ATLANTA — Until recently, astrophysicists studying exotic star systems pairing
a white dwarf and a red dwarf in very close proximity didn’t have much to go on.

Just five years ago, scientists knew of fewer than 100 such systems, called
pre-cataclysmic variables. But today a team of University of Washington
astronomers said that, with data from the Sloan Digital Sky Survey (SDSS), the
number has now grown to nearly 500.

That is significant because researchers are now able to study white dwarf and
red dwarf stars at different stages of their life cycles, giving scientists the
ability to compare them and develop an understanding of how the systems evolve
and change over the course of billions of years, possibly becoming supernovas.

"We’ve never had the opportunity to study a variety of these systems in detail
before now," said Nicole Silvestri, a University of Washington astronomy
researcher. Using this large sample from the SDSS, Silvestri and her colleagues
believe they can begin to answer some of the long-standing questions in
astronomy about pre-cataclysmic variables and their eventual end products,
cataclysmic variable systems.

Silvestri is lead author of a poster presentation on the findings presented
today (January 6, 2004) at the American Astronomical Society’s annual meeting in
Atlanta. Co-authors of the project are Suzanne Hawley and Paula Szkody of the
University of Washington’s Astronomy Department. The National Science Foundation
supported the research.

Pre-cataclysmic variable systems pair a red dwarf star about one-tenth the size
of our sun and a dense remnant of a star, called a white dwarf, in close orbit
around each other. When the two stars are close enough, orbiting one another in
less than four hours, the gravity of the denser white dwarf is able to pull
material off of the less dense red dwarf. Material from the red dwarf forms a
disk around the white dwarf that eventually accumulates on the surface of the
white dwarf. (Variability refers to the changing amount of light coming from the
stars as they orbit each other).

As the white dwarf gains mass, many small explosions, called cataclysmic events,
occur on the surface of the white dwarf. If the white dwarf gravity gets to a
critical point, it can collapse catastrophically. This heats up the white dwarf
tremendously and may cause it to explode as a supernova.

Pre-cataclysmic variables found so far in the SDSS data have orbital periods of
between four and 12 hours and are not close enough to have begun transferring
material between the stars.

Silvestri said the evolution of a pre-cataclysmic variable to a cataclysmic
variable takes billions of years and studying just one system as it evolves
would be impossible. But with nearly 500 pre-cataclysmic variables to study, "A
dataset of this size will allow us to take snapshots in time of the evolution of
the system," she said. "This will allow the researchers to study how properties
of each star change as the pair draw closer to each other, something that until
now, has never been investigated."

Silvestri and her colleagues are still at a loss to explain one oddity in the
research. Thousands of isolated white dwarfs have been observed and hundreds of
them have been found to be magnetic. And many white dwarfs in cataclysmic
variables are magnetic. But not one of the white dwarfs observed in the
pre-cataclysmic variable systems is magnetic.

"This makes the origin of magnetic cataclysmic variables (known as polars),
which do contain magnetic white dwarfs, exceedingly mysterious," added SDSS
researcher Suzanne Hawley of the University of Washington.

"That’s a question we’re still trying to find an answer to," Silvestri said.
"How do you get a magnetic white dwarf in a cataclysmic variable if it doesn’t
originate in one of these pairs that is evolving toward being a cataclysmic
variable?" The University of Washington team, James Liebert of the University of
Arizona and others are preparing a paper on that finding for the Astronomical


Nicole Silvestri
University of Washington, Astronomy Department
Seattle WA 98195-1580

Suzanne Hawley, University of Washington

Paula Szkody, University of Washington

[ (50KB)]
SDSS scientists recognize white-dwarf/red dwarf pairs by their spectrum, the
decomposition of the light into its component wavelengths. This figure shows the
spectrum of such a pair; the blue part of the spectrum on the left (smaller
wavelengths) is dominated by the white dwarf, while the red part on the right
(longer wavelengths) is dominated by the red dwarf (also known as an M dwarf).
The result is a composite spectrum that doesn’t look like the spectrum of any
single star. The two stars are close enough to each other that they appear as a
single unresolved point of light in a telescope; it is only through the spectrum
that the presence of two stars is revealed.

SOURCE: University of Washington, SDSS.