Planet formation in stellar binaries: global simulations of planetesimal growth

Kedron Silsbee,Roman R Rafikov

doi:10.1051/0004-6361/202141139

Abstract

Planet formation around one component of a tight, eccentric binary system such as γ Cephei (with semimajor axis around 20 AU) is theoretically challenging because of destructive high-velocity collisions between planetesimals. Despite this fragmentation barrier, planets are known to exist in such (so-called S-type) orbital configurations. Here we present a novel numerical framework for carrying out multi-annulus coagulation-fragmentation calculations of planetesimal growth, which fully accounts for the specifics of planetesimal dynamics in binaries, details of planetesimal collision outcomes, and the radial transport of solids in the disk due to the gas drag-driven inspiral. Our dynamical inputs properly incorporate the gravitational effects of both the eccentric stellar companion and the massive non-axisymmetric protoplanetary disk in which planetesimals reside, as well as gas drag. We identify a set of disk parameters that lead to successful planetesimal growth in systems such as γ Cephei or α Centauri starting from 1 to 10 km size objects. We identify the apsidal alignment of a protoplanetary disk with the binary orbit as one of the critical conditions for successful planetesimal growth: It naturally leads to the emergence of a dynamically quiet location in the disk (as long as the disk eccentricity is of order several percent), where favorable conditions for planetesimal growth exist. Accounting for the gravitational effect of a protoplanetary disk plays a key role in arriving at this conclusion, in agreement with our previous results. These findings lend support to the streaming instability as the mechanism of planetesimal formation. They provide important insights for theories of planet formation around both binary and single stars, as well as for the hydrodynamic simulations of protoplanetary disks in binaries (for which we identify a set of key diagnostics to verify).

Highlights

Planets have been discovered in a wide variety of stellar systems, including stellar binaries and systems of higher multiplicity (Marzari & Thebault 2019)
We run calculations in which we artificially turn off disk gravity, and vary the inspiral rate, to explore the role played by these physical processes in determining the outcome of planetesimal evolution
Studies that attempt to represent disk thermodynamics in a more realistic way (Marzari et al 2012; Gyergyovits et al 2014) find a dramatic reduction of disk eccentricity6, down to ed ∼ 0.02–0.05. Disk eccentricity at this level would allow planetesimal growth to proceed successfully starting from 6 Another work accounting for radiative effects by Picogna & Marzari (2013) is very different from the rest – it is a 3D Smoothed Particle Hydrodynamics (SPH) study – and found high disk eccentricity

Summary

Introduction

Planets have been discovered in a wide variety of stellar systems, including stellar binaries and systems of higher multiplicity (Marzari & Thebault 2019). A simple calculation (Heppenheimer 1978), including only the dominant secular effects of the stellar companion on planetesimal orbits, yields planetesimal collision velocities of a few km s−1 at the current location of the planet in the γ Cephei system (around 2 AU from the stellar primary). This is enough to destroy even planetesimals of hundreds of kilometers in size in catastrophic

Results

Discussion

Conclusion