The fate of planetesimals in turbulent disks with deadzones; constraints on planet formation

Chris Ormel

University of California, Berkeley, USA

Turbulence in protoplanetary nebulae affects planet formation in many ways. While small dust particles are mainly affected by the aerodynamical coupling with the turbulent gas velocity field, planetesimals and larger bodies are excited to a high eccentricity by gravitational interactions with turbulence-induced density fluctuations in the gas. This has several important implications. Firstly, planetesimals may not survive their mutual high-velocity collision. It has been advocated, therefore, that planet formation requires a dead zone, which suppresses the level of the turbulent activity in the nebular midplane. However, even in the presence of a collision-friendly dead zone there is another, more formidable constraint that turbulence imposes on planet formation: the condition for the onset of runaway growth accretion. Runaway growth requires that the random velocity (eccentricity) of planetesimals should fall below their escape velocity. This condition can be translated in terms of a minimum size (s_run) which planetesimals should have (see accompanying figure). A large s_run is problematic as it renders the timescale for planet formation longer than the lifetime of the nebula. However, we find that s_run increases significantly when we account for dust coagulation, which reduces or even removes the dead zone. Altogether, our findings imply that the classical, planetesimal-dominated, model for planet formation is not viable in the outer regions of a turbulent disk. This conclusion implies that either the disk has to become laminar or that other accretion mechanisms operate.
An example of the relative velocity turbulence induces on two similar-size particles (black-gray dashed curve). Red curves are strength curves and the region between the red dots gives the size range where collisions may fragment bodies. The blue dot gives the size at which planetesimals can start their runaway growth.

S. Okuzumi, Tokyo IoT, Japan
Key publication:
Ormel & Okuzumi (2013), submitted