Image Credit: Holland, W. S., Greaves, J. S., Zuckerman, B., et al. 1998, "Submillimetre images of dusty debris around nearby stars", Nature, vol. 392, pg. 788 Copyright, please do not reproduce without permission from the authors.
Fomalhaut at 850 microns
HD 216956 HR 8728
alfa PsA HIP 113368
RA (2000) = 22 57 39.0465
Dec (2000) = -29 37 20.050
SpT = A3V
V =1.16 mag d = 7.688 pc
Proper Motion (mas/yr) = +329.22 -164.21
This image of Fomalhaut essentially traces the thermal emission of dust. Notice that the black star the center represents the location of the star. The white peaks to the northwest and southeast of the star represents the peaks of dust emission. If the dust disk is a torus (doughnut-shape) surrounding the star, and if the doughnut is oriented to our line of sight roughly 20 degrees away from edge-on, then the resulting image will have two peaks to either side of the star because this is where our line of sight intercepts the most dust through the disk.
The most thorough analysis of this sub-millimeter image is presented in Dent et al. (2000). The relatively sharp peak in the spectral energy distribution (SED) at 100 microns indicates that the dust surrounding Fomalhaut is constrained in radius (and therefore temperature). Their best fit to the photometric data from the optical to the millimeter gives a dust temperature of 40 K, a grain size of 100 microns, and a total dust mass of 1.4 lunar mass. The shape of the SED constrains the mass of small grains (10 microns) as no greater than 10 percent the mass of large grains (100 microns).
Dent et al. find that the dust must lie between 100 and 140 AU radius, with a sharp cut-off in the dust distribution at the outer edge. The inner hole is relatively empty, with dust density that is less than 10 percent of the dust density in the torus. Note that in the image above, the emission peaks appear roughly 9.5 arcseconds radius from the star, which corresponds to 73 AU radius. The difference between the model and the observed torus radii is due to the fact that the observation is controlled by the 13.8 arcsecond resolution of the SCUBA beam. The disk material is actually located between 13" and 18" radius, but the blurring of the detector makes the emission peaks appear closer to the star than they really are. The Dent et al. model also shows that the vertical extent of the dust is 16 arcsecond or 120 AU (full-width at half-maximum).
The Dent et al. (2000) constraints on dust size and location can be used to calculate the lifetime of grains. They find that Poynting-Robertson drag would destroy 100 micron sized grains at 100 AU in 200 Myr, but grain-grain collisions at 100 AU would destroy these grains in 200,000 yr. Therefore collisions are the most significant process that destroys grains. If the age of Fomalhaut is 200 Myr, then the 100 micron grains observed with SCUBA have been replenished 1000 times over. The source of replenishment is thought to be collisions of larger dust grains or planetesimals. These objects are a parent population of primordial bodies that can survive up to the present age of the star. Dent et al. estimate that objects larger than 1.1 cm in size replenish the observed 100 micron grains.