Spitzer Space Telescope - Theoretical Research Proposal #50565


Identification of PAHs in Spitzer/IRS Spectra of the Icy Environments of YSOs:
A Laboratory Study of PAHs embedded in H2O and D2O Ices

Principal Investigator: Satyanarayana Gudipati
           Institution: Jet Propulsion Laboratory

     Technical Contact: Satyanarayana Gudipati, Jet Propulsion Laboratory

Co-Investigators: 
Michael Werner, Jet Propulsion Laboratory                             
Louis Allamandola, NASA Ames Research Center                          
Karl Stapelfeldt, Jet Propulsion Laboratory                           
Adwin Boogert, California Institute of Technology/IPAC                

Science Category: ISM 
Dollars Approved: 125000 

Abstract:
Ices are the main reservoir of volatiles in dense clouds and the envelopes and
disks surrounding Young Stellar Objects (YSOs). It is expected that the
universally observed Polycyclic Hydrocarbon molecules (PAHs) freeze out on these
ice grains. Laboratory experiments have shown that processing of these ices
forms species of astrobiological interest. Spitzer/IRS spectra of over 40 YSOs
have been analyzed to find several unassigned spectral features that may be due
to PAHs embedded in H2O ices [Boogert et al. 2007]. However, lack of
quantitative spectral data such as line widths, band shapes and absolute band
depths for neutral and ionized PAHs in H2O ices hindered further identification.
Recently we have shown that ionized PAHs are stable in astrophysical ices
[Gudipati and Allamandola, 2004]. Bernstein et al. [2007] have measured spectra
of neutral and ionized PAHs in H2O ices, covering only a narrow spectral region
(7-10 microns), omitting the strong 6.2 and 11.3 micron bands essential to
analyze the Spitzer/IRS spectra and the 3.3 micron band in complementary
ground-based spectra. In order to quantify the PAH content, absolute band depths
are needed, which were missing in this publication. We propose to extend the
measurements of Bernstein et al. to derive quantitative laboratory mid-infrared
(2.5-50 microns, including the Spitzer/IRS spectral range of 5?38 microns) data
such as absolute line strengths, positions and widths of neutral and ionized
PAHs in H2O and D2O ices. PAH spectra will be deconvolved from the ice-host
spectra in the low-extinction regions of the H2O and D2O hosts. A range of PAH
sizes will be used, one PAH at a time, embedded in the ice. The influence of CO2
on the PAH bands will also be quantified. These laboratory data will then be
used to fit the unidentified features of already processed Spitzer/IRS data of
icy mantles of over 40 YSOs and of spectra of background stars tracing quiescent
cloud material covered in numerous legacy, GTO, and GO programs.