PI: Benjamin Sargent (preferred name: Beth), sargent@stsci.edu, 410-338-6887, Muller N421D
Project Duration: This project is suitable for a graduate student, and it is suitable as a thesis project or as a 1 year rotation project
Project Abstract: We will investigate a series of cosmic dust candidates based on alumina polymorphs. We will (1) produce, characterize, and determine optical properties of stardust analogs composed of aluminum and oxygen; and (2) apply these optical properties to understanding dust formation around low- and intermediate-mass evolved stars. Dust in space is investigated via the analysis of analog materials in the laboratory and application of the results to astrophysical models. In particular, we compare lab-measured spectral features to those seen in observational (mostly infrared) spectra. A recent study (Sargent 2018, The Astrophysical Journal Letters, 866, L1) showed that various polymorphs (alternate crystal forms of the same composition) of alumina (Al2O3) give rise to previously unidentified emission features at 11, 13, 20, 28, and 32 microns wavelength in spectra of asymptotic giant branch (AGB) stars. However, relatively few laboratory measurements of the optical properties of alumina polymorphs exist. We propose to remedy this by preparing samples of alumina polymorphs by multiple different means and studying these samples using multiple laboratory techniques.
New dust analogs will be prepared using four synthesis methods: smoke production, heating of hydrated aluminous solid, laser ablation, and a hot filament reactor. These samples will be characterized structurally and chemically by electron microscopy, and spectroscopically in the visible to mid-IR, in order to determine their complex dielectric functions. Differential Scanning Calorimetry will provide thermodynamic information. Some of these polymorphs are metastable and thus difficult to generate on Earth but may be prevalent in space. Using measured alumina polymorph opacities and dielectric functions, we will construct models to compare with observed astronomical data spanning a range of wavelengths, from the optical to the mid-infrared. Existing data includes Spitzer Space Telescope and Infrared Space Observatory mid-infrared spectra of AGB stars. We will also use the spectroscopic and thermodynamic lab data to test and refine theoretical models for how dust forms around AGB stars.
Student Work: This project is arranged so that the work performed by our colleagues at the University of Texas at San Antonio (UTSA) will determine optical properties from laboratory measurements of samples, while the work performed at Johns Hopkins University will apply these optical properties to studying mass loss from AGB stars. It is anticipated that the graduate student would perform spectral modeling, including radiative transfer modeling, of astronomical spectroscopy and photometry of AGB stars. It is possible that the graduate student would also need to reduce spectroscopic and other astronomical data. After performing these studies, the graduate student would then publish the results of their studies in a peer-reviewed journal. Additionally, the graduate student would present their research to colleagues at conferences and/or other meetings, whether by poster or by talk.
