The Effect of the Radial Pressure Gradient in Protoplanetary Disks on Planetesimal Formation

TL;DR

This study investigates how the radial pressure gradient influences particle clumping via streaming instability in protoplanetary disks, revealing that smaller gradients promote clumping and planetesimal formation.

Contribution

The paper demonstrates that the radial pressure gradient magnitude significantly affects particle clumping, with smaller gradients favoring planetesimal formation, based on hybrid numerical simulations.

Findings

Smaller radial pressure gradients promote particle clumping.

Critical solid abundance increases with the pressure gradient.

Clumping occurs more readily for smaller particles at lower gradients.

Abstract

The streaming instability (SI) provides a promising mechanism for planetesimal formation because of its ability to concentrate solids into dense clumps. The degree of clumping strongly depends on the height-integrated solid to gas mass ratio Z in protoplanetary disks (PPDs). In this letter, we show that the magnitude of the radial pressure gradient (RPG) which drives the SI (characterized by $q\equiv{\eta}v_K/c_s$, where ${\eta}v_K$ is the reduction of Keplerian velocity due to the RPG and $c_s$ is the sound speed) also strongly affects clumping. We present local two-dimensional hybrid numerical simulations of aerodynamically coupled particles and gas in the midplane of PPDs. Magnetic fields and particle self-gravity are ignored. We explore three different RPG values appropriate for typical PPDs: $q=0.025, 0.05$ and 0.1. For each $q$ value, we consider four different particle size distributions ranging from sub millimeter to meter sizes and run simulations with solid abundance from Z=0.01 up to Z=0.07. We find that a small RPG strongly promotes particle clumping in that: 1) At fixed particle size distribution, the critical solid abundance $Z_{\rm crit}$ above which particle clumping occurs monotonically increases with $q$; 2) At fixed Z, strong clumping can occur for smaller particles when $q$ is smaller. Therefore, we expect planetesimals to form preferentially in regions of PPDs with a small RPG.