McDonald Observatory
Astronomers Discover New Type Of Pulsating White Dwarf Star
(1 May 2008) 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.
The 2.1-meter (82-inch) Otto Struve Telescope at the University of Texas McDonald Observatory. (courtesy: Marty Harris/McDonald Observatory.)
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.
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. (courtesy: Sloan Digital Sky Survey (SDSS) Collaboration (http://sdss.org))
"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 north-east 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.
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. (courtesy: K. Williams/T. Jones/McDonald Observatory)
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 ionised 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.
(source: McDonald Observatory)
