AU Microscopii - Debris Disk

Debris Disk

AU Microscopii harbors its own disk of dust, first resolved at optical wavelengths in 2003 by Paul Kalas and collaborators using the University of Hawaii 2.2-m telescope on Mauna Kea, Hawaii. This large debris disk faces the earth edge-on, and measures at least 200 AU in radius. At these large distances from the star, the lifetime of dust in the disk exceeds the age of AU Microscopii. The disk has a gas to dust mass ratio of no more than 6:1, much lower than the usually assumed primordial value of 100:1. The debris disk is therefore referred to as "gas-poor". The total amount of dust visible in the disk is estimated to be at least a lunar mass, while the larger planetesimals from which the dust is produced are inferred to has at least six lunar masses.

The spectral energy distribution of AU Microscopii's debris disk at submillimetre wavelengths indicate the presence of an inner hole in the disk extending to 17 AU, while scattered light images estimate the inner hole to be 12 AU in radius. Combining the spectral energy distribution with the surface brightness profile yields a smaller estimate of the radius of the inner hole, 1 - 10 AU.

The inner part of the disk is asymmetric and shows structure in the inner 40 AU. The inner structure has been compared with that expected to be seen if the disk is influenced by larger bodies or has undergone recent planet formation.

The presence of the inner hole and asymmetric structure has led a number of astronomers to search for planets orbiting AU Microscopii. By 2007, no searches had led to any detections of planets.

The surface brightness (brightness per area) of the disk as a function of projected distance from the star follows a characteristic shape. The inner 15 AU of the disk appear approximately constant in density. Around the density begins to decrease: first it decreases slowly as where ; then outside, the brightness drops more steeply, as where . This "broken power-law" shape is similar to the shape of the profile of β Pic's disk.