Attenuation of Millimeter Emission from Circumstellar Disks Induced by the Rapid Dust Accretion

From millimeter observations of classical T Tauri stars, it is suggested thatdust grains in circumstellar disks have grown to millimeter size or larger.However, gas drag on such large grains induces rapid accretion of the dust. Weexamine the evolution of dust disks composed of millimeter sized grains, andshow that rapid accretion of the dust disk causes attenuation of millimetercontinuum emission. If a dust disk is composed mainly of grains of 1 cm to 1 m,its millimeter emission goes off within 10^6 yr. Hence, grains in this sizerange cannot be a main population of the dust. Considering our results togetherwith grain growth suggested by the millimeter continuum observations, we expectthat the millimeter continuum emission of disks comes mainly from grains in anarrow size range of [1 mm -1 cm]. This suggests either that growth ofmillimeter sized grains to centimeter size takes more than 10^6 yr, or thatmillimeter sized grains are continuously replenished. In the former case,planet formation is probably difficult, especially in the outer disks. In thelatter case, reservoirs of millimeter grains are possibly large (> 10 m)bodies, which can reside in the disk more than 10^6 yr. Constraints on thegrain growth time-scale are discussed for the above two cases.Comment: Accepted by ApJ, 17 pages, 15 figure