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

TL;DR

This study reveals that dust mass and thermal emission in gravitationally unstable disks around Class 0/I YSOs are significantly underestimated, impacting the understanding of disk stability and planetesimal formation regions.

Contribution

It demonstrates dust depletion effects in unstable disks and estimates the maximum radius for planetesimal formation during early stellar evolution phases.

Findings

Dust mass decreases by a factor of 1/2 to 1/3 in steady state.

Thermal emission decreases by a factor of 1/3 to 1/5 due to dust growth.

Planetesimal formation radius is about 20 AU, representing a maximum limit.

Abstract

We investigate the dust structure of gravitationally unstable disks undergoing mass accretion from the envelope envisioning the application to Class 0/I young stellar objects (YSOs) We find that the dust disk quickly settles into a steady state and that, compared to a disk with interstellar medium (ISM) dust-to-gas mass ratio and micron-sized dust, the dust mass in the steady-state decreases by a factor of 1/2 to 1/3, and the dust thermal emission decreases by a factor of 1/3 to 1/5. The latter decrease is caused by dust depletion and opacity decrease owing to dust growth. Our results suggest that the masses of gravitationally unstable disks in the Class 0/I YSOs are underestimated by a factor of 1/3 to 1/5 when 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 disks in Class 0/I YSOs is gravitationally unstable than was previously believed. We also investigate the orbital radius $r_{\rm P}$ within which planetesimals form via coagulation of porous dust aggregates and show that $r_{\rm P}$ becomes $\sim 20$ AU for a gravitationally unstable disk around a solar mass star. Because $r_{\rm P}$ increases as the gas surface density increases and a gravitationally unstable disk has a maximum gas 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 to that in the Class II phase because large gas surface density is expected and large amount of dust is supplied by envelope-to-disk accretion.