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 _ { 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 .