We use a three-dimensional smoothed particle hydrodynamics code to simulate growth and dissipation of Be star discs in systems where the binary orbit is misaligned with respect to the spin axis of the primary star. We investigate six different scenarios of varying orbital period and misalignment angle, feeding the disc at a constant rate for 100 orbital periods, and then letting the disc dissipate for 100 orbital periods. During the disc growth phase, we find that the binary companion tilts the disc away from its initial plane at the equator of the primary star before settling to a constant orientation after 40–50 orbital periods. While the mass-injection into the disc is ongoing, the tilting of the disc can cause material to reaccrete on to the primary star prematurely. Once disc dissipation begins, usually the disc precesses about the binary companion’s orbital axis with precession periods ranging from 20 to 50 orbital periods. In special cases, we detect phenomena of disc tearing, as well as Kozai–Lidov oscillations of the disc. These oscillations reach a maximum eccentricity of about 0.6, and a minimum inclination of about 20^○ with respect to the binary’s orbit. We also find the disc material to have highly eccentric orbits beyond the transition radius, where the disc changes from being dominated by viscous forces, to heavily controlled by the companion star, in contrast to its nearly circular motion inwards of the transition radius. Finally, we offer predictions to how these changes will affect Be star observables.

We model the circumstellar disk of δ Sco using the three-dimensional Monte Carlo radiative transfer code HDUST in order to quantify the large-scale changes in the disk through the years 2000–2018, and to see if these changes can be attributed to the secondary star affecting the disk throughout its orbit. We determine our best-fitting models through matching simulated observations to actual Hα spectroscopy and V-band photometric observations. Our modeling results confirm previous findings that the disk of δ Sco was forming early in the century. We also find a period of disk dissipation when the companion is at apastron, as well as a significant growth of the disk between 2009 and 2011, prior to the periastron of 2011. Due to the steady-state nature of the disk after 2011, it is difficult to say whether the variations seen are due to the effect of the close passage of the binary companion.

Classical B emission (Be) stars are fast rotating, near-main-sequence B-type stars. The rotation and the presence of circumstellar discs profoundly modify the observables of active Be stars. Our goal is to infer stellar and disc parameters, as well as distance and interstellar extinction, using the currently most favoured physical models for these objects. We present BeAtlas, a grid of $61\, 600$ non-local thermodynamic equilibrium radiative transfer models for Be stars, calculated with the hdust code. The grid was coupled with a Monte Carlo Markov chain (MCMC) code to sample the posterior distribution. We test our method on two well-studied Be stars, α Eri and β CMi, using photometric, polarimetric, and spectroscopic data as input to the code. We recover literature determinations for most of the parameters of the targets, in particular the mass and age of α Eri, the disc parameters of β CMi, and their distances and inclinations. The main discrepancy is that we estimate lower rotational rates than previous works. We confirm previously detected signs of disc truncation in β CMi and note that its inner disc seems to have a flatter density slope than its outer disc. The correlations between the parameters are complex, further indicating that exploring the entire parameter space simultaneously is a more robust approach, statistically. The combination of BeAtlas and Bayesian-MCMC techniques proves successful, and a powerful new tool for the field: The fundamental parameters of any Be star can now be estimated in a matter of hours or days.

We use the three-dimensional Monte Carlo radiative transfer code hdust to model Be stars where the disc is tilted from the equatorial plane of the star. We compute 128 models across 4 spectral types, B0, B2, B5 and B8, tilting the disc by 0○, 10○, 20○, and 40○, while varying disc density according to spectral type. We also compute every model for an average and high stellar rotation rate. We first discuss non-tilted disc temperatures and show its non-linear dependence on stellar and disc parameters. We find that tilting the disc minimally affects the density-weighted average disc temperature, but tilting does create a temperature asymmetry in disc cross sections, which is more pronounced for a faster rotation rate. We also investigate the effect tilting has on V-band magnitude, polarization, and the H|$\rm \alpha$| line. Tilting the disc does affect these observables, but the changes are entirely dependent on the position of the observer relative to the direction of tilt. We find the observables that distinguish tilting from a change in density or geometry are the H|$\rm \alpha$| line shape, where it can transition between single-peaked and double-peaked, and the polarization position angle, whose value is dependent on the projected major elongation axis of the disc on the sky. We also present one early and one late-type model with warped discs. We find their temperature structure varies a small amount from the uniformly tilted models, and the different observables correspond to different tilt angles, consistent with their expected volume of origin within the disc.

We build on our previous work involving smoothed particle hydrodynamic simulations of Be stars, by using the model that exhibited disc tearing as input into the three-dimensional Monte Carlo radiative transfer code hdust to predict observables from a variety of viewing angles throughout the disc tearing process. We run one simulation at the start of each orbital period from 20 to 72 orbital periods, which covers two complete disc tearing events. The resulting trends in observables are found to be dependent on the relative position of the observer and the tearing disc. The H|$\rm \alpha$| equivalent width, V magnitude, and polarization can all increase or decrease in any combination depending on the viewpoint of the observer. The H|$\rm \alpha$| line profile also displays changes in strength and peak separation throughout the tearing process. We show how the outer disc of the torn system can have a large effect on the H|$\rm \alpha$| line profile, and also contributes to a wavelength-dependent polarization position angle, resulting in a similar sawtooth shape to the polarization percentage. Finally, we compare our predictions to Pleione (28 Tau) where evidence has suggested that a disc tearing event has occurred in the past. We find that our tearing disc model can broadly match the trends seen in Pleione’s observables, as well as produce the two-component H|$\rm \alpha$| lines observed in Pleione. This is the strongest evidence, thus far, of Pleione’s disc having indeed experienced a tearing event.

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.