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J.C. Weingartner (CITA, University of Toronto), B.T. Draine (Princeton University Observatory)
Photoelectric emission from dust grains dominates the heating of diffuse interstellar gas clouds as well as the photodissociation regions at the surfaces of molecular clouds. This mechanism therefore plays an important role in the dynamical evolution of the interstellar medium, and in shaping the line emission spectra of galaxies. Photoelectric heating was first considered by Spitzer (1948), and there have since been a number of reassessments of the heating rate.
For a given grain, the heating rate depends on the grain size, composition, and charge state, as well as on the spectrum of the illuminating radiation. Because photoelectric yields are enhanced for small grains (Watson 1972, 1973), estimates of the net photoelectric heating rate in interstellar gas are sensitive to the adopted grain size distribution, which should be consistent with the observed extinction curve (which shows strong regional variations) as well as with the observed emission from interstellar dust grains, from the near-infrared to the microwave. Previous estimates for photoelectric heating rates did not always use grain size distributions which were consistent with these constraints. While the measured extinction curve by itself does not suffice to uniquely specify the grain size distribution, in the present study we will consider size distributions which are consistent with the observed extinction in different regions, with either the minimum or maximum permissible population of ultrasmall grains.
In addition to using size distributions consistent with observations, we also model the photoelectric emission process and associated grain charging in detail, using realistic yields for graphitic and silicate grains, a realistic distribution of photoelectron kinetic energies, and new estimates for electron sticking efficiencies for small grains. The resulting photoelectric heating rates are calculated for grain size distributions consistent with extinction curves characteristic of diffuse clouds (RV\equiv AV/EB-V \approx 3.1), intermediate density regions (RV \approx 4.0), and dense clouds (RV \approx 5.5). We provide fitting functions which reproduce the numerical results.
This research was supported in part by NSF grant AST-9619429.
The author(s) of this abstract have provided an email address for comments about the abstract: weingart@cita.utoronto.ca