在极端红巨星周围由大的透明颗粒组成的紧密晕
A close halo of large transparent grains around extreme red giant stars
摘要：
中等质量的恒星通过缓慢的密集风将其外壳的大部分喷射回星际介质，以形成下一代恒星和行星，从而结束了它们的生命。恒星脉动被认为是将气体提升到足够冷的高度，以便尘埃凝结，然后由星光的辐射压力加速，夹带气体并驱动风。然而，由于难以观测到距离恒星数十毫角秒的稀薄气体和尘埃，因此对星光向外流传递足够动量的方式没有达成共识，因此质量损失的解释一直是一个问题。在这里，我们提出了空间分辨，多波长观测的三颗恒星的拱尘埃壳的渐近巨星分支的HR图。当在散射光下成像时，发现尘埃壳的半径非常小（<~ 2恒星半径），并且具有意想不到的大颗粒（~300 nm半径）。这种与光球层的接近证明了尘埃物质对星光是透明的，因此可以抵抗强烈辐射场的升华。虽然透明通常意味着不足以驱动风的辐射压力，但辐射场可以通过光子散射而不是吸收来加速这些大颗粒-这是低振幅脉动恒星的合理质量损失机制。
英文摘要：
Intermediate-mass stars end their lives by ejecting the bulk of their envelope via a slow dense wind back into the interstellar medium, to form the next generation of stars and planets. Stellar pulsations are thought to elevate gas to an altitude cool enough for the condensation of dust, which is then accelerated by radiation pressure from starlight, entraining the gas and driving the wind. However accounting for the mass loss has been a problem due to the difficulty in observing tenuous gas and dust tens of milliarcseconds from the star, and there is accordingly no consensus on the way sufficient momentum is transferred from the starlight to the outflow. Here, we present spatially-resolved, multi-wavelength observations of circumstellar dust shells of three stars on the asymptotic giant branch of the HR diagram. When imaged in scattered light, dust shells were found at remarkably small radii (<~ 2 stellar radii) and with unexpectedly large grains (~300 nm radius). This proximity to the photosphere argues for dust species that are transparent to starlight and therefore resistant to sublimation by the intense radiation field. While transparency usually implies insufficient radiative pressure to drive a wind, the radiation field can accelerate these large grains via photon scattering rather than absorption - a plausible mass-loss mechanism for lower-amplitude pulsating stars.
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