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.

中文翻译：

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古典星

在过去的十年中，关于经典Be恒星的性质已经达成共识：它们是非常快速旋转的主序B恒星，通过仍然未知但越来越受限制的过程，它们形成了向外扩散的气态，无尘的Keplerian圆盘。 。在这项工作中，首先将Be恒星的定义与相似的类别进行对比，并介绍了Be恒星获得的常见观测资料并解释了各自的形成机制。然后，我们回顾有关涉及非径向脉动物体和非磁性恒星的中心恒星的当前知识状态，只要它涉及大规模（即大部分为偶极子）全局场。局部的弱磁场仍然可能，但尚未得到证实。该BE-现象与一个或多个质量喷射过程​​相关的，作用于临界或以上大约75％的转速。可以很好地限制该过程的属性，仅留下很少的选择，最重要但非排他地是非径向脉动和小规模磁场。其中，很可能所有被实现：在不同的恒星中，不同的过程可能起作用。一旦材料被提升到开普勒轨道上，就不再保留对喷射过程细节的记忆，并且材料由粘度控制。在粘性减量磁盘模型的理论框架中，磁盘已被很好地理解。这不仅适用于磁盘结构，而且适用于其周期性和长期变化。二进制文件是在同伴可以与星际盘发生的各种类型的交互作用下进行审查的。最后，银河系Be星的金属含量较低，似乎更常见且旋转更快。