Disks in close binary stars: \gamma-Cephei revisited

TL;DR

This study uses high-resolution radiative hydrodynamical simulations to analyze the dynamics of disks in close binary systems, revealing their eccentricity, precession behavior, and sensitivity to physical parameters, which informs planet formation theories.

Contribution

It provides a detailed, robust analysis of disk dynamics in close binaries using improved simulation techniques and explores how physical parameters influence disk behavior.

Findings

Disks become eccentric with mean eccentricities between 0.06 and 0.27.

Disks exhibit slow retrograde precession with periods of 4-40 times the binary period.

Disk eccentricity is affected by opacity laws and physical parameters.

Abstract

Close binaries ($a_{bin} < 20$ au) are known to harbor planets, yet planet formation is unlikely to succeed in such systems. Studying the dynamics of disks in close binaries can help to understand how those planets could have formed. We study the impact that numerical and physical parameters have on the dynamics of disks in close binaries. We use the $\gamma$-Cephei system as an example and focus on disk quantities such as disk eccentricity and the precession rate as indicators for the dynamical state of the disks. We simulate disks in close binaries by performing two-dimensional radiative hydrodynamical simulations using a modified version of the Fargo code. First, we perform a parameter study for different numerical parameters to confirm that our results are robust. In the second part, we study the effects of different masses and different viscosities on the disks' dynamics. Previous studies on radiative disks in close binaries used too low resolutions and too small simulation domains, which impacted the disk's dynamics. We find that radiative disks in close binaries, after an initialization phase, become eccentric with mean eccentricities between 0.06 to 0.27 and display a slow retrograde precession with periods ranging from $4 - 40T_{bin}$ which depends quadratically on the disk's mean aspect ratio. In general, the disks show a coherent, rigid precession which can be broken, however, by changes in the opacity law reducing the overall eccentricity of the disk.