2001-2002 Colloquium Series
NAU Physical Sciences (Bldg 19, Rm 321), Thursday, 28 March 2002, 4:00 PM
(Refreshments at 3:45 PM)
THE LIFE CYCLE OF DUST: BIRTH AROUND EVOLVED STARS
Angela Speck , University of Illinois at Urbana/Champaign.
Abstract
The dust that forms around evolved stars is blown out into the interstellar medium (ISM) and becomes incorporated into molecular clouds and star forming regions, and is eventually included in new stellar systems and planets including our solar system. The aim of my research is to understand the origins and "life cycle" of dust in space.
Intermediate-mass stars (0.8 - 8.0 solar masses) eventually evolve off the main sequence and end their lives on the asymptotic giant branch (AGB). The intensive mass loss that characterizes the AGB produces a circumstellar shell of dust and neutral gas. During the AGB phase the mass loss is expected to vary, which can lead to changes in density within the dust shell. Furthermore, strong convection currents within the star allow newly-nucleosynthesized elements to be dredged up to the surface, thus changing the chemical composition of the dust shell. At the end of the AGB, the mass loss stops and the dust shell continues to drift off into space. The circumstellar dust shells of AGB and post-AGB stars contain the fossil record of AGB mass loss.
There are two observational aspects of dust around evolved stars that I will address: (1) the variations in mass loss with stellar evolution; and (2) the composition of the dust grains.
Using far-infrared observations obtained from the Infrared Space Observatory, I have discovered extremely large dust shells around two post-AGB stars (AFGL 2688 and AFGL 618). The radial profiles of these dust shells suggest that episodic mass loss has occurred with mass-loss enhancements on timescales corresponding to theoretical predictions of thermal pulses on the AGB.
In the second approach to understanding circumstellar dust, I investigated the composition and mineralogy of dust around evolved stars by comparing the infrared spectra of candidate materials in the laboratory to the observed spectra of dusty stars. Furthermore, pristine samples of circumstellar dust have been found in meteorites (presolar grains). By comparing the properties of these presolar grains with observed infrared properties of the circumstellar dust, I have succeeded in reconciling astronomical and meteoritic data with respect to silicon carbide, one of the most important astrominerals.
Using this dual approach to understanding the formation and evolution
of dust around evolved stars we begin to understand the nature of dust
as it begins its journey through the galaxy.
Local Host: Steve Tegler, (928) 523-9382.