A comet dust model for the β Pictoris disk

The 10μm silicate emission feature and the continuum emission from near infrared to millimeter of the dust in the disk of β Pictoris may be derived by assuming that the dust is continually replenished by comets orbiting close to the star. The basic, initial dust shed by the comets is taken to be the fluffy aggregates of interstellar silicate core-organic refractory mantle dust grains (with an additional ice mantle in the outer region of the disk). The heating of the dust is primarily provided by the organic refractory mantle absorption of the stellar radiation. The temperature of some of the particles close to the star is sufficient to crystallize the initially amorphous silicates. The dust grains are then distributed throughout the disk by radiation pressure. The steady state dust distribution of the disk then consists of a mixture of crystalline silicate aggregates and aggregates of amorphous silicate core-organic refractory mantle particles (without/with ice mantles) with variable ratios of organic refractory to silicate mass. The whole disk which extends inward to ∼ 1AU and outward to ∼ 2200AU is divided into three components which are primarily responsible respectively, for the silicate emission, the mid-infrared emission and the far infrared/millimeter emission. As a starting point, the grain size distribution is assumed to be like that observed for comet Halley dust while in the inner regions the distribution of small particles is relatively enhancedwhichmaybe attributed to the evaporation and/or fragmentation of large fluffy particles. The dust grains which best reproduce the observations are highly porous, with a porosity around 0.95 or as high as 0.975. The temperature distribution of a radial distribution of such particles provides an excellent match to the silicate 10μm (plus 11.2μm) spectral emission as well as the excess continuum flux from the disk over a wide range of wavelengths. These models result in a total mass of dust in the whole disk∼ 2× 1027 g of which only 10−5 – 10−4 is hot enough to give the silicate excess emission. The specific mineralogy of crystalline silicates has been discussed.