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.

中文翻译：

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渐近巨星分支上恒星的质量损失

随着低质量和中等质量的恒星到达渐近巨星分支（AGB），它们已经发展成为有趣而复杂的天体，成为宇宙气体/尘埃循环的主要参与者。在这个阶段，它们的出现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，气体和尘埃的大量流出是有其起源的，它们丰富了星际介质，并最终导致冷光巨星转变为白矮星。在大气层上层形成的尘埃颗粒在风加速过程中起着关键作用，通过散射和吸收恒星光子，并通过碰撞将它们向外的动量传递给周围的气体。高角分辨率仪器的最新进展，从视觉到无线电领域，正在导致对 AGB 恒星及其成风区域的复杂动力学大气层的宝贵新见解。观测揭示了光球层和尘埃形成层的不对称性和不均匀性，它们在数月的时间尺度上变化，以及星周包层中寿命更长的大尺度结构。高角分辨率观测表明在距恒星多远的距离发生尘埃凝结，并提供有关冷巨星附近尘埃颗粒的化学成分和大小的信息。这些是构建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约束，这是恒星和银河化学演化模型的关键组成部分。目前，还不能完全根据第一性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。我们通过讨论如何使用对星周分子和尘埃发射的观测来估计沿 AGB 和不同环境中质量损失的特征，以此来补充这一点。我们还简要讨论了二元性问题。这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性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。我们通过讨论如何使用对星周分子和尘埃发射的观测来估计沿 AGB 和不同环境中质量损失的特征，以此补充这一点。我们还简要讨论了二元性问题。这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性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。我们通过讨论如何使用对星周分子和尘埃发射的观测来估计沿 AGB 和不同环境中质量损失的特征，以此来补充这一点。我们还简要讨论了二元性问题。近年来取得了很大进展，本审查对此进行了说明。我们通过讨论如何使用对星周分子和尘埃发射的观测来估计沿 AGB 和不同环境中质量损失的特征，以此来补充这一点。我们还简要讨论了二元性问题。近年来取得了很大进展，本审查对此进行了说明。我们通过讨论如何使用对星周分子和尘埃发射的观测来估计沿 AGB 和不同环境中质量损失的特征，以此来补充这一点。我们还简要讨论了二元性问题。