Morphology of Emission Spectral Lines in Be Stars: The Entropy-Complexity Plane Approach

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Abstract
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.

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