[Paper Review] Classical Be Stars: Rapidly Rotating B Stars with Viscous Keplerian Decretion Disks

Th. Rivinius, A. C. Carciofi+1 more|arXiv (Cornell University)|Oct 15, 2013

Astrophysics and Star Formation StudiesPhysics and Astronomy363 references236 citations

TL;DR

This paper reviews classical Be stars as rapidly rotating B stars that form viscous, Keplerian decretion disks via unknown mass ejection mechanisms, primarily driven by non-radial pulsations or small-scale magnetic fields. The viscous decretion disk model successfully explains disk structure, variability (cyclic and secular), and interactions in binary systems, establishing Be stars as key laboratories for studying rapid rotation, disk viscosity, and stellar evolution across diverse metallicities.

ABSTRACT

In the past decade, a consensus has emerged regarding the nature of classical Be stars: They are very rapidly rotating main sequence B stars, which, through a still unknown, but increasingly constrained process, form an outwardly diffusing gaseous, dust-free Keplerian disk. In this work, first the definition of Be stars is contrasted to similar classes, and common observables obtained for Be stars are introduced and the respective formation mechanisms explained. We then review the current state of knowledge concerning the central stars as non-radially pulsating objects and non-magnetic stars, as far as it concerns large scale, i.e., mostly dipolar, global fields. Localized, weak magnetic fields remain possible, but are as of yet unproven. The Be phenomenon, linked with one or more mass ejection processes, acts on top of a rotation rate of about 75% of critical or above. The properties of the process can be well constrained, leaving only few options, most importantly, but not exclusively, non-radial pulsation and small scale magnetic fields. Of these, it is well possible that all are realized: In different stars, different processes may be acting. Once the material has been lifted into Keplerian orbit, memory of the details of the ejection process is lost, and the material is governed by viscosity. The disks are fairly well understood in the theoretical framework of the viscous decretion disk model. This is not only true for the disk structure, but as well for its variability, both cyclic and secular. Be binaries are reviewed under the aspect of the various types of interactions a companion can have with the circumstellar disk. Finally, extragalactic Be stars, at lower metallicities, seem more common and more rapidly rotating.

Motivation & Objective

• To establish a consensus on the physical nature of classical Be stars, distinguishing them from similar stellar classes.
• To clarify the mechanisms behind mass ejection and disk formation, focusing on non-radial pulsations and small-scale magnetic fields.
• To evaluate the role of viscosity in governing disk evolution and variability after mass ejection.
• To assess the impact of binarity and low metallicity on Be star properties and prevalence.
• To position Be stars as critical laboratories for studying rapid rotation, disk physics, and stellar evolution across diverse galactic environments.

Proposed method

• Comparative analysis of observational data from high-precision photometry, spectroscopy, polarimetry, and interferometry to characterize Be star properties.
• Application of the viscous decretion disk model to explain disk structure, secular and cyclic variability, and mass transport.
• Use of long-term photometric databases (e.g., OGLE, MACHO) and spectroscopic archives (e.g., BeSS) to study disk build-up and decay.
• Modeling of tidal interactions and disk truncation in Be binary systems using hydrodynamic and viscous disk theory.
• Incorporation of multi-wavelength data from space missions (JWST, GAIA, HERSCHEL) and ground-based facilities (VLT/VLTI, ALMA, E-ELT) to constrain disk and stellar parameters.
• Analysis of Be star frequency and rotational velocity trends across different metallicities, including extragalactic environments.

Experimental results

Research questions

• RQ1 What physical mechanisms drive the formation of Keplerian decretion disks in classical Be stars?
• RQ2 How do non-radial pulsations and small-scale magnetic fields contribute to mass ejection in Be stars?
• RQ3 To what extent do binary interactions modify the structure and dynamics of Be star disks?
• RQ4 Why are Be stars more prevalent in low-metallicity environments, and how does this affect their rotational properties?
• RQ5 How well can the viscous decretion disk model explain observed photometric and spectroscopic variability in Be stars?

Key findings

• Classical Be stars are rapidly rotating B stars with rotation rates at or above 75% of critical, and their disks form via an unknown ejection process that is increasingly constrained to non-radial pulsations or small-scale magnetic fields.
• The viscous decretion disk model successfully explains cyclic V/R variability, secular photometric variations, and disk truncation in binary systems, with material transported outward due to viscosity.
• Observations show that Be stars in low-metallicity environments are more common and rotate faster, with Be star fractions potentially reaching 100% in very low-metallicity regions.
• The disk's memory of the ejection process is lost quickly, and once in Keplerian orbit, disk evolution is governed solely by viscous processes, not the initial ejection mechanism.
• Be stars are increasingly recognized as ideal laboratories for studying rapid rotation, disk turbulence, and stellar evolution across the Hertzsprung-Russell diagram and at different metallicities.
• Future facilities such as JWST, GAIA, and the E-ELT will enable multi-wavelength, multi-technique studies of Be stars in the Milky Way and beyond, advancing understanding of disk physics and stellar evolution.