive flares are common on the Sun and expected for stars that have
internal structures similar to our Sun's. Stars as small as VB10 are
predicted to have a simpler internal structure than that of the Sun and
so are not expected to generate the electric currents required for
magnetic fields that drive flares.
Besides leading to a clearer understanding of the interior structure of
the smallest red dwarf stars known, these unexpected results might
possibly shed light on brown dwarf stars. A brown dwarf is a
long-sought class of astronomical object that is too small to shine
like a star through nuclear fusion processes, but is too large to be
considered a planet.
"Since VB10 is nearly a brown dwarf, it is likely brown dwarfs also
have strong magnetic fields," says Linsky. "Additional Hubble searches
for flares are needed to confirm this prediction."
A QUARTER-MILLION DEGREE TORCH
The star VB10 and its companion star Gliese 752A make up a binary
system located 19 light-years away in the constellation Aquila. Gliese
752A is a red dwarf that is one-third the mass of the Sun and slightly
more than half its diameter. By contrast, VB10 is physically smaller
than the planet Jupiter and only about nine percent the mass of our
Sun. This very faint star is near the threshold of the lowest possible
mass for a true star (.08 solar masses), below which nuclear fusion
processes cannot take place according to current models.
A team led by Linsky used Hubble's Goddard High Resolution Spectrograph
(GHRS) to make a one-hour long exposure of VB10 on October 12, 1994.
No detectable ultraviolet emission was seen until the last five
minutes, when bright emission was detected in a flare. Though the
star's normal surface temperature is 4,500 degrees Fahrenheit,
Hubble's GHRS detected a sudden burst of 270,000 degrees Fahrenheit in
the star's outer atmosphere. Linsky attributes this rapid heating to
the presence of an intense, but unstable, magnetic field.
THE INTERIOR WORKINGS OF A STELLAR DYNAMO
Before the Hubble observation, astronomers thought magnetic fields in
stars required the same dynamo process which creates magnetic fields on
the Sun. In the classic solar model, heat generated by nuclear fusion
reactions at the star's center escapes through a radiative zone just
outside the core. The heat travels from the radiative core to the
star's surface through a convection zone. In this region, heat
bubbles to the surface by motions similar to boiling in a pot of
water.
Dynamos, which accelerate electrons to create magnetic forces, operate
when the interior of a star rotates faster than the surface. Recent
studies of the Sun indicate its convective zone rotates at nearly the
same rate at all depths. This means the solar dynamo must operate in
the more rapidly rotating radiative core just below the convective
zone.
The puzzle is that stars below 20 percent the mass of our Sun do not
have radiative cores, but instead transport heat from their core
through convection only. The new Hubble observations suggest a
magnetic dynamo perhaps of a new type can operate inside these stars.
These results are being reported at the 185th meeting of the American
Astronomical Society in Tucson, Arizona.
* * * * * * * * * * * *
The Space Telescope Science Institute is operated by the Association of
Universities for Research in Astronomy, Inc. (AURA) for NASA, under
contract with the Goddard Space Flight Center, Greenbelt, MD. The
Hubble Space Telescope is a project of international cooperation
between NASA and the European Space Agency (ESA).
JILA is a joint institute of the University of Colorado and the
National Institute of Standards and Technology (NIST). Dr. Linsky is a
staff member of the Quantum Physics Division of NIST.