High-spatial-resolution simulations of Be star disks in binary systems

I. Structure and kinematics of coplanar disks

¹ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85748 Garching b. München, Germany
² Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, Rua do Matão 1226, Cidade Universitária, 05508-900 São Paulo, SP, Brazil
³ European Organisation for Astronomical Research in the Southern Hemisphere (ESO), Karl-Schwarzschild-Str. 2, 85748 Garching b. München, Germany
⁴ Ritter Observatory, MS113, Department of Physics and Astronomy, University of Toledo, Toledo, OH 43606-3390, USA
⁵ Center for Development Policy Studies, Hokkai-Gakuen University, Toyohiraku, Sapporo, Hokkaido 062-8605, Japan
⁶ Department of Physics and Astronomy, Western University, London, ON N6A 3K7, Canada

^⋆ Corresponding author: amanda.rubio@usp.br, rubio@mpa-garching.mpg.de

Received: 23 October 2024
Accepted: 8 April 2025

Abstract

Context. In recent decades, binarity among O and B stars has been shown to be key in our understanding of their birth, evolution, and death. For the particular case of Be stars, binarity might be the cause for their spinup, which comprises a piece to the puzzling mechanism behind their mass outbursts, leading to the formation of their viscous decretion disks. Detecting companions in systems with Be stars can be challenging, making it difficult to obtain observational constraints on the binary fraction of Be stars.

Aims. We explore the effects of a binary companion in a system with a Be star, from the moment when the disk first begins to form until it reaches quasi steady-state. The tidal forces considerably affect the Be disk, leading to the formation of distinct regions in the system. These effects bring on observational consequences that can be used to infer the presence of a otherwise undetectable companion.

Methods. We used smoothed particle hydrodynamics (SPH) simulations of coplanar, circular binary systems with fixed Be parameters (star mass of 12.9 M_⊙, equatorial radius of 5.5 R_⊙, and effective temperature of 26 000 K). We also varied the orbital periods (30, 50 and 84 days), disk viscosities (α = 0.1, 0.5, and 1), and mass ratios (q = 0.16, 0.33, and 0.5). A high spatial resolution was achieved by adopting particle splitting in the SPH code, along with a more realistic description of the secondary star and the disk viscosity.

Results. With the upgraded code, we were able to probe a region approximately four times larger than previously possible. Our models show that the disk can be divided into five regions of interest: an inner Be disk, spiral dominated disk, and bridge, as well as the previously unseen circumsecondary and circumbinary regions. These features were revealed thanks to the increased resolution of the simulation. In this work, we describe the configuration and kinematics of each region and provide a summary of their expected observational signals. In all simulations, mass transfer takes place from the Be disk into the Roche lobe of the companion via the bridge. In other words, the disk is not sharply truncated at a given radius; rather, it suffers a strong decrease in density in a region spanning several stellar radii. This truncation region is azimuthally variable, elongated along the Roche potentials of the binary system. Material that enters the Roche lobe of the companion is partially captured by it, effectively forming a rotationally supported, disk-like structure. Part of the material is accreted by the companion in all simulations, but the expected X-ray emission of this accretion is faint. Material that does not get accreted then escapes the Roche lobe of the companion and forms a circumbinary, one-armed spiral around the system. This is the first work to describe the region beyond the truncation region of the Be disk and its observational consequences in detail.

Conclusions. All five regions are present for all models explored in this work. The effects of orbital period, viscosity, and mass ratio on the structure of Be binary systems are significant and have an anticipated impact the observables. Based on our models, 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 enable not only an improved interpretation of the existing data, but also aid in the planning of future observations.