Using hydrodynamic principles we investigate the nature of the disk viscosity following the parameterization by Shakura & Sunyaev adopted for the viscous decretion model in classical Be stars. We consider a radial viscosity distribution including a constant value, a radially variable α assuming a power-law density distribution, and isothermal disks, for a late-B central star. We also extend our analysis by determining a self-consistent temperature disk distribution to model the late-type Be star 1 Delphini, which is thought to have a nonvariable, stable disk as evidenced by Hα emission profiles that have remained relatively unchanged for decades. Using standard angular momentum loss rates given by Granada et al., we find values of α of approximately 0.3. Adopting lower values of angular momentum loss rates, i.e., smaller mass loss rates, leads to smaller values of α. The values for α vary smoothly over the Hα emitting region and exhibit the biggest variations nearest the central star within about five stellar radii for the late-type, stable Be stars.

The IR spectra of Be stars display numerous hydrogen recombination lines, constituting a great resource for obtaining information on the physical and dynamic structures of different regions within the circumstellar envelope. Nevertheless, this spectral region has not been analysed in depth, and there is a lack of synthetic spectra with which to compare observations. Therefore, we computed synthetic spectra with the HDUST code for different disc parameters. Here, we present our results on the spectral region that includes lines of the Brackett series. We discuss the dependence of the line series strengths on several parameters that describe the structure of the disc. We also compared model line profiles, fluxes, and EWs with observational data for two Be stars (MX Pup and $\pi$ Aqr). Even though the synthetic spectra adequately fit our observations of both stars and allow us to constrain the parameters of the disc, there is a discrepancy with the observed data in the EW and flux measurements, especially in the case of MX Pup. It is possible that by including Brackett lines of higher terms or adding the analysis of other series, we may be able to better constrain the parameters of the observed disc.

With the aim of better understanding the physical conditions under which Be stars form and evolve, it is imperative to further investigate whether poorly studied young open clusters host Be stars. In this work, we explain how data from Gaia DR2 and DR3 can be combined to recover and characterize active Be stars in open clusters. We test our methodology in four open clusters broadly studied in the literature, known for hosting numerous Be stars. In addition, we show that the disk formation and dissipation approach that is typically used to model long term Be star variability, can explain the observed trends for Be stars in a (${\mathrm{G}}_{DR3}$-${\mathrm{G}}_{DR2}$) versus ${\mathrm{G}}_{DR3}$ plot. We propose that extending this methodology to other open clusters, and, in particular, those that are poorly studied, will help to increase the number of Be candidates. Eventually, Be stars may eclipse binary systems in open clusters.

Stellar rotation is central to understanding stellar structure and evolution. It plays a particularly important role in Be stars, whose high rotational velocities contribute to the formation of circumstellar gaseous disks. These disks produce characteristic emission lines that are prominent observational tracers of the circumstellar material. The morphology of the emission lines encodes relevant information concerning the structure, geometry, and orientation of the disks. Since astronomical observations reveal only how an object appears projected on the plane of the sky, recovering its actual orientation is an important and perennial problem in observational astronomy. This is especially relevant for Be stars, as their disks can exhibit very different line morphologies depending on their inclination angle. Determining this inclination can, in turn, provide constraints on the stellar rotational velocity under the assumption that the star and disk share the same rotational plane. In the present contribution, we apply Shannon entropy and the LMC structure indicator (also known as the LMC complexity measure) to the analysis of publicly available synthetic Hα emission-line profiles taken from the Lailey-Sigut grid. These profiles are computed from state-of-the-art radiative transfer models of a circumstellar disk surrounding an intermediate-mass star, representative of a Be star system. Using the entropy–complexity plane as a diagnostic tool, we focus on the effects associated with the disk mass and its orientation. Our results show that the entropy-complexity plane can be used to infer both the mass of the disk and the inclination angle of Be star-disk systems. This suggests that the entropy-complexity diagram, which has so far been seldom employed in astronomy, holds significant potential as a practical quantitative tool for interpreting spectroscopic data and extracting key information on astrophysical systems.

Angular momentum transport is a fundamental process shaping the structure, evolution, and lifespans of stars and disks across a wide range of astrophysical systems. Be stars offer a valuable environment for studying viscous transport of angular momentum in massive stars, thanks to their rapid rotation, observable decretion disks, and likely absence of strong magnetic fields. This study analyzes angular momentum loss in 40 Be binary simulations spanning a range of orbital separations and companion masses, using a smoothed-particle hydrodynamics (SPH) code. A novel framework is introduced to define the outer disk edge based on the behavior of the azimuthal velocity, streamlining the analysis of angular momentum transport within the system. Applying this framework reveals that systems with smaller truncation radii tend to reaccrete a larger fraction of their angular momentum during dissipation, thereby inhibiting the stars ability to regulate its surface rotation. Modification of this rate may alter the star's mass-injection duty cycle or long-term evolutionary track. Finally, a subset of the simulations were post-processed using the Monte Carlo radiative transfer code \texttt{HDUST}, generating synthetic observables including H$α$ line profiles, V-band polarization, and UV polarization. Suggestions for observational verification of the dynamical results are demonstrated using the simulated data.