Solving for the 2D Water Snowline with Hydrodynamic Simulations. Emergence of gas outflow, water cycle and temperature plateau (2502.08936v1)

Abstract: In protoplanetary disks, the water snowline marks the location where 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 forming planetesimals. However, previous studies exploring the pile-up conditions typically employ 1D, vertically-averaged and isothermal assumptions. In this work, we investigate how a 2D flow pattern and realistic temperature structure affect the pile-up of pebbles at the snowline and how latent heat effects can leave observational imprints. We perform 2D (R-Z) multifluid hydrodynamic simulations, tracking chemically heterogeneous pebbles and the released vapor. With a recent-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 under various opacities and stellar luminosity. 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 active disks, we 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 promotes ice-trapping at snowline. Latent heat exchange flattens the temperature gradient across the snowline, broadening the width while reducing the peak solid-to-gas ratio of pile-ups. Due to the water cycle, active disks are more conducive to planetesimal formation than passive disks. The significant temperature dip (~ 40K) caused by latent heat cooling manifests as an intensity dip in the dust continuum, presenting a new channel to identify the water snowline in outbursting systems.

• The paper employs 2D hydrodynamic simulations with a phase change module to solve for the water snowline, revealing how vapor outflow, a water cycle, and temperature plateaus emerge.
• Vapor outflow from the snowline significantly enhances ice pile-up outside the snowline through diffusion and advection, a previously unmodeled 2D effect.
• Latent heat flattens temperature gradients near the snowline, broadening pile-up extent and decreasing peak solid ratios.

Solving for the 2D Water Snowline with Hydrodynamic Simulations

The paper investigates the intricate dynamics surrounding the water snowline in protoplanetary disks through 2D hydrodynamic simulations, emphasizing the interplay between snowline morphologies, vapor transport processes, and latent heat effects. The motivation behind this research lies in understanding the localized enhancement of solids at the snowline, a region pivotal for planetesimal formation. Traditional studies have primarily relied on vertically-averaged, one-dimensional models which may overlook significant vertical dynamical effects. This paper endeavors to elucidate how a comprehensive two-dimensional analysis can reveal more about the snowline's influence on pebble accumulation and solid pile-ups.

Methodology

The paper employs multifluid hydrodynamic simulations in the radial-vertical plane using Athena++, employing the novel phase change module to capture mass transfer and latent heat exchange during pebble sublimation. This approach allows the authors to track chemically heterogeneous pebbles along with the released vapor. The paper distinguishes between (1) passive disks, dominated by stellar irradiation, resulting in warm upper layers and cooler midplanes, (2) active disks, where viscous heating results in hotter midplanes, and (3) vertically-isothermal disks with uniform vertical temperature distribution due to efficient radiative cooling. The radiative model is sophisticatedly developed using a two-stream radiation transfer method.

Results

Snowline Morphology and Gas Dynamics:

The simulations reveal that vapor released at the snowline can drive a previously unaccounted outflow in the two-dimensional flow mechanics. This outflow enhances ice pile-up outside the snowline through a combination of outward vapor diffusion and advection. The results indicate that the midplane water-cycle in active disks traps a higher mass of ice due to strong vertical temperature gradients, promoting effective recondensation on pebbles. The midplane headwind is significantly reduced due to vapor injection, which may lower the threshold for streaming instabilities conducive to planetesimal formation.

Latent Heat Effects:

The latent heat of sublimation significantly impacts the temperature profile across the snowline, particularly in active disks. Latent heat absorption flattens the radial temperature gradient, broadening the pile-up extent while decreasing the peak solid-to-gas ratios. On the other hand, the effect in passive and vertically-isothermal disks is less pronounced due to the higher optical thickness or a strong stellar irradiation component.

Implications

Planetesimal Formation:

The paper identifies the water snowline as a favorable site for planetesimal formation due to the high solids-to-gas ratios achieved through vapor diffusion and midplane outflow. Although the obtained peak ratio of solids to gas does not inherently reach the unity necessary for streaming instabilities, the findings suggest that adding the backreaction from pebble dynamics could significantly enhance these ratios.

Observational Relevance:

The thermodynamic conditions found around snowlines, including the latent heat-induced temperature plateau, can produce observable signatures such as dust continuum intensity dips, particularly in systems experiencing accretion outbursts. These thermally altered regions can potentially be identified in young stellar objects or outbursting systems like FU Ori-type stars.

Future Directions

The outcomes of this research suggest further exploration into the variability of parameters such as pebble sizes, ice-to-gas flux ratios, and multi-phase opacities, which could alter snowline characteristics. Additionally, integrating MHD effects could more accurately capture the vertical temperature gradient influences and accretion dynamics, potentially refining predictions on planetesimal composition and formation scales. Understanding the 3D dynamics and considering non-steady states in evolving disks could also provide a more comprehensive understanding of snowline behavior.

This paper represents an essential step in unraveling the complexities inherent in protoplanetary disk chemistry and dynamics, offering new insights into the underlying processes influencing planet formation.