[meteorite-list] Dust Around Nearby Star Like Powder Snow

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 11 Jan 2007 16:47:44 -0800 (PST)
Message-ID: <200701120047.QAA10269_at_zagami.jpl.nasa.gov>

Media Relations
University of California-Berkeley

Media Contacts:
Robert Sanders
(510) 643-6998, (510) 642-3734

FOR IMMEDIATE RELEASE: Monday, January 08, 2007

Dust around nearby star like powder snow
By Robert Sanders, Media Relations

BERKELEY -- Astronomers peering into the dust surrounding a nearby red dwarf
star have found that the dust grains have a fluffiness comparable to that of
powder snow, the ne plus ultra of skiers and snowboarders.

This is the first definitive measurement of the porosity of dust outside our
solar system, and is akin to looking back 4 billion years into the early
days of our planetary system, say researchers at the University of
California, Berkeley. That was the era after the formation of planets, but
before the remaining snowball- or softball-sized rubble was ground into dust
by collisions and blown out of the inner solar system.

"We believe that this porosity is primordial, and reflects the agglomeration
process whereby interstellar grains first assembled to form macroscopic
objects," said James Graham, UC Berkeley professor of astronomy.

The grains are probably microscopic dirty snowballs, a mixture of ice and
rock.

"The difference between a snowflake and a hailstone -- both are ice but with
very different porosities -- occurs because they form very differently," he
added. "Hailstones grow in violent thunderstorms; snowflakes grow under much
more sedate meteorological conditions. Similarly, we conclude that the dust
grains in the AU Mic debris disk formed by gentle agglomeration."

Graham and Paul Kalas, a UC Berkeley assistant adjunct professor of
astronomy, discussed their findings on the AU Microscopii (AU Mic) system at
a press conference yesterday (Sunday, Jan. 7) during the Seattle meeting of
the American Astronomical Society.

Graham, Kalas and former UC Berkeley post-doctoral fellow Brenda C.
Matthews, now at the Herzberg Institute of Astrophysics in Victoria, British
Columbia, Canada, also presented their findings yesterday during a poster
session at the meeting. Their paper on the dust in the AU Mic disk was
published in the Jan. 1, 2007, issue of The Astrophysical Journal.

Objects in our solar system also are porous -- comet grains that have lost
their ice are like birds' nests, while some asteroids have been shown to be
half-empty rubble piles -- but none are as full of nothingness as the dust
in AU Mic, which is more than 90 percent vacuum.

"Most things we see have been compactified or compressed so that the vacuum
has been squeezed out and filled in. Once you get to macroscopic objects a
few inches across, those interstices are compressed and go away. So, 97
percent is a very high value," Graham said.

The astronomers were studying the closest known star with a dusty debris
disk and possible planetary system, which were discovered around AU Mic by
Kalas nearly three years ago. Red dwarfs like AU Mic, with a mass less than
half that of the sun, are the most common stars in the Milky Way Galaxy. And
at 33 light years distance, AU Mic is close enough for the Hubble Space
Telescope to image with exquisite spatial resolution.

Hubble observations have previously shown that the 12 million-year-old AU
Mic system bears a strong resemblance to our much older solar system, with a
ring of debris around it analogous to our Kuiper Belt of comets and
Pluto-sized objects. This outer belt starts about 40 to 50 astronomical
units (AU) from the central star, where an AU is 93 million miles, the
average distance of the Earth from the sun. The inside of this region
appears devoid of dust, hence the suspicion that the star has planets and
other orbiting debris that have removed the dust.

The UC Berkeley researchers, however, were curious about the dust grains far
smaller than the rocks and planets.

"The big question in planet formation is how dust grains grow from
interstellar sizes -- about 100 nanometers -- to macroscopic objects,"
Graham said. A 100 nanometer grain is one-tenth of a micron; a thousand such
grains would span the diameter of a human hair. "We know that interstellar
grains exist; we know that planets exist, but what we don't know is how they
grow."

On August 1, 2004, the Hubble telescope slipped Polaroid glasses over its
Advanced Camera for Surveys and snapped pictures of the nearly edge-on AU
Mic disk as the polarizing filters rotated, sampling different linear
polarizations.

"We use the polarizing filters to measure how the light reflects and
scatters off the dust," Graham said. "The degree of polarization is useful
for the same reason that polarizing sunglasses are useful to reduce the
glare of reflected sunlight from the ocean."

By comparing the brightness of the scattered light at different
polarizations, the researchers were able to calculate the porosity of the
dust, which turned out to be greater than 90 percent, analogous to powder
snow common in California's Sierra Nevada. The most porous dust is similar
to the driest powder snow on Earth, termed "champagne powder," which is 97
percent air and only 3 percent ice.

These dust grains, which are on the order of a micron across, the size of
soot or smoke particles, are quickly blown out of the inner disk by the
stellar wind, which means that the dust is continually being replenished by
colliding bodies in the inner system.

"These colliding bodies must be fairly fluffy, too," Graham said. "These are
the 10- to 20-centimeter snowballs, which are weakly bound together. Two of
them have a glancing collision and release a puff of ice that we get to see
in reflected light from the star."

The findings are consistent with a theory of planet formation whereby gas
and dust coalesce into rocks and planets within the first 10 million or so
years. While planet-size bodies continue to sweep up some of the remaining
dust and debris, the debris also collides and creates small dust grains
small enough for the stellar wind to blow it out of the inner system,
leaving a hole dominated by larger objects, like the planets, dwarf planets
and asteroids of our solar system. UC Berkeley theoretical astronomer Eugene
Chiang coined the term "birth ring" to indicate the ring of objects around a
star that divides a planetary system into an inner region devoid of small
dust grains and an outer region into which these grains have been blown and
still orbit the star in a belt like the Kuiper Belt.

"This gives quite a lot of credence to Chiang's theory," Graham said. "The
thought is that these debris disks are in the cleanup phase, where all the
small particles are colliding and being reduced to small dust grains and
being blown away. So what is left in a few 100 million years is meter-sized
objects and above. And, of course, the planetary mass objects."

The work was supported by the National Aeronautics and Space Administration.
Received on Thu 11 Jan 2007 07:47:44 PM PST