The news from AU Microscopii couldn’t be more interesting, but I’m late getting to it simply because the recent American Astronomical Society meeting left us with so many good things to talk about (wish I could have been there!). But we’ll be examining this red dwarf, 33 light years away in the constellation Microscopium (the Microscope) for a long time because it’s so useful for study. It’s close enough for the Hubble Space Telescope to image it with excellent resolution, and we know from such studies that the star is encircled by a debris disk. Check the image below, where you can see that the disk is nearly edge on as seen from Earth.

Image: The dust and debris disk surrounding the star AU Microscopii, as imaged by the Hubble Space Telescope. The lines indicate the polarization of starlight reflected from the disk, which reveals the porosity or fluffiness of the dust grains. The disk is about 120 astronomical units (AU) across, where one AU is equivalent to the distance between the Earth and sun, or 93 million miles. Credit: NASA, ESA and James Graham/UC Berkeley.

Inside the disk, which begins between 40 and 50 AU from the star, there is a region that is evidently free of dust, leading to the assumption that orbiting debris and possibly planets have scoured out the inner system. AU Mic thus becomes an interesting laboratory in the study of planetary formation. And it may help us solve a key question, for the most recent Hubble observations show that the dust surrounding the star is fluffy, something like powdered snow.

That porosity is helpful because it tells us something about how these interstellar grains — microscopic mixtures of ice and rock — probably formed. Here’s James Graham (UC Berkeley) on the issue:

“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.”

This work, which was presented to the AAS in Seattle, shows that the dust around AU Mic is so porous that it is more than 90 percent vacuum. That ‘gentle agglomeration’ Graham refers to above evidently is the process that allows interstellar dust grains — about 100 nanometers in size — to grow into the larger grains the team is seeing.

For the grains in AU Mic’s debris disk are ten times as large as interstellar grains, about a micron across. Even so, that’s still small enough that these particles are readily blown away from the inner disk by the star’s stellar wind. And something is replenishing the disk, presumably collisions between still larger bodies the size of softballs as the ‘birth ring’ around AU Mic gradually clears itself out.

“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.”

Centauri Dreams‘ take: As we expand our exoplanetary observations, we’ll be able to sample star after star in different stages of planetary formation. This one offers the significant benefit that it gives us clues as to how primordial materials begin to grow into larger ones in the first place. As the paper puts it: “AU Mic may exhibit the signature of the primordial agglomeration process whereby interstellar grains first assembled to form macroscopic objects.”

The paper is Graham et al., “The Signature of Primordial Grain Growth in the Polarized Light of the AU Microscopii Debris Disk,” Astrophysical Journal Vol. 654, pp. 580-594 (1 January 2007), with abstract here.