Calling Dr. Frankenstein! : Interactive Binaries Show Signs ofInduced Hyperactivty (Forwarded)

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RELEASE NO: NOAO 07-01

Calling Dr. Frankenstein! : Interactive Binaries Show Signs of Induced
Hyperactivty

Astronomers studying highly energetic binary stars called polars have
obtained the first observational evidence that the intense magnetic fields
produced by the white dwarf half of the interacting pair can induce
flares, sunspots and other explosive activity in its otherwise
low-wattage, low-mass partner.

"Like Dr. Frankenstein zapping an inert corpse, the white dwarfs in these
systems produce very strong electrical currents inside the bodies of their
partner star, which can create violent eruptions where there otherwise
would be very little if any," says Stella Kafka, an astronomer at the
National Optical Astronomy Observatory (NOAO) and lead author of one of
two related poster papers presented today in Seattle at the 209th meeting
of the American Astronomical Society. "These transitory phenomena occur on
human timescales, lasting from minutes to years."

Decades ago, astronomers found evidence that other Sun-like stars show
large optical flares, star-spots, x-ray emission and other energetic
activity cycles, especially when they are part of binary systems. In
binaries, fast rotation rates and tidal interactions between the two
stellar components are the primary contributors to the observed activity.

By contrast, the low-mass partners in polars (also known as Magnetic
Cataclysmic Variables) can be as small as planet Jupiter, and range in
mass from about 20 percent of the Sun down to brown dwarf-like objects
with 5 percent or less of a solar mass. The masses of these companions are
theoretically too low for conventional Sun-like internal dynamos to be
possible.

Thus, the surface activity detected by these studies is likely greatly
enhanced by the white dwarf's strong magnetic field passing through the
secondary low-mass star, causing large-scale electric currents in its
interior. This flow of charged particles creates an effective dynamo
mechanism.

"This discovery points to a new mechanism for the generation of stellar
activity by forces outside of the star itself, a phenomenon that we have
dubbed 'hyperactivity,' " says co-author Steve B. Howell of NOAO and the
WIYN observatory.

Over the past two years, a team of astronomers consisting of Kafka,
Howell, R. Kent Honeycutt (Indiana University), Fred Walter (State
University of New York), Thomas Harrison (New Mexico State University) and
Jeff Robertson (Arkansas Tech University) have carefully observed four
polars (in particular, EF Eridanus and ST Leo Minor) using the 2.1-meter,
4-meter and WIYN 3.5-meter telescopes at Kitt Peak National Observatory,
the Magellan 6.5-meter telescope and the ESO Very Large Telescope in
Chile, for more than 20 nights of observing.

"Careful analysis of the resulting data shows strong evidence for the
formation and structure of star-spots and gigantic prominences and loops
in the low-mass partner in these polars," says Kafka. "Furthermore, we
found that this activity seems to be concentrated toward the white dwarf
and on both sides of the cool red star."

This is the first time that astronomers have strong observational evidence
that strong magnetic-field interactions between the stars in a close
binary system may be the primary ingredient for the formation of large
starspots and flares.

Polars are binaries consisting of a white dwarf (an old star with a mass
of one-half to one times that of the Sun but a diameter approximately
equal to the Earth), and a very cool, red, low-mass stellar object. The
two stars are trapped in a close orbit about each other (separated by less
than the diameter of the Sun), completing a full circle in only 80 to 180
minutes. A special characteristic of these systems is that the white dwarf
contains a very strong magnetic field in the range of 13 to 66 million
gauss (13-66 megagauss).

For comparison, the magnetic field at the Earth's surface is 0.3-0.6
gauss. The magnetic field strength of the Sun averages one gauss, but can
reach values as high as 3,000 gauss in active sunspot regions. Rapidly
rotating solar-like stars are known to have increased levels of starspot
activity and higher average magnetic field strengths. Their fast rotation
makes the star's internal dynamo rotate rapidly, leading to stronger
stellar magnetic fields, more starspots on the star's surface, and
energetic activity like flares.

In polars, the low-mass companion is "locked" in its orbit by tidal
interactions with the white dwarf; very similar to the way that the Moon
always keeps nearly the same face toward Earth. Therefore, the low-mass
star spins around its axis with a period of only a few hours (compared to
the 25-day rotation of the Sun).

Since rotation is a key ingredient of stellar activity, ultra-fast
rotation of the red star in 1-3 hours is expected to increase its average
magnetic field strength to values near 2,000-6,000 gauss (2-6 kilogauss).
"When mixed with the enormous magnetic field of the white dwarf, the
interaction between the two stars creates a spaghetti-like pattern of
magnetic field lines between the two stars," Howell says. "These magnetic
fields confine gas around and between the two components and are
responsible for triggering the enhanced activity on the low-mass star."

The artist's concept shown in the figure visualizes such an effect: it
shows a cool low-mass red star with a highly magnetic white dwarf locked
in a tight orbit by gravity. The interacting magnetic field lines (blue)
produce large coronal loops on the low-mass red star, allowing for
high-temperature material to flow along them as well as become trapped in
them, similar to large loop-like prominences observed on the Sun.

The increased stellar activity and large loop structures represent one of
the findings that the team presents today during this AAS meeting (see
posters 9.16 and 9.17).

"The systems studied by our team can, in some ways, be looked at as
scaled-up versions of the 'hot Jupiter' type of extrasolar planetary
systems," Howell says. These exoplanet systems consist of a solar-like
star and a massive Jupiter-like planet in close orbit. As the planet
orbits around its parent star, the outer atmosphere (or chromosphere) of
the star responds to the passage of the planet.

Observations suggest that the magnetic field of the star permeates the
planet and allows magnetic loops to reconnect by using the planet as a
conductor. As a result, energetic activity would be induced in the
planet's atmosphere, resulting in small flares and events similar to an
aurora (northern lights) on Earth. The similar (though higher-level)
phenomena in magnetic cataclysmic variables is easier to study and
therefore can provide more detailed information about such interactions,
eventually leading to a comprehensive model.

The color artwork that illustrates this result is available at
http://www.noao.edu/outreach/press/p...01.html#images

The National Optical Astronomy Observatory, based in Tucson, Arizona,
includes Kitt Peak National Observatory, Cerro Tololo Inter-American
Observatory and the NOAO Gemini Science Center. It is operated by the
Association of Universities for Research in Astronomy Inc. (AURA), under a
cooperative agreement with the National Science Foundation.

IMAGE CAPTION:
[http://www.noao.edu/outreach/press/p...g-binaries.jpg
(1.06MB)]
This artist's concept shows an interacting binary star system known as a
polar (or a magnetic cataclysmic variable). The white star is a very
dense, highly magnetic white dwarf in which the magnetic poles of the star
are not aligned with its rotation axis. The cool, low-mass red star is
distorted due to the strong gravity of the much more massive white dwarf.
New research has provided the first direct observational evidence that
significant stellar activity in the red star (such as large starspots,
prominences, and flares) can be induced by interactions with the strong
magnetic field of the white dwarf (blue lines), a phenomenon dubbed
hyperactivity.

Credit: P. Marenfeld and NOAO/AURA/NSF