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.