Residence Times of Particles in Diffusive Protoplanetary Disk Environments I. Vertical Motions

TL;DR

This paper develops a method to analyze particle residence times and vertical trajectories in diffusive protoplanetary disks, revealing how different diffusivity profiles influence material evolution and chemical processes.

Contribution

A novel method for calculating particle paths in diffusive disks, applicable to both uniform and layered diffusivity profiles, linking dynamics with chemical evolution.

Findings

Particle paths vary significantly with diffusivity profiles.

Long-term evolution is independent of diffusivity form.

Different trajectories affect chemical and photochemical processes.

Abstract

The chemical and physical evolution of primitive materials in protoplanetary disks are determined by the types of environments they are exposed to and their residence times within each environment. Here a method for calculating representative paths of materials in diffusive protoplanetary disks is developed and applied to understanding how the vertical trajectories that particles take impact their overall evolution. The methods are general enough to be applied to disks with uniform diffusivity, the so-called "constant-$\alpha$" cases, and disks with a spatially varying diffusivity, such as expected in "layered-disks." The average long-term dynamical evolution of small particles and gaseous molecules is independent of the specific form of the diffusivity in that they spend comparable fractions of their lifetimes at different heights in the disk. However, the paths that individual particles and molecules take depend strongly on the form of the diffusivity leading to a different range of behavior of particles in terms of deviations from the mean. As temperatures, gas densities, chemical abundances, and photon fluxes will vary with height in protoplanetary disks, the different paths taken by primitive materials will lead to differences in their chemical and physical evolution. Examples of differences in gas phase chemistry and photochemistry are explored here. The methods outlined here provide a means of integrating particle dynamics with chemical models to understand the formation and evolution of primitive materials in protoplanetary disks over timescales of $10^5$-$10^6$ years.