Letter to the Editor

Embryos grown in the dead zone^*

Assembling the first protoplanetary cores in low mass self-gravitating circumstellar disks of gas and solids

¹ Department of Physics and Astronomy, Uppsala Astronomical Observatory, Box 515, 75120 Uppsala, Sweden e-mail: wlyra@astro.uu.se
² Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
³ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany

Received: 16 July 2008
Accepted: 10 October 2008

Abstract

Context. In the borders of the dead zones of protoplanetary disks, the inflow of gas produces a local density maximum that triggers the Rossby wave instability. The vortices that form are efficient in trapping solids.

Aims. We aim to assess the possibility of gravitational collapse of the solids within the Rossby vortices.

Methods. We perform global simulations of the dynamics of gas and solids in a low mass non-magnetized self-gravitating thin protoplanetary disk with the Pencil Code. We use multiple particle species of radius 1, 10, 30, and 100 cm. The dead zone is modelled as a region of low viscosity.

Results. The Rossby vortices excited in the edges of the dead zone are efficient particle traps. Within 5 orbits after their appearance, the solids achieve critical density and undergo gravitational collapse into Mars sized objects. The velocity dispersions are of the order of 10 m s^-1 for newly formed embryos, later lowering to less than 1 m s^-1 by drag force cooling. After 200 orbits, over 300 gravitationally bound embryos were formed, 20 of them being more massive than Mars. Their mass spectrum follows a power law of index -2.3 ± 0.2.