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B.T. Draine (Princeton University Observatory)
The infrared emission spectrum of interstellar clouds requires the existence of a large number of ultrasmall dust grains, so small that single-photon heating can produce vibrational temperaturess as high as ~300 K. These grains must therefore be extremely small, consisting of only ~30 -- 1000 atoms, and they must be sufficiently numerous to account for ~0% of the total absorption of starlight in diffuse clouds. It is obviously immaterial whether one chooses to call these ``grains'' or ``molecules'' -- they are both, of course.
In addition to being vibrationally excited by starlight, these ultrasmall grains will be rotationally excited by a number of processes. Since they will generally have electric dipole moments, they will produce rotational electric dipole emission. The spectrum of this emission is expected to peak in the 10-30 GHz region (Draine & Lazarian 1998, ApJ 508, 157). The emission will consist of many discrete lines, but they will be difficult to resolve. The rotational excitation of these grains will be discussed.
Sensitive experiments to measure the angular structure in the cosmic microwave background radiation appear to have already detected this rotational emission from ultrasmall interstellar dust grains. Future CMBR missions, such as MAP and Planck, will have much to tell us about this interstellar grain population.
This research was supported in part by NSF grant AST-9619429.
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