Dusty Old Star Offers Window to Our Future/Texas astronomers, othersfind dead stars collecting dust (Forwarded)

Gemini Observatory
Hilo, Hawaii

Media Contacts:
Peter Michaud
Gemini Observatory, Hilo HI
(808) 974-2510 (Office)

Stuart Wolpert
UCLA Media Relations Representative
(310) 206-0511 (Office)

Science Contact:
Inseok Song
Gemini Observatory, Hilo HI
(808) 974-2609 (Office)

For Immediate Release: Thursday, 08 September 2005

Dusty Old Star Offers Window to Our Future, Astronomers Report

Astronomers have glimpsed dusty debris around an essentially dead star
where gravity and radiation should have long ago removed any sign of
dust. The discovery might provide insights into our own solar system's
eventual demise several billion years from now.

The results are based on mid-infrared observations made with the Gemini
8-meter Frederick C. Gillett Telescope (Gemini North) on Hawaii's Mauna
Kea in May of 2005. The Gemini observations reveal a surprisingly high
abundance of dust orbiting an ancient stellar ember named GD 362.

"This is not an easy one to explain," said Eric Becklin, UCLA astronomer
and Principal Investigator for the Gemini observations. "Our best guess
is that something similar to an asteroid or possibly even a planet
around this long-dead star is being ground up and pulverized to feed the
star with dust. The parallel to our own solar system's eventual demise
is chilling."

The Gemini research team includes scientists from UCLA, Carnegie
Institution and Gemini Observatory. The results are scheduled for
publication in an upcoming issue of the Astrophysical Journal. (Paper
available at http://arxiv.org/abs/astro-ph/0509193). The results will be
published concurrently with complementary near-infrared observations by
a University of Texas (UT) team led by Mukremin Kilic made at the NASA
Infrared Telescope Facility (IRTF) also on Mauna Kea. See companion (UT)
press release here [NOTE: See below].

"We now have a window to the future of our own planetary system," said
Benjamin Zuckerman, UCLA professor of physics and astronomy, member of
NASA's Astrobiology Institute, and a co-author on the Gemini-based
paper. "For perhaps the first time, we have a glimpse into how planetary
systems like our own might behave billions of years from now."

"The reason why this is so interesting is that this particular white
dwarf has by far the most metals in its atmosphere of any known white
dwarf," Zuckerman added. "This white dwarf is as rich in calcium,
magnesium and iron as our own sun, and you would expect none of these
heavier elements. This is a complete surprise. While we have made a
substantial advance, significant mysteries remain."

"We have confirmed beyond any doubt that dust never does sleep!" quips
Gemini Observatory's Inseok Song, a co-author on the paper. "This dust
should only exist for hundreds of years before it is swept into the star
by gravity and vaporized by high temperatures in the star's atmosphere.
Something is keeping this star well stocked with dust for us to detect
it this long after the star's death."

"There are just precious few scenarios that can explain so much dust
around an ancient star like this," said UCLA's Mike Jura who led the
effort to model the dust environment around the star. "We estimate that
GD 362 has been cooling now for as long as five billion years since the
star's death-throes began and in that time any dust should have been
entirely eliminated." Jura likens the disk to the familiar rings of
Saturn and thinks that the dust around GD 362 could be the consequence
of the relatively recent gravitational destruction of a large "parent
body" that got too close to the dead star.

GD 362 is a white dwarf star. It represents the end-state of stellar
evolution for stars like the Sun and more massive stars like this one's
progenitor, which had an original mass about seven times the Sun's.
After undergoing nuclear reactions for millions of years, GD 362's core
ran out of fuel and could no longer create enough heat to counterbalance
the inward push of gravity. After a short period of instability and mass
loss, the star collapsed into a white-hot corpse. The remains are
cooling slowly over many billions of years as the dying ember makes its
slow journey into oblivion.

Based on its cooling rate, astronomers estimate that between two to five
billion years have passed since the death of GD 362. This long time
frame would explain why there is no sign of a shell of glowing gas known
as a planetary nebula from the expulsion of material as the star died.
During its thermonuclear decline, GD 362 went through an extensive
period of mass loss, going from a mass of about seven-times that of the
Sun to a smaller, one-solar-mass shadow of its former self. See sidebar
for more background text and image.

Although about one-quarter of all white dwarfs contain elements heavier
than hydrogen in their atmospheres, only one other white dwarf is known
to contain dust. The other dusty white dwarf, designated G29-38, has
about 100 times less dust density than GD 362.

According to Gemini Astronomer and team member Jay Farihi, "White dwarf
researchers have been looking extensively for systems like this for
almost 20 years. I expect that ongoing space-based mid-infrared surveys
will soon tell us more about these uncommon, dusty, dead stars. What
makes this discovery particularly exciting is that this is indrect
evidence that planetary systems can survive the death of their star!"

The Gemini observations were made with the MICHELLE mid-infrared
spectrograph on the Gemini North telescope on Mauna Kea Hawai'i. "These
data are phenomenal," said Alycia Weinberger of the Carnegie
Institution. "Observing this star was a thrill! We were able to find
the remnants of a planetary system around this star only because of
Gemini's tremendous sensitivity in the mid-infrared. Usually you need a
spacecraft to do this well."

The Gemini mid-infrared observations were unique in their ability to
confirm the properties of the dust responsible for the "infrared excess"
around GD 362. The complementary IRTF near-infrared observations and
paper by the UT team provided key constraints on the environment around
the star. UT faculty and co-author Ted von Hippel describes how the IRTF
observations complement the Gemini results. "The IRTF spectrum rules out
the possibility that this star could be a brown dwarf as the source of
the "infrared excess." von Hippel continues, "The combination of the two
data sets provides a convincing case for a dust disk around GD 362."

