High-spatial-resolution simulations of Be star disks in binary systems: I. Structure and kinematics of coplanar disks

TL;DR

This study uses high-resolution SPH simulations to analyze the structure and kinematics of Be star disks in binary systems, revealing how tidal forces and material flows create observable features that indicate the presence of companions.

Contribution

It provides the first detailed description of the region beyond the Be disk truncation zone and its observational signatures in coplanar binary systems.

Findings

Tidal forces create distinct regions in Be star disks with observable consequences.

Material entering the Roche lobe forms a disk-like structure around the companion.

Circumbinary disks form from material not accreted by the companion.

Abstract

Binarity in massive stars has proven to be an important aspect in the their evolution. For Be stars, it might be the cause of their spin up, and thus part of the mechanism behind the formation of their viscous decretion disks. Detecting companions in systems with Be stars is challenging, making it difficult to obtain observational constraints on their binary fraction. We explore the effects of a binary companion in a system with a Be star, from disk formation to quasi steady-state using smoothed particle hydrodynamics (SPH) simulations of coplanar, circular binary systems. High spatial resolution is achieved by adopting particle splitting in the SPH code, as well as a more realistic description of the secondary star and the disk viscosity. The tidal forces considerably affect the Be disk, forming distinct regions in the system, with observational consequences that can be used to infer the presence of a otherwise undetectable companion. With the upgraded code, we can probe a region approximately 4 times larger than previously possible. We describe the configuration and kinematics of each part of the system, and provide a summary of their expected observational signals. Material that enters the Roche lobe of the companion is partially captured by it, forming a rotationally supported, disk-like structure. Material not accreted escapes and forms a circumbinary disk around the system. This is the first work to describe the region beyond the truncation region of the Be disk and its observational consequences with detail. We argue that observational features of previously unclear origin, such as the intermittent shell features and emission features of HR 2142 and HD 55606, originate in areas beyond the truncation region. This new understanding of the behavior of disks in Be binaries will allow not just for better interpretation of existing data, but also for the planning of future observations.