Dr. Christian Johnson is awarded an NSF Astronomy and Astrophysics Postdoctoral Fellowship to carry out a program of research and education at the University of California, Los Angeles. Dr. Johnson will increase dramatically the number of spectroscopic observations of red giant branch (RGB) stars in the Galactic bulge along both minor axis and off-axis fields. This program will for the first time extend the determination of chemical abundances to light odd-Z elements and heavy neutron-capture elements in a sample of ~1,000 bulge giants. These data will be combined with the results of Dr. Johnson's dissertation work, a comprehensive chemical analysis of ~1,000 RGB stars in Omega Centauri, to provide insight into the chemical evolution of two independent "spheroidal" systems exhibiting signs of significant self-enrichment but that span a metallicity range of nearly a factor of 1,000.

Dr. Johnson will also develop a spectrum synthesis webtool designed to help make stellar abundance work more accessible to the general public, undergraduate non-science majors, and advanced undergraduates. The webtool is intended to be a guided tutorial that allows students and the general public to actively participate in determining the chemical composition of stars in the context of learning about stellar evolution and nucleosynthesis. Dr. Johnson will develop activities associated with the webtool that are appropriate for use in both non-science major and advanced undergraduate classes.

Project Report

The principal purpose of this funded research project was to investigate the chemical composition and formation history of stars near the inner portion of the Milky Way known as the Galactic bulge. This was done by obtaining new observations (high resolution spectra) of Galactic bulge red giant branch (RGB) stars using primarily the Hydra multi-fiber spectrographs on the Blanco 4m and WIYN 3.5m telescopes at the Cerro Tololo and Kitt Peak U.S. national observatories in Chile and Arizona. The project produced 13 publications in professional, peer-review journals and in particular provided the first detailed look at the production and enrichment of heavy elements (e.g., Europium) in Galactic bulge stars. Elements of different masses are often produced through different nucleosynthesis processes and over different time scales. Therefore, analyzing the abundance ratios of multiple elements is critical to piecing together the formation history and time scale of stellar populations like the Galactic bulge. For example, an open problem in understanding the formation of the bulge is whether the stars formed in a single burst (and thus have a single, old age) or whether star formation was more extended like the Galactic disk. It was found from this work that the ratio of Lanthanum (produced mostly in stars that live >1 billion years) to Europium (produced mostly in stars that live <100 million years) is both constant and very low compared to the Sun. This suggests that the overwhelming majority of bulge stars formed "quickly" and are much older than those like the Sun. A serendipitous discovery in this project’s data set was that of a star that is extremely enhanced in elements heavier than about Iron. While these kinds of stars are well-documented in the Galactic halo population, this bulge star is anywhere from ~3-10x more metal-rich, and has only one known analog: an RGB star with different light element chemistry in a satellite dwarf galaxy. It is unclear how these stars are able to form at such a "high" metallicity. Another interesting discovery from this project relied instead on the abundance ratios of Sodium and Aluminum, which trace a different time scale than the heavier elements. These elements appear to indicate subtle chemical differences between stars in the inner and outer portions of the bulge. Furthermore, it was found that the oldest and most "metal-poor" outer bulge stars have a light element chemistry more similar to the Galactic halo while the same inner bulge stars have light element chemistry more similar to the Galactic disk. This suggests that the bulge likely did not form without some influence, perhaps via gas inflow, from the halo and disk. Additionally, it was found that the slope of the Sodium-to-Aluminum ratio as a function of Iron abundance was different for the halo, disk, and bulge. This may point to Sodium and Aluminum being sensitive tracers of bulk star formation rate or efficiency. In addition to the research carried out by the PI, this project was also instrumental in training 7 undergraduate/graduate students in the field of stellar spectroscopy. The PI developed teaching material for training students, and the general public, in the field of stellar spectroscopy. These materials were used both for training students at the PI’s institution and two other institutions in the UK and Germany. Two important coding efforts came out of this work as well: the parallelization of a code that produces synthetic stellar spectra and a machine learning algorithm for analyzing absorption lines in stellar spectra. Both of these codes are being packaged for general use and will decrease the amount of time a researcher takes to determine abundances in a star by about an order of magnitude.

Agency
National Science Foundation (NSF)
Institute
Division of Astronomical Sciences (AST)
Application #
1003201
Program Officer
Harshal Gupta
Project Start
Project End
Budget Start
2010-07-15
Budget End
2013-06-30
Support Year
Fiscal Year
2010
Total Cost
$253,000
Indirect Cost
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