Apparent disk-mass reduction and planetesimal formation in gravitationally unstable disks in Class 0/I YSOsAuthors:Tsukamoto et alAbstract:We investigate the dust structure of gravitationally unstable gas disks undergoing mass accretion from the envelope, envisioning application to Class 0/I YSOs. We compute evolution of the surface density and dust size by taking into account dust collisional growth and radial drift. We find that the dust disk quickly settles into the steady state and the dust mass in the steady-state disk decreases by a factor of 1/2 to 1/3, while the radiative flux of the dust thermal emission also decreases by a factor of 1/3 to 1/5, both compared to that for a disk with ISM dust-to-gas mass ratio and micron-sized dust. We suggest that the disk mass in the Class 0/I YSOs is underestimated by factor of 1/3 to 1/5 when it is calculated from the dust thermal emission assuming an ISM dust-to-gas mass ratio and micron-sized dust opacity, and that a larger fraction of the disks in Class 0/I YSOs is gravitationally unstable than previously considered. We derive an empirical formula for the disk-mass reduction rate, which can be used to test whether or not the disks observed in Class 0/I YSOs are gravitationally unstable. We also investigate the orbital radius of planetesimal formation rP and show that rP becomes ∼20 AU. Because rP increases as the gas surface density increases and a gravitationally unstable disk has a theoretical maximum gas surface density, rP∼20 AU is the theoretical maximum radius for planetesimal formation. We find that the dust particles migrate inwardly in the form of "pebble", and we suggest that planet formation via pebble accretion in Class 0/I phase is preferable to that in the Class II phase because the dust is supplied by envelope accretion and a significant amount of dust particles (Mdust∼10−2M⊙) pass through the disk during the Class 0/I phase.