Students

My most recent Ph. D. student is Colin Wallace, currently a post-doc with Dr. Ed Prather at the University of Arizona. His thesis was the first comprehensive study of students’ ideas about Cosmology.  He produced tutorials that achieved the largest gains in student understanding of the Big Bang, Dark Matter, and Hubble’s Law that any instructor has achieved.  His thesis is appearing in a number of papers in the journal Astronomy Education Review.

My other recent Ph. D. students include Francesca Primas (now at the European Southern Observatory, in Germany and Chile ), Luisa Rebull (now at NASA’s Infrared Processing and Analysis Center, Caltech, Pasadena, CA).

Here’s some of the work:

The Evolution of Galactic Boron and the Production Site of the Light Elements

Since the 1970s it has been believed that the light elements Li, Be, and B are made by the spallation of cosmic rays (CRs; primarily energetic protons and alpha particles) on nuclei of C, N, and O in the interstellar medium. Abundances of the light elements have in fact been used to infer the existence of a large flux of low energy CRs which are kept from our direct detection by solar modulation but which should efficiently produce Li, Be, and B. Our data suggests that the “reverse” process of energetic C and O particles hitting ambient H and He, probably in the vicinity of supernovae, have played a much larger role in the formation of the light elements than previously recognized.

The Hubble Space Telescope has been used to obtain spectra of the boron 2500 Ang. region in eight galactic halo stars ranging from [Fe/H] = -0.4 to -3.0. The sample includes the most metal-poor (and presumably oldest) star ever observed for B. Spectrum synthesis using latest Kurucz model atmospheres has been used to determine B abundances for each star, and particular attention paid to the errors of each point, to permit judgement of the goodness-of-fit of models of galactic chemical evolution.

Usual models of galactic chemical evolution do not fit the data well; their predicted increase of light elements with time is too rapid. A straight line of slope approximately 1 gives an excellent fit to a plot of log (Boron abundance) vs. [Fe/H], and if NLTE abundances are used the slope is approximately 0.7. Plotting B vs. [O/H] rather than [Fe/H] increases the slope of either plot about 0.2. The observed relation suggests that the production of light elements such as B and Be is directly related to the production of heavier elements.

Our data do not show a change in slope between halo and disk metallicities, but the number of stars near the disk-halo transition is small and a modest change is not precluded. The NLTE B/Be ratio is typically about 15 throughout the lifetime of the Galaxy, a ratio naturally produced by cosmic ray (CR) spallation. Our data support a model in which most light element production comes from low-energy CR spallation of C and O nuclei onto protons and alpha particles, probably in the vicinity of massive supernovae in star-forming regions. This process was discussed in the original work of Reeves, Fowler, & Hoyle (1970), but until recently most models have emphasized light element production in the general ISM from the spallation of high-energy protons and alpha particles onto CNO nuclei. Especially during the Galaxy’s early history, when the metallicity of the ISM was low, the spallation of protons and alpha particles onto CNO nuclei cannot account for as much B as we observe unless the CR flux was sufficiently higher to compensate. The observed relation also constrains any direct production of B by the neutrino process in supernovae (Woosley & Weaver 1995) to be at most a small part of total B production.