2007-2008 Colloquium Series
NAU Physical Sciences (Bldg 19, Rm 321), Monday, March 3, 2008, 4:00 PM
(Refreshments at 3:45pm)
The Nature of Stardust: Astromineralogy and Circumstellar Dust Around Evolved Stars
Angela Speck
University of Missouri
Abstract
At the beginning of the Universe, all matter was in the form of hydrogen and helium: all elements bigger than helium form via nuclear fusion in stars. These newly-formed elements are ejected from stars either explosively (in the case of supernovae) or more gently over a few hundred thousand years for lower mass star like the sun. These new elements then become part of the interstellar medium, from which new stars and their planets form. Dust is a vital ingredient in understanding many astrophysical processes. It is an essential part of star formation processes; it is the key to understanding mass loss from aging stars; and it contributes to several aspects of interstellar processes such as gas heating and the formation of molecules. Such a crucial and ubiquitous constituent of our Universe needs to be well understood in its own right, if we are to understand its contributions to many aspects of astrophysics.
Intermediate-mass stars (0.8-8.0 solar masses) are major contributors of new elements to interstellar space. These stars eventually evolve into asymptotic giant branch (AGB) stars. During the AGB phase, these stars suffer intensive mass loss leading to the formation of circumstellar shells of dust and neutral gas, including the new elements formed during the star's life. Eventually the star runs out of material to lose, and the central core collapses and heats up. Meanwhile the material around the star (the circumstellar shell) drifts away from the star. Once the central star is hot enough to have significant ultraviolet (UV) emission it will begin to ionize the surrounding medium, and it becomes a planetary nebula.
Using a combination of observing techniques (e.g. infrared (IR) spectroscopy, visible, IR and sub-mm imaging) and laboratory IR studies, together with theoretical considerations (e.g. kinetics and thermodynamics of the dust-forming region; nucleosynthesis models and changing stellar chemistries) and meteoritic evidence, it is possible to investigate the structure and evolution of the circumstellar dust and its environment and how the evolution of AGB stars (in terms of chemistry, mass-loss rates, dust shell dispersion and the change from benign AGB star to ionized planetary nebulae), leads to changes in the dust composition and distribution.
Local Host: Dave Cornelison, (928) 523-7641.