Probing mass loss mechanisms in AGB and post-AGB stars

国际天体物理研究所将继续研究中等质量恒星演化后期的质量损失过程，重点是所谓的渐近巨型分支恒星，以及随后的膨胀导致行星状星云形成的阶段。PI将努力了解这些演化恒星中的物理过程，通过这些过程，脉动、激波、非平衡化学和尘埃形成驱动恒星质量损失，并确定抛出的气体和尘埃的组成。从这些恒星附近的尘埃颗粒反射的偏振光是距离恒星非常近的新形成的尘埃的诊断。模型和数据的比较应该给出这些恒星周围环境中形成的尘埃的位置和性质的独特的强有力的约束。利用选定的分子发射线拍摄附近明亮巨星的额外~1角秒分辨率图像，这些谱线是气体温度和密度的示踪物，将提供PI先前在其环绕恒星的包层中探测到的小尺度结构中的物理条件的信息：这些结果不仅在科学文献中传播，而且通过由斯图尔德天文台定期向我们社区提供的公开讲座和展示向更广泛的社区传播，重点是使感兴趣的公众能够获得科学知识，并刺激进一步的学习。

AST 0307687Bieging, John H.The PI will continue a research program on mass loss processes in intermediate mass stars in the late stages of their evolution, with emphasis on the so-called asymptotic giant branch stars, and the subsequent stages where expansion leads to formation of planetary nebulae. The PI will endeavor to understand the physical processes in these evolved stars by which pulsation, shocks, non-equilibrium chemistry, and dust formation drive the stellar mass loss and determine the composition of the ejected gas and dust. The PI will use optical telescopes with a sensitive, wide-band spectropolarimeter to measure the optical polarization as a function of wavelength for a selection of evolved stars. Polarized light reflected off dust grains near such stars is a diagnostic of newly-formed dust very close to the star. Comparison of models and data should give uniquely powerful constraints on the location and properties of dust formed in the circumstellar environment of these stars. Furthermore the PI will obtain mm and submm observations of key molecular emission lines to constrain the spatial distribution of molecules in the circumstellar envelopes of the evolved stars. One key goal of this study will be to test whether the molecular abundances and excitation conditions imply that pulsation-driven shocks in the stellar atmosphere determine the chemical composition of the ejected matter. Additional ~1 arcsec resolution images of a small sample of bright nearby giant stars in light of selected molecular emission lines that are tracers for gas temperature and density will give information on the physical conditions in the small-scale structures detected previously by the PI in their circumstellar envelopes Broader impact: The results are disseminated not only in the scientific literature, but also to the wider community via public lectures and displays, routinely provided by the Steward Observatory to our community, with an emphasis on making the science accessible to the interested public, and on stimulating further learning. This research has an important education function by engaging both undergraduate and graduate students, and by allowing and encouraging them to attend scientific meetings to present the results of the work in which they participated.