McDonald Observatory Astronomers Discover New Type of PulsatingWhite Dwarf Star (Forwarded)

McDonald Observatory
University of Texas

Contact:
Rebecca Johnson
ph: 512-475-6763
fax: 512-471-5060

Additional Media Contact:

Diane Benegas
National Science Foundation
703-292-4489

1 May 2008

McDonald Observatory Astronomers Discover New Type of Pulsating White
Dwarf Star

AUSTIN, Texas -- University of Texas at Austin astronomers Michael H.
Montgomery and Kurtis A. Williams, along with graduate student Steven
DeGennaro, have predicted and confirmed the existence of a new type of
variable star with the help of the 2.1-meter Otto Struve Telescope at
McDonald Observatory. The discovery will be announced in today's issue of
Astrophysical Journal Letters.

Called a "pulsating carbon white dwarf," this is the first new class of
variable white dwarf star discovered in more than 25 years. Because the
overwhelming majority of stars in the universe -- including the Sun --
will end their lives as white dwarfs, studying the pulsations (i.e.,
variations in light output) of these newly discovered examples gives
astronomers a window on an important endpoint in the lives of most stars.

A white dwarf star is the leftover remnant of a Sun-like star that has
burned all of the nuclear fuel in its core. It is extremely dense, packing
half to 1.5 times the Sun's mass into a volume about the size of Earth.
Until recently, there have been two main types of white dwarfs known:
those that have an outer layer of hydrogen (about 80 percent), and about
those with an outer layer of helium (about 20 percent), whose hydrogen
shells have somehow been stripped away.

Last year, University of Arizona astronomers Patrick Dufour and James
Liebert discovered a third type of white dwarf star, still more rare. For
reasons that are not understood, these "hot carbon white dwarfs" have had
both their hydrogen and helium shells stripped off, leaving their carbon
layer exposed. Astronomers suspect these could be among the most massive
white dwarfs of all, and are the remnants of stars slightly too small to
end their lives in a supernova explosion.

After these new carbon white dwarfs were announced, Montgomery calculated
that pulsations in these stars were possible. Pulsating stars are of
interest to astronomers because the changes in their light output can
reveal what goes on in their interiors -- similar to the way geologists
study seismic waves from earthquakes to understand what goes on in Earth's
interior. In fact, this type of star-study is called "asteroseismology."

So, Montgomery and Williams' team began a systematic study of carbon white
dwarfs with the Struve Telescope at McDonald Observatory, looking for
pulsators. DeGennaro discovered that a star about 800 light-years away in
the constellation Ursa Major, called SDSS J142625.71+575218.3, fits the
bill. Its light intensity varies regularly by nearly two percent about
every eight minutes.

"The discovery that one of these stars is pulsating is remarkably
important," said National Science Foundation astronomer Michael Briley.
"This will allow us to probe the white dwarf's interior, which in turn
should help us solve the riddle of where the carbon white dwarfs come from
and what happens to their hydrogen and helium." The research was funded by
NSF and the Delaware Asteroseismic Research Center.

The star lies about ten degrees east northeast of Mizar, the middle star
in the handle of the Big Dipper. This white dwarf has about the same mass
as our Sun, but its diameter is smaller than Earth's. The star has a
temperature of 35,000 degrees Fahrenheit (19,500 C), and is only 1/600th
as bright as the Sun.

None of the other stars in their sample were found to pulsate. Given the
masses and temperatures of the stars in their sample, SDSS
J142625.71+575218.3 is the only one expected to pulsate based on
Montgomery's calculations.

The astronomers speculate that the pulsations are caused by changes in the
star's carbon outer envelope as the star cools down from its formation as
a hot white dwarf. The ionized carbon atoms in the star's outer layers
return to a neutral state, triggering the pulsations.

There is a chance that the star's variations might have another cause.
Further study is needed, the astronomers say. Either way, studying these
stars will shed light on the unknown process that strips away their
surface layers of hydrogen and helium to lay bare their carbon interiors.

IMAGE CAPTIONS:

[Image 1:
http://mcdonaldobservatory.org/news/...age.php?id=18]
The 2.1-meter (82-inch) Otto Struve Telescope at the University of Texas
McDonald Observatory. Photo by Marty Harris/McDonald Observatory.

[Image 2:
http://mcdonaldobservatory.org/news/...ge.php?id=144]
McDonald Observatory astronomers Michael Montgomery, Kurtis Williams, and
Steven DeGennaro discovered that the star SDSS J142625.71+575218.3 is the
first pulsating carbon white dwarf. Credit: Sloan Digital Sky Survey
(SDSS) Collaboration (http://sdss.org)

[Image 3:
http://mcdonaldobservatory.org/news/...ge.php?id=142]
[Image 4:
http://mcdonaldobservatory.org/news/...ge.php?id=143]
This 'light curve' shows the changes in light output over time, or
'pulsations,' of the first-discovered pulsating carbon white dwarf, as
measured by the Argos instrument on the 2.1-meter Otto Struve Telescope at
McDonald Observatory. Credit: K. Williams/T. Jones/McDonald Observatory