TPF-I SWG Report - Exoplanet Exploration Program - NASA

G ENERAL A STROPHYSICS
Figure 3-5. Schematic of the atmospheric spatial structure and temperature profile of a
typical Mira star (Reid and Menten 1997).
be responsible for anomalous metal-abundances in white dwarf atmospheres. TPF-I/Darwin
interferometric imaging may resolve these disks, determine their structure, and constrain their
compositions. Such observations may shed independent light on the abundances of planetary systems and
that have been destroyed millions to billions of years ago.
AGB mass-loss (e.g., Zijlstra et al. 2006) determines the mass distribution of stellar remnants, including
the lower mass limit of type II supernovae progenitors. Stellar mass loss also drives Galactic evolution
through replenishment and chemical enrichment of the ISM. Such mass loss contributes roughly half the
total gas recycled by all stars (Maeder 1992), creates an amount of carbon roughly equal to that produced
by supernovae and Wolf-Rayet stars (Dray et al. 2003; Gavilán, Buell, and Mollá 2005), and is the main
source of carbonaceous interstellar dust (Dwek 1998; Edmunds 2001).
TPF-I/Darwin will resolve AGB stars at the distance of the Galactic center. These observations will
directly measure the impacts of the high-pressure Galactic-center environments, radiation, and outflows
on the structure of AGB star envelopes. VLA and ground-based [NeII] observations have already
revealed cometary tails around some evolved stars such as IRS7 (Yusef-Zadeh and Morris 1991).
Interferometry will enable the study of such tails to be used as diagnostics of the environments.
There are about 200 moderately evolved AGB stars (Mira variables) known within 1 kpc. Both oxygenrich
(M-type) and carbon-rich (C-type) AGB stars show spectral features around 9.7 µm and 11.3 µm due
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