Undergraduate Honors Theses

Thesis Defended

Spring 2019

Document Type


Type of Thesis

Departmental Honors



First Advisor

Philip Armitage

Second Advisor

Jacob Simon


The streaming instability is a mechanism that produces regions of particle overdensity in protoplanetary disks. These over-densities gravitationally collapse to form planetesimals. Although it is well known that the extent of particle clumping is dependent on the radial gas pressure gradient, the relationship between pressure gradient and planetesimal properties is not known. We carry out very high resolution local, shearing box simulations (i.e., a small co-rotating patch) of a protoplanetary disk to study the effect of the radial pressure gradient on the streaming instability and resulting planetesimal properties. We find that the pre-collapse structure of particles grows increasingly axisymmetric with increasing pressure gradient, and for relatively small radial pressure gradients, smaller filaments form with a non-axisymmetric web-like structure. The initial mass distribution can be fit to a single power-law, where we measure a power-law index of p = 1.6 for every non-zero pressure gradient. An exponentially truncated power law provides a better fit; here, we find a power-law index of p’ = 1.3. We also find that the largest planetesimal masses have a weak, and possibly negligible, dependence on pressure gradient. This result rules out a cubic scaling of planetesimal mass with the pressure gradient, as suggested by linear theory. A simulation initialized with zero pressure gradient, which is not subject to the streaming instability, also yields a top-heavy mass function but with a noticeably different shape. These results point towards a initial planetesimal mass distribution that is at best very weakly dependent on the properties of the disk.