Spitzer Space Telescope - General Observer Proposal #40848


Crystalline silicates in Bulge AGB stars

Principal Investigator: Jan Cami
           Institution: SETI Institute

     Technical Contact: Jan Cami, SETI Institute

Co-Investigators: 
Joris Blommaert, Insititute for Astronomy, KU Leuven                  
Alexander Tielens, NASA Ames Research Center                          
Kay Justtanont, Stockholm Observatory                                 
Evelien Vanhollebeke, Institute for Astronomy, KU Leuven              
Ciska Kemper, University of Manchester                                

Science Category: evolved stars/pn/sne 
 Observing Modes: IrsStare 
  Hours Approved: 16.5 

Abstract:
Mid-IR spectroscopic observations of O-rich AGB stars generally show the
presence of copious amounts of circumstellar dust, often characterized by strong
emission or absorption due to amorphous silicates. Detectable fractions of
crystalline silicates have so far only be found in stars with high mass loss
rates. Here we propose to spectroscopically observe -- using Spitzer-IRS from
9.9 -- 37.2 micron at high-resolution -- a homogeneous sample of 13 O-rich AGB
stars spanning a large range in mass loss rates, and which all show tantalizing
evidence for the presence of crystalline silicates in their low resolution
spectra. The target stars are selected from a sample of O-rich AGB stars in the
Galactic Bulge that was observed with IRS at low resolution with the aim to
study the variations in the dust composition as a 1.5 solar mass star evolves
along the AGB. The proposed observations will allow us to i) unambiguously
confirm the presence of crystalline silicates in AGB stars with low- to
intermediate mass loss rates; ii) determine at what phase in the AGB crystalline
silicates are first formed and hence where they fit in the dust condensation
sequence for O-rich AGB stars iii) determine the temperature and chemical
composition of these crystalline silicates and hence iv) understand how these
crystalline silicates are formed and how they evolve as they sail away from the
central star; and finally v) what stellar parameters are important for the
efficient formation of crystalline silicates.