Solving for the 2D water snowline with hydrodynamic simulations: Emergence of the gas outflow, water cycle, and temperature plateau

Context. In protoplanetary disks, the water snowline marks the location where inwardly drifting, ice-rich pebbles sublimate, releasing silicate grains and water vapor. These processes can trigger pile-ups of solids, making the water snowline a promising site for the formation of planetesimals, for instance, via streaming instabilities. However, previous studies exploring the dust pile-up conditions have typically employed 1D, vertically averaged, and isothermal assumptions. Aims. In this work, we investigate how the 2D flow pattern and a realistic temperature structure affect the accumulation of pebbles at the snowline. Furthermore, we explore how latent heat imprints snowline observations. Methods. We performed 2D multifluid hydrodynamic simulations in the disk's radial-vertical plane with Athena++, tracking chemically heterogeneous pebbles and the released vapor. With a recently-developed phase change module, the mass transfer and latent heat exchange during ice sublimation are calculated self-consistently. The temperature is calculated by a two-stream radiation transfer method with various opacities and stellar luminosity. Results. We find that vapor injection at the snowline drives a previously unrecognized outflow, leading to a pile-up of ice outside the snowline. Vapor injection also decreases the headwind velocity in the pile-up, promoting planetesimal formation and pebble accretion. In actively heated disks, we are able to identify a water cycle: after ice sublimates in the hotter midplane, vapor recondenses onto pebbles in the upper, cooler layers, which settle back to the midplane. This cycle enhances the trapped ice mass in the pile-up region. Latent heat exchange flattens the temperature gradient across the snowline, broadening the width, while reducing the peak solid-to-gas ratio of pile-ups. Conclusions. Due to the water cycle, active disks are more conducive to planetesimal formation than passive disks. The significant temperature dip (up to 40 K) caused by latent heat cooling is manifested as an intensity dip in the dust continuum, presenting a new channel for identifying the water snowline in outbursting systems.