Planetary Nebulae as Agents of Neutron-Capture Element Enrichment of the Galaxy

在所谓的渐近巨支演化阶段，行星状星云显示出一系列在其前体恒星内合成的轻中子捕获元素的化学富集。从锌到氪的原子序数范围内的四种元素的新的光谱示踪剂已经通过红外，光学和紫外光谱鉴定，并得到了它们的丰度。一些行星状星云显示出高达10到20倍的增强，而另一些则显示出正常的（太阳系）值。通过确定行星状星云中中子捕获元素的增强，我们直接测量了从中等质量恒星流入星际介质的富集物质的组成，这是驱动星系化学演化过程的重要因素。我们还了解了在进化的恒星中挖掘核合成产物的效率。这位研究者正在进行一项多波长光谱研究，将更充分地探索和建立行星状星云中中子捕获富集的人口统计学。该项目包括将已经确定的光学和红外线的观测扩展到这些物体的更大更有代表性的样本，并从其他离子和元素中寻找新的线。这项研究正在测试恒星演化后期的现有模型，包括关键的新物理，如对流超调和旋转（它们也以其他方式影响恒星的演化，例如演化轨迹），并澄清最初几个中子捕获元素的核合成位点，这些元素位于铁峰区域和较重的中子捕获元素之间，这些元素在恒星中得到了很好的研究。更广泛的影响：该项目解决了2002年美国国家科学院宇宙物理委员会报告中确定的一个关键问题：从铁到铀的元素是如何产生的？行星状星云的研究在多次挖掘后建立了富集的终点，并且，由于它提供了进入元素周期表中几乎未被探索的区域的途径，该项目还为原子光谱领域提供了独特的数据，例如，通过确定低能级离子的精确能量，碰撞横截面值和相关的原子参数。根据不同元素的比例，可以测试核反应的横截面，也可以限制核合成发生的条件。该项目还对未来科学家的发展和科学知识的普及产生了影响。两名研究生获得了资金支持，本科生，包括少数民族和妇女，接受了特定技术的培训，以便他们能够从事定义明确的小型项目。这项工作的兴奋之处传达给初中生和高中生，他们刚刚开始熟悉化学元素的概念，通过与当地学校的磁铁科学项目合作，包括研究生或本科生参与这项工作。

Planetary nebulae have been recently shown to display a range of chemical enrichments in light neutron-capture elements synthesized within their precursor stars during the so-called Asymptotic Giant Branch evolutionary phase. New spectroscopic tracers of the four elements in the atomic number range from Zinc to Krypton have been identified through infrared, optical, and ultraviolet spectroscopy, and their abundances derived. Some planetary nebulae show enhancements of up to factors of 10 to 20, while others show normal (solar system) values. By determining the enhancements of neutron-capture elements in planetary nebulae, we measure directly the composition of enriched material that is flowing into the interstellar medium from intermediate-mass stars, an important agent in driving the process of galactic chemical evolution. We also learn about the efficiency of dredge-up of nucleosynthetic products in evolved stars. This investigator is undertaking a multi-wavelength spectroscopic study that will more fully explore and establish the demographics of neutron-capture enrichments in planetary nebulae. The project involves both extending observations of the already identified optical and infrared lines to larger and more representative samples of these objects and searching for new lines from additional ions and elements. This study is testing current models for the late stages of stellar evolution that include crucial new physics such as convective overshoot and rotation (which also affect the stars' evolution in other ways, e.g. evolutionary tracks) and clarifying the nucleosynthetic sites for the first few neutron-capture elements, that lie between the iron-peak region and the heavier neutron-capture elements that are well-studied in stars.Broader Impact: The project addresses a key question identified in a 2002 report by the National Academy of Sciences Committee on the Physics of the Universe: How were the elements from iron to uranium made? The planetary nebula study establishes the end-point of enrichment after multiple dredge-up episodes, and, because it provides access to a nearly unexplored region of the periodic table, the project also provides unique data for the field of atomic spectroscopy, e.g. by determining precise energies for low-lying energy levels of poorly-studied ions, values for collisional cross-sections, and related atomic parameters. From the ratios of different elements, it may be possible to test nuclear reaction cross-sections, as well as to constrain the conditions under which the nucleosynthesis occurs. The project is also having an impact on the development of future scientists and a scientifically informed population. Two graduate students receive financial support, and undergraduate students, including ethnic minorities and women, are trained in specific techniques so that they can work on well-defined small projects. The excitement of this work is conveyed to middle and high school students, who are just becoming acquainted with the idea of chemical elements, through coordination with local schools with magnet science programs, and involve either a graduate or undergraduate student in this effort.