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.
