Dr. Nicholas Sterling is awarded an NSF Astronomy and Astrophysics Postdoctoral Fellowship to carry out a program of research and education at Michigan State University. Dr. Sterling will conduct a spectroscopic investigation of planetary nebulae (PNe) and Wolf-Rayet (WR) nebulae, in order to investigate the production of neutron(n)-capture elements (atomic number Z > 30) by s-process nucleosynthesis in the progenitor stars of these objects. He will observe PNe with a wide range of progenitor masses in the optical spectral region, which provides access to several n-capture elements that are not detectable in stellar spectra. These data will be used to derive s-process enrichment factors, determine key physical conditions and constrain the element-by-element pattern of s-process enrichments in PN progenitor stars with different initial masses, and reveal the s-process neutron source in intermediate-mass AGB stars. In addition, Dr. Sterling will observe WR nebulae in the infrared in order to detect emission lines of Se and Kr, two of the most highly enriched elements by the weak s-process. This will potentially provide some of the first observational evidence of weak s-process enrichments in massive stars. These observations will be complemented by a laboratory astrophysics study to determine atomic data needed to accurately derive the abundances of the n-capture elements Br and Rb in ionized nebulae. The atomic data will be added to the atomic databases of state-of-the-art photoionization codes and made accessible to the broad astrophysical community. This research has implications for the origin and chemical evolution of trans-iron elements in the Universe, and nucleosynthesis and convective mixing during the late stages of stellar evolution. Moreover, this work will establish an observational and atomic physics foundation upon which future nebular studies can be built, enabling chemical evolution investigations of these elements in Galactic and extragalactic environs.
Dr. Sterling will also manage an educational outreach program at Michigan State University comprised of designing and teaching seminars for students, co-teaching Physics and Astronomy Department courses, and advising an undergraduate student research project. The content of the seminars, which will be offered at a range of levels, will draw from the research activities of Dr. Sterling and make them accessible to university students.
Elements heavier than zinc (atomic number >30) cannot be created via nuclear fusion in stellar interiors. Rather, they are formed when a nuclear reaction produces neutrons, which are captured by iron-peak elements that beta-decay (i.e., eject an electron) to form heavier elements. This process is called neutron(n)-capture nucleosynthesis, and elements heavier than zinc are referred to as n-capture (or trans-iron) elements. The production of n-capture elements in stars is not as well as understood as that of lighter elements. It is believed that about half of the n-capture nuclei in the Universe are formed in supernova explosions, with the other half produced deep inside low-mass giant stars and in the cores of massive stars. Historically, trans-iron elements have been studied via spectroscopy of stars, but this reveals information for only a handful of elements. Moreover, some types of stars can be difficult to study when they become giants, because they are surrounded by dense, dusty gas. Nebular spectroscopy provides access to many n-capture elements that cannot be studied in giant stars, and to classes of stars that are difficult to study during the giant stage. The detection of trans-iron elements in a large number of planetary nebulae (PN; the gas ejected at the end of a low-mass star's life), H II regions (gas surrounding massive young stars) and even in other galaxies demonstrates the potential of nebular spectroscopy for studying heavy-element nucleosynthesis. By studying how much of an element (its abundance) there is in a planetary nebula, it is possible to determine how much of that element was produced (its enrichment) in the giant star that preceded the PN stage. The production of n-capture elements in low-mass stars is intimately linked to carbon production, since they are produced in the same layer of low-mass giant stars. Therefore, accurately determining n-capture element enrichment factors in PN helps constrain the carbon enrichment, which is difficult to accurately determine in nebulae. Carbon is a key element for life, and finding out what kinds of stars produce carbon and by how much helps constrain when life in the Universe could arise. This program focused on improving the accuracy of n-capture element abundance determinations in astrophysical nebulae, both through observations and through atomic physics. We studied the atomic properties of heavy elements such as Se, Br, Kr, and Rb to understand how they interact with light and electrons, and how they are ionized. This information is critical, since we must understand the physics of n-capture elements in astrophysical nebulae in order to accurately determine their abundances. Observationally, we took very sensitive spectra of 24 PN to search for emission lines of n-capture elements. Multiple ions of various n-capture elements (e.g., Se, Br, Kr, Rb, and Xe) can be detected at visual wavelengths. The more ions of each element that can be detected, the more accurate the abundance determination. These data complement previous infrared observations of Se and Kr ions in PN, in which only one ion of each element was detected. In our optical data, we detected Kr in all but 2-3 objects, with several PN exhibiting emission from 2-3 Kr ions. Xe emission was detected in about one-third of our sample, while Se, Br, and Rb (which have fainter optical emission lines) were detected in 4--6 PN each. We also observed three nebulae surrounding massive stars called ``Wolf-Rayet'' stars, which have ejected a large portion of their atmospheres. The nebulae are enriched in material formed inside the stars. We searched for infrared Se and Kr emission lines to determine whether these elements are enriched in the nebulae. Such a finding would be the first observational evidence of n-capture nucleosynthesis in massive stars before they explode as supernovae. Unfortunately, we did not detect Se or Kr in any of these objects. Wolf-Rayet star nebulae are much fainter and larger than PN, making the detection of weak emission lines very difficult. We computed upper limits to the Se and Kr abundances (i.e., the maximum abundance based on the spectra that we obtained) for these nebulae. In most cases, the upper limits did not exclude enrichments due to n-capture nucleosynthesis, though in one object we found that Se was not enriched. These observational and atomic physics studies helped further astronomers' understanding of how the heaviest elements are formed in the Universe. Better determinations of trans-iron element abundances also helps us understand the production of the biologically important atom carbon. Our atomic physics work has implications not only for astrophysical investigations, but also for other areas such as nuclear fusion studies.
