Spitzer Space Telescope - Theoretical Research Proposal #40674


Modeling the Infrared Emission Spectra of Specific PAH Molecules in Interstellar
Space

Principal Investigator: Aigen Li
           Institution: University of Missouri, Columbia

     Technical Contact: Aigen Li, University of Missouri, Columbia

Science Category: ISM 
Dollars Approved: 87146 

Abstract:
The 3.3, 6.2, 7.7, 8.6 and 11.3 micron emission features ubiquitously seen in a
wide variety of Galactic and extragalactic objects, are generally attributed to
polycyclic aromatic hydrocarbon (PAH) molecules. Although the PAH hypothesis is
quite successful in explaining the general pattern of the observed emission
spectra, so far there is no actual precise identification of a single specific
PAH molecule in interstellar space. Therefore, when modeling the observed PAH
emission spectra, astronomers usually take an empirical approach by constructing
"astro-PAHs" which do not represent any specific material, but approximate the
actual absorption properties of the PAH mixture in astrophysical regions. We
propose a Spitzer Theory Program to study the photoexcitation of specific PAH
molecules and their ions in interstellar space, taking a statistical-mechanical
(instead of thermal) approach. For most of the specific PAH molecules selected
for this research (with a small number of vibrational degrees of freedom),
thermal approximation is not valid. Using available laboratory and
quantum-chemical data (e.g. vibrational frequencies, UV/visible/IR absorption
cross sections), we will calculate the emission spectra of 21 representative
specific PAH molecules and their ions, ranging from naphthalene to
circumcoronene, illuminated by interstellar radiation fields of a wide range of
intensities. This program will create a web-based "library" of the emission
spectra of 21 specific PAH molecules and their ions as a function of starlight
intensities. This "library" will be made publicly available by October 2008 on
the internet at http://www.missouri.edu/~lia/. By comparing observed PAH spectra
with model spectra produced by co-adding the emission spectra of different PAH
molecules available in this "library" (with different weights for different
species), one will be able to estimate the total PAH mass and relative
abundances of each PAH species, using real PAH properties.