Feb 11

"The End of Nucleosynthesis"
Ian Roederer, University of Texas at Austin

We are all familiar with the standard processes of stellar nucleosynthesis - protons, neutrons, alpha-particles, etc. being added to existing nuclei to form heavier atoms. But what is necessary to form the very heaviest atoms at the bottom of the periodic table, and why does nucleosynthesis fail to continue to produce infinitely heavier atoms? We'll explore the issue in more detail, focusing on the heaviest observable products of stellar nucleosynthesis: lead (Pb), thorium (Th), and uranium (U). WARNING: MAY CONTAIN RADIOACTIVE MATERIAL!

Feb 18

"Parallaxes of the Newly Dead: Improved Distances to Planetary Nebulae Central Stars"
G. Fritz Benedict, University of Texas at Austin

I present absolute parallaxes and relative proper motions for the central stars of the planetary nebulae DeHt 5, Abell 31, NGC 7293 (The Helix), and NGC 6853 (The Dumbell). We obtain these with astrometric data from white-light interferometers on HST. I describe our analysis, using DeHt 5 as an example. Weighted averaging of our new absolute parallaxes with previous independent planetary nebula parallax measurements yields an average parallax precision of 5%. Some rudimentary astrophysics yields estimates of the radii of these central stars, all larger than your garden-variety white dwarf. The seminar closes with some coming astrometric attractions, near-term future parallax results from HST.

May 6

"Disk structure, grain growth and mineralization: observations of the evolution of dust in protoplanetary disks"
Dan M. Watson, University of Rochester, NY

During the past six years, the Spitzer Space Telescope and improved ground-based facilities have enabled a huge increase in the number of complete samples of circumstellar disks, around young stars of Solar type or smaller, in which the composition of the solid component has been studied with complete mid-infrared spectra. With these samples we can assess observationally the evolution of the vertical and radial structure of disks through the planet-forming era, in parallel with the evolution of the composition the dust grains. Here we will review the progress in this endeavour, with emphasis on objects in nearby associations and star-formation regions. Special features include the concordance of minerals observed in protoplanetary disks with those observed in the fossil record of our Solar system's formation, and the evidence for giant-planetary formation in 1-3 Myr-old disks.