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

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EMBARGOED FOR RELEASE: 9:20 a.m. PST, Sunday, January 7, 2007

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