Cornell University (1971)
Joined UW in
My primary interests are the formation, evolution, and chemistry of planetary nebulae (“PNe”)*. The processes that form and shape PNe remain amount the least understood facets of stellar evolution. I use images and spectroscopy to understand an interpret their shapes and internal motions. From this my colleagues and I generate hydrodynamical models of their evolution and probe their significance in the cosmic enrichment of helium, carbon and nitrogen.
*95% of all stars that we see in our own galaxy, the Milky Way, will ultimately become “planetary nebulae”. This includes the Sun. Much as a butterfly emerges when its chrysalis is ejected, planetary nebulae are formed when a red giant star ejectes its outer layers as clouds of luminescent gas, revealing the dense, hot, and tiny white dwarf star at its core. The other 5% of stars — that is, those born with masses more than eight times larger than our Sun — end their lives as supernovae.
Planetary nebulae (“PNe”) are formed as an aging star becomes unstable, vibrates, and ejects its outer envelope containing up to about half of its original mass or more. PNe come in many wonderful shapes and structures. The shaping process is not well understood. The reason is simple: round stars should eject their envelopes as spherical bubbles. Most pNe are highly symmetric but not round or spherical.
The answer to the ejection process begins with careful models of the outflow patterns of the ejected gas. To this end I run “hydrodynamic” models of gas flows using the same physics that Boeing uses to model air flows over airplane wings. The biggest difference in these types o models is that the speeds of ejected gas are up to 400 miles per second rather than 400 miles per hour. The differences are important.
The image below shows the structure of pne outflow. To its right are panels that show the results of two different types of models that we use to explain them. Compare the predicted density structure (negative black & white inset) to the images. The models also predict the temperatures of the flows (up to 10,000,000 degrees, the outflow speed pattern, and the way in which gas from the ejected gas at the head of the flow mixes with ambient gas the predated the ejection and its outflows.
Another facet of my research is the chemical abundances of PNe as measured from observations of features of carbon, nitrogen helium, hydrogen, oxygen, and other elements found in their spectra. PNe are by far them most prodigious producers of carbon in the Universe. Our work obviously relates to the questions of the chemical origins of life and the supply of fresh dust particles from which planets are formed.