Planet formation in stellar binaries I: planetesimal dynamics in massive protoplanetary disks

TL;DR

This paper analytically investigates how the gravity of eccentric protoplanetary disks influences planetesimal dynamics in binary star systems, revealing its critical role in collision velocities and planet formation processes.

Contribution

It provides the first analytical solutions for planetesimal eccentricity and relative velocities considering disk gravity, binary perturbations, and gas drag in eccentric disks.

Findings

Disk gravity significantly affects relative velocities of planetesimals.

Aligned disks lead to lower collision velocities compared to misaligned disks.

Small planetesimals (~1 km) are favored in high-velocity collisions.

Abstract

About $20\%$ of exoplanets discovered by radial velocity surveys reside in stellar binaries. To clarify their origin one has to understand the dynamics of planetesimals in protoplanetary disks within binaries. The standard description, accounting for only gas drag and gravity of the companion star has been challenged recently, as the gravity of the protoplanetary disk was shown to play a crucial role in planetesimal dynamics. An added complication is the tendency of protoplanetary disks in binaries to become eccentric, giving rise to additional excitation of planetesimal eccentricity. Here, for the first time, we analytically explore secular dynamics of planetesimals in binaries such as $\alpha$ Cen and $\gamma$ Cep under the combined action of (1) gravity of the eccentric protoplanetary disk, (2) perturbations due to the (coplanar) eccentric companion, and (3) gas drag. We derive explicit solutions for the behavior of planetesimal eccentricity ${\bf e}_p$ in non-precessing disks (and in precessing disks in certain limits). We obtain the analytical form of the distribution of relative velocities of planetesimals, which is a key input for understanding their collisional evolution. Disk gravity strongly influences relative velocities and tends to push sizes of planetesimals colliding with comparable objects at the highest speed to small values, $\sim 1$ km. We also find that planetesimals in eccentric protoplanetary disks apsidally aligned with the binary orbit collide at lower relative velocities than in mis-aligned disks. Our results highlight a decisive role that disk gravity plays in planetesimal dynamics in binaries.