Spitzer Space Telescope - Archive Research Proposal #50257


IR Spectroscopy of PANHs in Dense Clouds

Principal Investigator: Louis Allamandola
           Institution: NASA Ames Research Center

     Technical Contact: Louis Allamandola, NASA Ames Research Center

Co-Investigators: 
Andrew Mattioda, NASA Ames Research Center                            
Scott Sandford, NASA Ames Research Center                             

Science Category: ISM 
Dollars Approved: 75000 

Abstract:
Interstellar PAHs are likely to be frozen into ice mantles on dust grains in
dense clouds. These PAHs will produce IR absorption bands, not emission
features. A couple of very weak absorption features in ground based spectra of a
few objects in dense clouds may be due to PAHs. It is now thought that aromatic
molecules in which N atoms are substituted for a few of the C atoms in a PAH's
hexagonal skeletal network (PANHs) may well be as abundant and ubiquitous
throughout the interstellar medium as PAHs. Spaceborne observations in the 5 to
8 um region, the region in which PAH spectroscopy is rich, reveal unidentified
new bands and significant variation from object to object. It is not possible to
analyze these observations because lab spectra of PANHs and PAHs condensed in
realistic interstellar ice analogs are lacking. This lab data is necessary to
interpret observations because, in ice mantles, the surrounding molecules affect
PANH and PAH IR band positions, widths, profiles, and intrinsic strengths.
Further, PAHs (and PANHs?) are readily ionized in pure H2O ice, further altering
the spectrum. This proposal starts to address this situation by studying the IR
spectra of PANHs frozen in laboratory ice analogs that reflect the composition
of the interstellar ices observed in dense clouds. Thanks to Spitzer Cycle-4
support, we are now measuring the spectra of PAHs in interstellar ice analogs to
provide laboratory spectra that can be used to interpret IR observations. Here
we propose to extend this work to PANHs. We will measure the spectra of these
interstellar ice analogs containing PANHs before and after ionization and
determine the band strengths of neutral and ionized PANHs in these ices. This
will enable a quantitative assessment of the role that PANHs can play in the 5-8
um spectrum of dense clouds and address the following two fundamental questions
associated with dense cloud spectroscopy and chemistry: 1- Can PANHs be detected
in dense clouds? 2- Are PANH ions components of interstellar ice?