The Gemini observations included time made available as part of an
exchange of instrument time with the W.M. Keck Observatory.

Publication-quality images and audio available at

http://www.gemini.edu/index.php?opti...1&limitstart=1

SIDEBAR
White Dwarfs and the Evolution of Stars

Observations and theory give strong evidence that all stars expel a
large fraction of their mass throughout their lifetimes. The most
intensive phase of this mass loss takes place during a star's late
evolutionary phase. When a star nears the end of its life, its central
region contains less fuel for hydrogen burning and the core region
contracts until the temperature gets high enough for the nuclear fusion
of helium to begin. The structure of the star changes dramatically as
hydrogen in the shell surrounding the core also begins burning.
Depending on the star's initial mass on the main sequence (where a star
spends most of its hydrogen-burning lifetime), it then enters an
unstable period where variations in its temperature, radius and
luminosity occur. This can result in structural changes in the star,
such as the loss of its external layers.

For stars more than about eight times the mass of the Sun, the ultimate
effect of these instabilities is a spectacular supernova explosion.
Stars less than about eight solar masses enter a short phase (a few
thousand years) that includes successive episodes of mass loss. This
eventually leaves behind a stripped stellar core of carbon and oxygen
mixed with a degenerate gas of electrons that determine the structure of
the remnant. What's left is called a white dwarf, an important end
product of stellar evolution.

White dwarfs have an average diameter of about 10,000 kilometers, about
the size of the Earth. However, their final masses are about half that
of the Sun, which makes their density about a million times that of most
common solid elements found on Earth. The properties of these stellar
corpses are fascinating because of the curious nature of the degenerate
electrons that provide the pressure to support them. The compressed
electrons behave like a solid because of their high conductivity and
incompressibility, but they are truly what is known as a degenerate gas.
This incompressible quality of white dwarfs has led some call them, "the
largest diamonds in the universe."

The white dwarf phase of a star can last for billions of years, and
during this time the object does not generate energy by thermonuclear
reactions. The energy that radiates is sustained simply by cooling, just
as a hot piece of iron metal emits radiation as it cools.

*****

McDonald Observatory
University of Texas

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

08 September 2005

Texas astronomers, others find dead stars collecting dust

MAUNA KEA, Hawaii -- Two independent teams of astronomers using
telescopes on Mauna Kea, Hawaii have glimpsed dusty debris around an
essentially dead star where gravity and radiation should have long ago
removed any sign of dust. The discovery might provide insights into our
own solar system's demise billions of years from now.

The observations, made with the NASA Infrared Telescope Facility (IRTF)
and the Gemini 8-meter Frederick C. Gillett Telescope, reveal a
surprisingly high abundance of dust orbiting an ancient stellar ember
named GD 362.

GD 362 is a white dwarf star. It represents the end-state of stellar
evolution for almost all stars, including the Sun and more massive stars
like this one's progenitor, which had an original mass about seven times
the Sun's. After undergoing nuclear reactions for millions of years, GD
362's core ran out of fuel and could no longer create enough heat to
counterbalance the inward push of gravity. After a short period of
instability and mass loss, the star collapsed into a white-hot corpse.
The remains will cool slowly over many billions of years as the dying
ember makes its final, slow journey into oblivion.

In its former life, GD 362 may have been home to a planetary system. The
dust disk may be evidence of that.

According to University of Texas graduate student Mukremin Kilic, who
led the team making the IRTF observations, "The best explanation for the
disk around GD 362 is that a planet or asteroid-like object was tidally
disrupted by the white dwarf and ground up to tiny particles that ended
up in a debris disk around the star. It is likely that we are witnessing
the destruction of a planetary system and that a similar fate may await
our own planetary system in about five billion years."

These results are exploring new ground in the search for planetary
systems. "This is only the second white dwarf star known to be
surrounded by a debris disk," Kilic said. The other is called G29-38.

"Both of these stars' atmospheres are continuously polluted by metals --
that is, heavy chemical elements -- almost surely accreted from the
disk," Kilic said. "If the accretion from a debris disk can explain the
amounts of heavy elements we find in white dwarfs, it would mean that
metal-rich white dwarfs -- and this is fully 25% of all white dwarfs --
may have debris disks, and therefore planetary systems, around them," he
said, concluding, "Planetary systems may be more numerous than we thought."

The IRTF team also includes Ted von Hippel and Don Winget from The
University of Texas and Sandy Leggett of the Joint Astronomy Centre. The
Gemini team is led by Eric Becklin of UCLA, and includes researchers
from the Carnegie Institution and Gemini Observatory.

The IRTF and Gemini data are "beautifully complementary," Texas' von
Hippel said. "Both data sets support the idea of a dust disk around GD
362, but they do so based on different evidence."
He explained that the Gemini data provide "measurements at longer
wavelengths that are more sensitive to the dust temperature," while the
IRTF results provide higher-resolution spectroscopy in the
near-infrared, thus "excluding an orbiting brown dwarf as the source" of
the excess infrared light from the white dwarf.

The teams will publish back-to-back papers in an upcoming issue of
Astrophysical Journal Letters.

The IRTF is a 3-meter telescope, optimized for infrared observations,
operated and managed for NASA by the University of Hawaii Institute for
Astronomy. Gemini is an international partnership managed by the
Association of Universities for Research in Astronomy under a
cooperative agreement with the National Science Foundation.