Spiral Density Enhancements in Be Binary Systems

TL;DR

This study uses SPH simulations to analyze how binary companions influence the spiral density structures in Be star disks, predicting observable interferometric signatures and their dependence on system parameters.

Contribution

It provides new insights into the formation and observational detection of spiral density enhancements caused by binary companions in Be star disks, considering various viscosities and misalignment angles.

Findings

Larger misalignment angles produce more tightly wound spiral arms.

Spiral features can be detected via interferometry in Hα and K-band.

Disk density enhancements influence observed axis ratios depending on orientation.

Abstract

We use a smoothed particle hydrodynamics (SPH) code to examine the effects of a binary companion on a Be star disk for a range of disk viscosities and misalignment angles, i.e. the angle between the orbital plane and the primary's spin axis. The density structures in the disk due to the tidal interaction with the binary companion are investigated. Expanding on our previous work, the shape and density structure of density enhancements due to the binary companion are analyzed and the changes in observed interferometric features due to these orbiting enhancements are also predicted. We find that larger misalignment angles and viscosity values result in more tightly wound spiral arms with densities that fall-off more slowly with radial distance from the central star. We show that the orbital phase has very little effect on the structure of the spiral density enhancements. We demonstrate that these spiral features can be detected with an interferometer in H$\alpha$ and K-band emission. We also show that the spiral features affect the axis ratios determined by interferometry depending on the orientation of these features and the observer. For example, our simulations show that the axis ratios can vary by 20% for our co-planar binary disk system depending on the location of the disk density enhancements.