Mechanisms of Mass Loss from Oxygen-Rich AGB Stars

在观测质量为太阳0.7到8倍的恒星的质量损失率方面，人们付出了相当大的努力。当恒星成为一颗明亮的红巨星时，大部分质量就会消失，事实上，大部分质量是在核燃烧寿命更长得多的最后20万年里消失的。然而，质量损失过程很难建模，因为它涉及的动力学过程往往远离平衡。这项工作的重点是为渐近巨星分支末端的富氧恒星提供一个可靠的质量损失公式。最令人担忧的是欧洲质量损失代码的最新结果，其中增加了硅酸盐颗粒不透明度的波长依赖性，使它们无法驱动风（直到这一点，人们一直认为这种尘埃在导致风和质量损失方面发挥了主要作用）。在这里，威尔逊博士和她的团队将采用该团队已经开发或正在使用的流体动力学建模代码，并通过辐射改进他们对尘埃不透明和非局部热力学平衡冷却/再加热速率的处理，同时探索脉动期间内部大气反馈的影响。尘埃不透明问题将通过这项工作以及与美国宇航局和欧洲的尘埃研究人员的合作直接解决。对质量损失模型/公式的广泛测试将通过各种观测来完成，包括经验质量损失研究，大气特征的干涉测量，包括内部尘埃半径附近的“分子壳”，以及一个世纪以来500多颗恒星的数据，这些数据大多是由业余天文学家收集的，并存档于美国变星观测者协会。这个项目将教育和培养一名研究生，他的博士论文将以这项工作为基础。另外，爱荷华州立大学的2到3名本科生也将参与其中。生成的模型网格将被放置在一个网站上，其他天文学家可以通过一个“智能界面”访问它们。这将有助于人口研究和对干涉测量和红外观测数据的分析。网站上的模型和数据也将包括在天文学入门和高级课程的练习中。除此之外，地球的最终命运以及太阳演化过程中地球将会遇到的情况等问题具有公众吸引力，这项工作将为天文馆展览、公共讲座、流行文章以及业余天文学家的目标出版物提供材料。

Considerable effort has gone into observing mass loss rates from stars with masses from 0.7 to 8 times the Sun's mass. Most of this mass is lost when the star is a luminous red giant, and most in fact during the last 200,000 years of a much longer nuclear-burning lifetime. The mass loss process is however hard to model, because it involves dynamical processes often far from equilibrium. This work is focused on providing a reliable prescription of this mass loss for oxygen-rich stars at the end of the Asymptotic Giant Branch. Of fundamental concern are recent results from European mass loss codes where adding the wavelength dependence of the opacity of silicate grains made them unable to drive winds (until this point it had always been thought that such dust played a major role in enabling winds and mass loss). Here Dr. Willson and her team will take hydrodynamical modeling codes already developed or in use by the team and improve their treatment of dust opacities and non-local thermodynamic equilibrium cooling/reheating rates by radiation as well as explore the effects of interior-atmosphere feedback during pulsations. The dust opacity problem will be directly addressed by this work and through collaboration with dust researchers at NASA and in Europe. Extensive testing of the resulting models/prescriptions for mass loss will be done with a variety of observations, including empirical mass loss studies, interferometric measurements of atmospheric features including 'molecular shells' near the inner dust radius, and a century of data for over 500 stars collected by mostly amateur astronomers and archived at the American Association of Variable Star Observers.This project will educate and train a graduate student whose Ph.D. thesis will be based on this work. An additional 2 or 3 undergraduate students at Iowa State University will also be involved. The resulting model grids will be placed on a web site where they will be accessible to other astronomers through an 'intelligent interface.' These will be useful for population studies and the analysis of data from interferometry and infrared observations. The models and data on the website will also be included in exercises for introductory and advanced courses in astronomy. Beyond this, the question of the ultimate fate of the Earth and the conditions that Earth will encounter as the Sun evolves have public appeal, and this work will provide material for planetarium shows, public lectures, and popular articles as well as targeted publications for amateur astronomers.