Spitzer Space Telescope - Theoretical Research Proposal #30197


Modeling the PAH Emission Spectra of Protoplanetary and Debris Disks

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

Co-Investigators: 
J.I. Lunine, University of Arizona                                    

Science Category: circumstellar/debris disks 
Dollars Approved: 82239.0 

Abstract:
The 3.3, 6.2, 7.7, 8.6 and 11.3 micron emission features of polycyclic
aromatic hydrocarbon (PAH) molecules have been detected in
protoplanetary disks around Herbig Ae/Be stars and T Tauri stars and
in debris disks around main-sequence stars.  PAHs play an important
role in the thermal budget and chemistry of the gas in the disk, by
providing photoelectrons for heating the gas and large surface areas
for chemical reactions.  Stochastically heated by a single UV/visible
photon, the PAH emission is spatially more extended than large grains
and therefore, the disks can be more easily resolved at the PAH
emission bands.  We propose to model the PAH emission spectra of
protoplanetary and debris disks obtained by Spitzer and ISO.  We will
first calculate the temperature probability distribution functions
dP/dT for both neutral and ionized PAHs of a wide range of sizes, at a
wide range of radial distances (from the central star) in disks
illuminated by stars of a wide range of spectral types.  By modeling
the PAH emission of dust disks, we will be able (1) to derive the
abundance, size and spatial distributions of PAHs;  (2) to derive the
PAH photoelectric heating rates which dominate the gas heating in the
disk surface layers;  and (3) to see how the abundance and properties
of the PAHs vary among disks at different evolutionary stages and
illuminated by stars of different parameters (e.g.  luminosity,
spectral type).  This program will create a web-based ``library'' of
the temperature distribution functions dP/dT of PAHs (and their
emission spectra and photoelectric heating rates) as a function of
size, charge state, and radial distance in disks illuminated by stars
of different spectral types.  This library, a useful tool for
interpreting the PAH emission features of dust disks obtained by
Spitzer and for understanding the disk chemistry, will be made
publicly available by April 2007 via the WWW at
http://www.missouri.edu/~lia/.