PROTOPLANETARY DUST CLOUDS IN DISKS OF HERBIG AE BE STARS/

The very large brightness decrease of late-type Herbig Ae/Be stars is believed to be caused by obscuring dust clouds orbiting in the outer p arts of their circumstellar disks. The distances of the dust clouds to the central stars have been estimated using the wavelength at maximum flux of the excess near-IR radiation, Wien's displacement law, and a formula derived by Rowan-Robinson (1980). The critical masses of these clouds were calculated employing Chandrasekhar's (1943) formula. The minimum size of the dust grains in the obscuring clouds was estimated using Aumann et al.'s (1984) formula they had applied to the star alph a Lyr. However, it can be about ten times smaller if the dust grains a re situated at the back of the cloud. The average size of these grains has been determined by assuming a size distribution similar to that i n the asteroidal belt (Dohnanyi, 1969) and in the interstellar medium (Mathis et al., 1977). Their number density was determined by means of the extinction power of the dust cloud at the V pass-band. The result s of our calculations show that above parameters are similar to those in our solar system. Therefore, we believe that most probably (a) the formation of planetesimals in the circumstellar disks of Herbig Ae/Be stars is on-going; and (b) the obscuring clouds will, in the long run, become planet-like objects.