Spitzer Space Telescope - General Observer Proposal #30430


Do s-Process Enhanced Planetary Nebulae Have Unusual Dust Emission
Spectra?

Principal Investigator: Harriet Dinerstein
           Institution: The University of Texas at Austin

Co-Investigators: 
Nicholas Sterling, The University of Texas at Austin                  
Kris Sellgren, Ohio State University                                  

Science Category: evolved stars/pn/sne 
 Observing Modes: IrsStare 
  Hours Approved: 7.5 

Abstract:
We propose to obtain IRS observations of the mid-infrared dust
emission of a sample of Galactic planetary nebulae (PNs) which are
known to have enrichments of elements produced in the precursor star
by slow neutron-capture nucleosynthesis (the "s-process").  These
enhanced abundances result from captures of free neutrons by Fe-peak
nuclei following by convective mixing during the AGB;  this "third
dredge-up" is also responsible for increasing the surface abundance of
carbon.  Since PNs are the descendants of AGB stars and are often
C-rich, it is not surprising that we find substantial enrichments of
s-process products such as Ge, Se, and Kr in some PNs.  Despite their
low initial abundances, 1e-9 to 1e-10  times H, modest enrichments of
neutron-capture elements can have observable effects.  The spectral
type S, a transitional class between O-rich and C-rich AGB stars, is
characterized by prominent ZrO bands;  Zr is produced in the
s-process.  We have attempted, without success, to detect gas-phase Zr
in PNs.  However, Zr is highly refractory.  It can condense into ZrO2
or be incorporated into high-temperature rocky condensates in O-rich
environments, while in C-rich environments it may form metallic
carbides (i.e.  ZrC, an analog of TiC).  Indeed, Zr-Mo carbide
inclusions found in some meteoritic presolar grains are thought to
originate in the atmospheres of C-rich AGB stars.  Other refractory
s-process products (e.g.  Sr, Ba) may also be incorporated into
grains.  High-quality Spitzer spectra of the dust emission in a set of
PNs with known s-process enhancements - determined by us from
gas-phase measurements of undepleted elements - will be valuable for
comparison with laboratory spectroscopy of grain analogs.  These
comparisons will help determine whether the dredge-up of n-capture
products affects the dust chemistry of PNs and may offer some new
insights into the dust composition.