Turbulence Regulates the Rate of Planetesimal Formation via Gravitational Collapse [EPA]

http://arxiv.org/abs/2001.10000


We study how the interaction between the streaming instability and intrinsic gas-phase turbulence affects planetesimal formation via gravitational collapse. Turbulence impedes the formation of dense particle clumps by acting as an effective turbulent diffusivity, but it can also promote planetesimal formation by concentrating solids, for example in zonal flows. We quantify the effect of turbulent diffusivity using numerical simulations of the streaming instability in small local domains, forced with velocity perturbations that establish approximately Kolmogorov-like fluid turbulence. We find that planetesimal formation is strongly suppressed by turbulence once velocity fluctuations exceed a threshold value of $\delta v^2 \simeq 10^{-3.5} – 10^{-3} c_s^2$. Turbulence whose strength is just below the threshold reduces the rate of solid material being converted into bound clumps. The main effect of turbulence is to thicken the mid-plane solid layer. Our results are thus consistent with planetesimal formation requiring a mid-plane solid-to-gas ratio $\epsilon \gtrsim 0.5$. We describe a method for tracking bound clumps in our simulations, and use this method to construct a mass function of planetesimals that is measured for each clump shortly after its collapse. Adopting this definition of the initial mass, instead of measuring masses at a fixed time, reduces planetesimal masses by a factor of three. For models in which planetesimals form, we show that the initial mass function is well-described by a broken power law, whose parameters are robust to the inclusion and strength of imposed turbulence. Turbulence in protoplanetary disks is likely to substantially exceed the threshold for planetesimal formation at radii where temperatures $T \gtrsim 10^3 \ {\rm K}$ lead to thermal ionization. Fully in situ planetesimal and planet formation may therefore not be viable for the closest-in exoplanets.

Read this paper on arXiv…

D. Gole, J. Simon, R. Li, et. al.
Wed, 29 Jan 20
46/46

Comments: Submitted to ApJ, Comments welcome