As low- and intermediate-mass stars reach the asymptotic giant branch (AGB), they have developed into intriguing and complex objects that are major players in the cosmic gas/dust cycle. At this stage, their appearance and evolution are strongly affected by a range of dynamical processes. Large-scale convective flows bring newly-formed chemical elements to the stellar surface and, together with pulsations, they trigger shock waves in the extended stellar atmosphere. There, massive outflows of gas and dust have their origin, which enrich the interstellar medium and, eventually, lead to a transformation of the cool luminous giants into white dwarfs. Dust grains forming in the upper atmospheric layers play a critical role in the wind acceleration process, by scattering and absorbing stellar photons and transferring their outward-directed momentum to the surrounding gas through collisions. Recent progress in high-angular-resolution instrumentation, from the visual to the radio regime, is leading to valuable new insights into the complex dynamical atmospheres of AGB stars and their wind-forming regions. Observations are revealing asymmetries and inhomogeneities in the photospheric and dust-forming layers which vary on time-scales of months, as well as more long-lived large-scale structures in the circumstellar envelopes. High-angular-resolution observations indicate at what distances from the stars dust condensation occurs, and they give information on the chemical composition and sizes of dust grains in the close vicinity of cool giants. These are essential constraints for building realistic models of wind acceleration and developing a predictive theory of mass loss for AGB stars, which is a crucial ingredient of stellar and galactic chemical evolution models. At present, it is still not fully possible to model all these phenomena from first principles, and to predict the mass-loss rate based on fundamental stellar parameters only. However, much progress has been made in recent years, which is described in this review. We complement this by discussing how observations of emission from circumstellar molecules and dust can be used to estimate the characteristics of the mass loss along the AGB, and in different environments. We also briefly touch upon the issue of binarity.

当低质量和中等质量恒星到达渐近巨星分支（AGB）时，它们已经发展成为有趣且复杂的天体，成为宇宙气体/尘埃循环的主要参与者。在这个阶段，它们的外观和进化受到一系列动力学过程的强烈影响。大规模对流将新形成的化学元素带到恒星表面，并与脉动一起在扩展的恒星大气中引发冲击波。在那里，大量的气体和尘埃流出，丰富了星际介质，最终导​​致冷发光巨星转变为白矮星。在高层大气层中形成的尘埃颗粒在风加速过程中起着至关重要的作用，它们会散射和吸收恒星光子，并通过碰撞将其向外的动量传递给周围的气体。从视觉到射电领域，高角分辨率仪器的最新进展正在为AGB恒星及其形成风区域的复杂动态大气带来有价值的新见解。观测揭示了光球层和尘埃形成层的不对称性和不均匀性，这些层在数月的时间尺度上变化，以及星周包层中更长寿的大型结构。高角分辨率观测表明尘埃凝结发生在距离恒星多远的地方，并且提供了有关冷巨星附近尘埃颗粒的化学成分和大小的信息。这些是建立真实的风加速模型和开发 AGB 恒星质量损失预测理论的基本限制，而 AGB 恒星是恒星和星系化学演化模型的重要组成部分。 目前，还不能完全根据第一原理对所有这些现象进行建模，并仅根据基本恒星参数来预测质量损失率。然而，近年来已经取得了很大进展，本综述对此进行了描述。我们通过讨论如何利用对星周分子和尘埃发射的观测来估计沿 AGB 和不同环境中的质量损失特征来补充这一点。我们还简要讨论了二元性问题。