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

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

2010-05-27

arxiv.org/abs/1005.4981v1

Astronomy and Astrophysics

Astrophysics

Earth and Planetary Astrophysics

10 pages, submitted to ApJL

Scientific paper

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.

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