An Interferometric Snapshot Survey to Constrain Mass-Loss Dynamics and Physics in AGB Stars

We propose Spitzer IRS observations of a unique sample of 25 AGB (mira) variable stars with interferometrically determined K-band angular diameters and spectrophotometry taken simultaneously using the Palomar Testbed Interferometer. These high-resolution spectral and spatial observations, in conjunction with amateur light-curves (e.g. AAVSO), will be used together to place constraints on the location of the IR stellar photosphere and the nature of the molecules and dust in the circumstellar environment (CSE). We can then develop complete models of the stars and CSE, using our state-of-the-art hydrodynamic atmosphere codes (ATLAS/PANDORA) and radiative transfer code (DUSTY). This will allow us to undertake the most accurate modeling of these highly dynamic environments that has yet been done for evolved stars. With results from these observations we will answer several key questions about these stars including: 1) how important are the roles of NLTE atmospheres in the formation of dust?; 2) what dust species are relevant to a given AGB dust morphology and do these change as a result of the pulsational cycle?; 3) is dust created or destroyed during the pulsational cycle and how does this relate to the abundances of other molecules in the CSE?; and finally, 4) what is the physical location of the dust production around an AGB star which acts to define the age-dependent spectral-energy distribution? These types of observations require a high-level of spectroscopic fidelity, repeatability and sensitivity which is unable to be executed under conditions of telluric contamination. Therefore these types of observations can only be performed with Spitzer's IRS instrument.