Evolution of the disk in the Be binary $\delta$ Scorpii probed during three periastron passages

TL;DR

This study investigates how the circumstellar disk of the Be star in the eccentric binary system δ Scorpii evolves during three periastron passages using observations and modeling, revealing increasing asymmetries and transient perturbations.

Contribution

It provides a detailed observational and modeling analysis of disk evolution in δ Scorpii during periastron passages, highlighting the transient nature of perturbations and the impact of orbital dynamics.

Findings

Hα emitting region expanded after each periastron

Disk asymmetries became more pronounced over time

Perturbations propagated inward near periastron

Abstract

We examine the evolution of the disk surrounding the Be star in the highly eccentric binary system $\delta$ Scorpii over its three most recent periastron passages. $V$-band and $B-V$ photometry, along with H$\alpha$ spectroscopy are combined with a new set of extensive multi-band polarimetry data to produce a detailed comparison of the disk's physical conditions during the time periods surrounding each closest approach of the secondary star. We use the three-dimensional Monte Carlo radiative transfer code \textsc{HDUST} and smoothed particle hydrodynamics (\textsc{SPH}) code to support our observations with models of disk evolution, discussing the behaviour of the H$\alpha$ and He\,\textsc{i} 6678 lines, $V$-band magnitude, and polarization degree. We compare the characteristics of the disk immediately before each periastron passage to create a baseline for the unperturbed disk. We find that the extent of the H$\alpha$ emitting region increased between each periastron passage, and that transient asymmetries in the disk become more pronounced with each successive encounter. Asymmetries of the H$\alpha$ and He\,\textsc{i} 6678 lines in 2011 indicate that perturbations propagate inward through the disk near periastron. When the disk's direction of orbit is opposite to that of the secondary, the parameters used in our models do not produce spiral density enhancements in the H$\alpha$ emitting region because the tidal interaction time is short due to the relative velocities of the disk particles with the secondary. The effects of the secondary star on the disk are short-lived and the disk shows independent evolution between each periastron event.