Spitzer Space Telescope - General Observer Proposal #20654


EP Aqr:  the Rosetta rocks of astro-archeology

Principal Investigator: Jan Cami
           Institution: NASA Ames Research Center

Co-Investigators: 
Xander Tielens, Kapteyn Institute Groningen (The Netherlands)         
Els Peeters, NASA Ames Research Center                                

Science Category: evolved stars/pn/sne 
 Observing Modes: IrsMap IrsStare 
  Hours Approved: 2.1 

Abstract:
The evolution of stars on the Asymptotic Giant Branch (AGB) is
characterized by substantial mass loss, producing large amounts of
circumstellar dust and a dust-driven outflow, which injects the
stellar ejecta into the interstellar medium.  The composition of this
newly formed dust is known to vary substantially from source to
source, which is generally thought to reflect differences in the mass
loss rate (e.g., density).  During the evolution of the star on the
AGB, the mass loss rate is known to vary substantially leading to
large density variations and detached shell structures.  Observations
of bright examples of such shells provide us thus with the opportunity
to dig into the past history of the mass loss processes from these
objects.  We propose to spectroscopically map the extended but bright
outflow of the nearby AGB star, EP Aqr, with Spitzer-IRS from 5.2 --
38 micron.  In this way we will be able to trace the composition and
mass of the dust formed at different times (e.g., corresponding to
thousands of years of mass loss history).  The IR spectrum of EP Aqr
contains the spectral fingerprints of nearly all minerals (alumina,
spinel, magnesium-iron oxides, amorphous and possibly crystalline
silicates, gehlenite, ...) found in the dust shells surrounding O-rich
AGB stars, which makes it an ideal target for such a dust
astro-archeology study.  The proposed observations will therefore
directly link for the first time the spectral differences in dust
composition to the mass loss history of the central star.  Such
observations will also provide important clues to large-scale
structure of AGB star outflows as well as to theoretical models for
dust formation and mass-loss mechanisms.