Herschel Space Observatory - Theoretical Research Proposal #1015

Modeling the Far-IR/Submm Emission Properties of Interstellar Dust

Principal Investigator: Aigen Li       
           Institution: University of Missouri

Dollars Approved: 110637

Abstract:
Interstellar dust is a powerful source of far-infrared (FIR) and submm emission. An
accurate knowledge of the dust FIR/submm emissivity, an important parameter
characterizing its physical properties, is crucial for interpreting the FIR/submm
observations of emission from dust, tracing the physical conditions and determining
the masses of the emitting regions, and for understanding the heating, cooling and
dynamical behavior of the interstellar medium, as well as the process of star
formation.
Various observations have recently suggested an emissivity enhancement or
flattening at long wavelengths above that from the canonical frequency squared
power-law. This emissivity enhancement or flattening is generally attributed to (i)
grain coagulational growth, and formation of ices on grain surfaces in dense
regions, (ii) amorphous dust with disordered internal structure, or (iii) impurity
atoms in bulk grains. The latter has been invoked to explain the so-called "very
cold emission" (about 4-7 K) detected on galactic scales both in the Galaxy and in
external galaxies.  
We propose a Herschel Theory Program (1) to construct the dielectric functions from
X-ray to millimeter wavelengths for cosmic dust candidate materials of a range of
temperatures; (2) to calculate the FIR/submm emissivities of porous aggregated dust
grown by agglomeration in dense regions; and (3) to perform, by the first time, a
quantitative study of the effects of impurities on the FIR/submm emissivities of
dust. These results will be used to study (i) the FIR/submm emissivity increase and
flattening observed in dense clouds and protoplanetary disks; and
(ii) the nature of the "very cold" submm/mm excess emission observed in the Milky
Way and other galaxies. These results will serve as a useful tool for interpreting
Herschel observations of dusty regions. 
This project will create a web-based
library of opacity/dielectric function database for porous aggregated dust and dust
with impurities. This library will be made publicly available by December 2009 on
the internet at http://www.missouri.edu/~lia/. We note that the widely-used
opacities of Pollack et al. (1994) and Henning & Stognienko (1996) were obtained
using the optical constants of tholin organics (made from DC discharge of gas
mixture of 90% N_2 and 10% CH_4) which are optically very different from amorphous
carbon considered in this research.