Impacts of planet migration models on planetary populations

Effects of saturation, cooling and stellar irradiation^⋆

¹ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
e-mail: dittkrist@mpia.de
² Physikalisches Institut, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
³ Institut UTINAM, CNRS-UMR 6213, Observatoire de Besançon, BP 1615, 25010 Besançon Cedex, France

Received: 19 August 2013
Accepted: 16 February 2014

Abstract

Context. Several recent studies have found that planet migration in adiabatic disks differs significantly from migration in isothermal disks. Depending on the thermodynamic conditions, that is, the effectiveness of radiative cooling, and on the radial surface density profile, planets migrate inward or outward. Clearly, this will influence the semimajor-axis-to-mass distribution of planets predicted by population-synthesis simulations.

Aims. Our goal is to study the global effects of radiative cooling, viscous torque desaturation, gap opening, and stellar irradiation on the tidal migration of a synthetic planet population.

Methods. We combined results from several analytical studies and 3D hydrodynamic simulations in a new semi-analytical migration model for the application in our planet population synthesis calculations.

Results. We find a good agreement of our model with torques obtained in 3D radiative hydrodynamic simulations. A typical disk has three convergence zones to which migrating planets move from the in- and outside. This strongly affects the migration behavior of low-mass planets. Interestingly, this leads to a slow type II like migration behavior for low-mass planets captured in these zones even without an ad hoc migration rate reduction factor or a yet-to-be-defined halting mechanism. This means that the new prescription of migration that includes nonisothermal effects makes the previously widely used artificial migration rate reduction factor obsolete.

Conclusions. Outward migration in parts of a disk helps some planets to survive long enough to become massive. The convergence zones lead to potentially observable accumulations of low-mass planets at certain semimajor axes. Our results indicate that more studies of the mass at which the corotation torque saturates are needed since its value has a main impact on the properties of planet populations.