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Cosmic Weight Loss: The Lowest Mass White Dwarf (Forwarded)



 
 
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Old April 18th 07, 10:18 PM posted to sci.space.news
Andrew Yee[_1_]
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Default Cosmic Weight Loss: The Lowest Mass White Dwarf (Forwarded)

Public Affairs Office
Harvard-Smithsonian Center for Astrophysics
Cambridge, Massachusetts

For more information, contact:

David A. Aguilar, Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
617-495-7462

Christine Pulliam, Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics
617-495-7463

For Release: Tuesday, April 17, 2007

Release No.: 2007-08

Cosmic Weight Loss: The Lowest Mass White Dwarf

Cambridge, MA -- Astronomers have found the lowest mass white dwarf known in
our galaxy: a Saturn-sized ball of helium containing only about one-fifth
the mass of the Sun. In addition, they have spotted the source of the white
dwarf's radical weight-loss plan. An unseen companion, likely another white
dwarf, has sucked away much of the tiny white dwarf's material, leaving it a
shadow of its former self.

"This star is bizarre," said Warren Brown of the Harvard-Smithsonian Center
for Astrophysics. "It takes extraordinary circumstances to make such a low
mass white dwarf."

When a Sun-like star ages and dies, it becomes a white dwarf. The newfound
white dwarf, with the unwieldy designation of SDSS J091709.55+463821.8
(hereafter J0917+46) lies about 7,400 light-years from Earth near the border
of the constellations Lynx and Ursa Major. Where a typical white dwarf holds
about half a Sun's worth of material, the newfound white dwarf contains only
a fraction of that mass.

"Our white dwarf is skinny in terms of mass, yet it looks fat in terms of
its physical size," stated first author Mukremin Kilic of Ohio State
University. "It's about nine times bigger than a typical white dwarf in
diameter."

When the astronomers first found J0917+46, they predicted that it must have
an unseen companion that had aided its weight loss. A subsequent radial
velocity search, which looked for signs that the white dwarf wobbled when
tugged by a companion's gravity, confirmed the prediction. The astronomers
ruled out the possibility that the companion is either a low-mass main
sequence star or a black hole. It must be either another white dwarf or a
neutron star, with a white dwarf being the more likely candidate.

"No star is old enough to produce such an extremely low-mass white dwarf by
itself," explained Brown. "Therefore, we knew that mass must have been
stripped from the white dwarf by a companion."

"Finding the companion means that stellar evolution theories have passed a
major test," added co-author Scott Kenyon of the Smithsonian Astrophysical
Observatory. "The fact that the companion is a more massive white dwarf or
neutron star is also consistent with theory."

The team also described the oddball pair's history. This binary system began
with one star about twice the mass of the Sun and a second star slightly
less massive than the Sun. The more massive star was the first to evolve,
becoming a white dwarf weighing perhaps one-third as much as the Sun. Ten
billion years later, its companion became another white dwarf. In each step,
the puffed-up outer layers of the evolving star enveloped the companion,
causing friction that moved the two stars closer together. They now orbit
each other every 7.6 hours at a distance of about 650,000 miles and a
stunning speed of 335,000 miles per hour.

"The relation between our white dwarf and its companion is like a cosmic
marriage in which both people have to give a lot," said Kilic. "Two stars
start out close to each other. One of them engulfs the other (like a hug)
and gives continuously (losing mass), and they get closer. Then the other
star evolves and becomes a giant and engulfs the first star (hugging back)
and now it has to give a lot, or lose a lot of mass. They get closer and
closer and end up dancing continuously."

The astronomers predict that the two white dwarfs eventually will merge.
However, that merger will not take place for 10 billion years or more.

Key observations were made with the MMT Observatory in Arizona, which is
operated jointly by the Smithsonian Astrophysical Observatory and the
University of Arizona.

These findings are reported online at
http://arxiv.org/abs/0704.1813
and
http://arxiv.org/abs/astro-ph/0611498

Note to editors: Images to accompany this release are online at
http://cfa-www.harvard.edu/press/200...08_images.html
 




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