Joseph C. Weingartner 1 & Bruce T. Draine 2
1 Physics Dept., Jadwin Hall, Princeton University, Princeton, NJ 08544, USA
2 Princeton University Observatory, Princeton, NJ 08544, USA
Observations of molecular hydrogen with the ISO SWS have yielded valuable information about the physical conditions within photodissociation regions (PDRs). Of particular interest have been the surprisingly high values of gas temperature (500 to 1000 K) inferred from the H2 rotational distribution function. It is not yet clear what heating processes are able to maintain partially-molecular gas at such high temperatures.
Photoelectric emission from dust grains is expected to be a major heating mechanism in PDRs. Here we consider the possibility of enhanced dust to gas ratios in PDRs. The anisotropic ultraviolet radiation field illuminating the PDR will result in a force on the grains, due both to the direct absorption and scattering of radiation and to the recoil accompanying anisotropic photoelectric emission.
The photoelectric recoil force resulting from anisotropic photoelectric emission has been estimated using Mie theory for the grain optics and a simple model for the photoelectric emission process. The force depends on the grain charge, which has been evaluated self-consistently. We find that the total force resulting from anistropic illumination may be large enough to produce substantial gas-grain drift, resulting in an increased dust-to-gas ratio in the PDR. For some well-known PDRs, we estimate the magnitudes of these forces and determine whether or not they are important for the grain dynamics, and estimate the resulting rate of photoelectric heating.
We explore the complicated dynamics which results when the total force on grains located at the ionization front is large enough to push the grains away from the ionization front, deeper into the PDR. We estimate the amount of gas heating expected due to photoelectric emission in these circumstances.