Anahi Granada

Context . Long-term H α variability provides a direct probe of the dynamical evolution of Be-star circumstellar disks, but interpreting these changes in terms of physical disk properties remains challenging. Aims . We explore the entropy–complexity ( H–C ) plane as a compact diagnostic to link changes in the H α emission-line morphology to the physical evolution of Be-star disks. Methods . We applied entropy and López–Ruiz–Mancini–Calbet complexity diagnostics to H α line profiles from a grid of viscous decretion disk (VDD) models and from time-resolved observations of the late-type Be star β Psc. We compared their respective evolutionary pathways in the H–C plane. Results . The star exhibits an abrupt excursion and a subsequent secular recovery in the H–C plane, associated with rapid changes in the disk mass and apparent inclination within the adopted framework, before returning to a spectroscopic state similar to that observed prior to the major disk event around 2007. Conclusions . The H–C diagram provides a simple diagnostic that is capable of tracing long-term structural evolution and rapid transitions in Be-star disks using the morphology of the H α line profile alone.

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.

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.