Planetesimal formation in an evolving protoplanetary disc: Effects of evaporation cooling from water ice inside the snow line

Abstract

Considering the evaporation cooling of icy particles, we propose a mechanism for planetesimal formation in an evolving protoplanetary disc. Because of gas drag, icy particles undergo orbital decay. As these particles cross the snow line, they begin to evaporate. When they move to an enough hot region, a positive feedback process will begin near the evaporation front. Because of rapid evaporation near the front, particles rapidly cool the surrounding gas. The disc structure near the front is significantly changed after enough particles evaporate there. A prominent feature is the pressure gradient becomes shallow. Subsequent inward moving particles will pile up near the front. As they evaporate, the pressure gradient becomes shallower and a pressure maximum forms finally. Particles will pile up there further. When there are enough particles near the evaporation front, planetesimals can form through gravitational instability. In the calculation, we consider the total gas disc evolution, the water vapour disc evolution, the icy particle disc evolution, and interactions among the three discs. It is shown that, for an icy particle disc initially made up of metre-sized particles, in ~102 yr, the particle density near the evaporation front will become high enough to form planetesimals, while only a small fraction of the icy particle disc mass is consumed. In addition, we study the effects of initial particle radius, the water-to-gas ratio, the disc viscosity, and the initial position of the snow line.