Dust around nearby star like powder snow

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.

An artist's concept of the birth ring of debris encircling the 12-million-year-old star AU Microscopii. Porous, snowball-sized bodies collide within the birth ring. Stellar winds disperse dust grains away from the star beyond the birth ring to the outer debris disk. View full-size graphic (Credit: NASA, ESA an A. Feild STScI)

“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.