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We investigate the dust structure of gravitationally unstable disksundergoing mass accretion from the envelope envisioning the application toClass 0/I young stellar objects (YSOs) We find that the dust disk quicklysettles into a steady state and that, compared to a disk with interstellarmedium (ISM) dust-to-gas mass ratio and micron-sized dust, the dust mass in thesteady-state decreases by a factor of 1/2 to 1/3, and the dust thermal emissiondecreases by a factor of 1/3 to 1/5. The latter decrease is caused by dustdepletion and opacity decrease owing to dust growth. Our results suggest thatthe masses of gravitationally unstable disks in the Class 0/I YSOs areunderestimated by a factor of 1/3 to 1/5 when calculated from the dust thermalemission assuming an ISM dust-to-gas mass ratio and micron-sized dust opacity,and that a larger fraction of disks in Class 0/I YSOs is gravitationallyunstable than was previously believed. We also investigate the orbital radius$r_{\rm P}$ within which planetesimals form via coagulation of porous dustaggregates and show that $r_{\rm P}$ becomes $\sim 20$ AU for a gravitationallyunstable disk around a solar mass star. Because $r_{\rm P}$ increases as thegas surface density increases and a gravitationally unstable disk has a maximumgas surface density, $r_{\rm P}\sim 20$ AU is the theoretical maximum radius.We suggest that planetesimals formation in the Class 0/I phase is preferable tothat in the Class II phase because large gas surface density is expected andlarge amount of dust is supplied by envelope-to-disk accretion.

Apparent disk-mass reduction and planetesimal formation in gravitationally unstable disks in Class 0/I YSOs