AAS 195th Meeting, January 2000
Session 74. ISM: Dust

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[74.03] Microwave Emission from Interstellar Grains: The Rotational Distribution Function

B. T. Draine, A. Li (Princeton University Observatory)

Studies to map the cosmic microwave background radiation have discovered "galactic foreground" microwave emission correlated with interstellar matter. Draine & Lazarian (1998a,b; hereafter DL98a,b) have proposed that this emission is electric dipole radiation from rapidly-rotating ultrasmall a \lesssim 15Å\ interstellar grains, and have shown that the population of ultrasmall grains previously indicated by infrared observations should in fact radiate in the microwave with approximately the observed intensity.

The rotational excitation and damping of these ultrasmall grains is due to a variety of processes, including damping by electric dipole radiation, excitation and damping by collisions with ions and neutrals, excitation and damping due to long-range interactions with passing ions (``plasma drag''), and excitation and damping resulting from absorption of optical-UV photons and emission of IR photons. DL98b estimated the rotational emission by assuming that the grains of a given radius would have a Maxwellian distribution of rotational velocities, but this is only a rough approximation since the excitation and damping of rotation is nonthermal.

We obtain the rotational distribution function using a semiclassical calculation, including all of the important physical processes. We show how, for each grain size, the different physical processes contribute to the "rotational transition matrix", and we then solve numerically for the steady-state rotational populations. The microwave emission spectrum is computed from the rotational distribution functions. The overall emission spectra are similar to the estimates of DL98b, but there are some differences.

The microwave emission spectrum is predicted for realistic grain size distributions, and found to be in good agreement with recent observations.

This research was supported in part by NSF grant AST-9619429 and NASA grant NAG5-7030.

References: Draine, B.T., & Lazarian, A. 1998a, ApJ, 494, L19 (DL98a); Draine, B.T., & Lazarian, A. 1998b, ApJ, 508, 157 (DL98b)

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