A COMET DUST MODEL FOR THE BETA-PICTORIS DISK

The 10 mu m silicate emission feature and the continuum emission from near infrared to millimeter of the dust in the disk of beta Pictoris m ay 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 man tle in the outer region of the disk). The heating of the dust is prima rily provided by the organic refractory mantle absorption of the stell ar radiation. The temperature of some of the particles close to the st ar is sufficient to crystallize the initially amorphous silicates. The dust grains are then distributed throughout the disk by radiation pre ssure. The steady state dust distribution of the disk then consists of a mixture of crystalline silicate aggregates and aggregates of amorph ous silicate core-organic refractory mantle particles (without/with ic e mantles) with variable ratios of organic refractory to silicate mass . The whole disk which extends inward to similar to 1 AU and outward t o similar to 2200 AU is divided into three components which are primar ily responsible respectively, for the silicate emission, the mid-infra red emission and the far infrared/millimeter emission. As a starting p oint, the grain size distribution is assumed to be like that observed for comet Halley dust while in the inner regions the distribution of s mall particles is relatively enhanced which may be attributed to the e vaporation and/or fragmentation of large fluffy particles. The dust gr ains which best reproduce the observations are highly porous, with a p orosity 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 10 mu m (plus 11.2 mu m) spectral emission as well as the excess continuum flux from the disk over a wide range of waveleng ths. These models result in a total mass of dust in the whole disk sim ilar to 2 x 10(27) g of which only 10(-5) - 10(-4) is hot enough to gi ve the silicate excess emission. The specific mineralogy of crystallin e silicates has been discussed.