Spinning Dust Emission: Effects of irregular grain shape, transient heating and comparison with WMAP results

Abstract

Planck is expected to answer crucial questions on the early Universe, but it also provides further understanding on anomalous microwave emission. Electric dipole emission from spinning dust grains continues to be the favored interpretation of anomalous microwave emission. In this paper, we present a method to calculate the rotational emission from small grains of irregular shape with moments of inertia $I_{1}> I_{2}> I_{3}$. We show that a torque-free rotating irregular grain with a given angular momentum radiates at multiple frequency modes. The resulting spinning dust spectrum has peak frequency and emissivity increasing with the degree of grain shape irregularity, which is defined by $I_{1}:I_{2}:I_{3}$. We discuss how the orientation of dipole moment \bmu in body coordinates affects the spinning dust spectrum for different regimes of internal thermal fluctuations. We show that the spinning dust emissivity for the case of strong thermal fluctuations is less sensitive to the orientation of \bmu than in the case of weak thermal fluctuations. We calculate spinning dust spectra for a range of gas density and dipole moment. The effect of compressible turbulence on spinning dust emission intensity is investigated. We show that the emission intensity in a turbulent medium increases by a factor from 1.2-1.4 relative to that in a uniform medium, as sonic Mach number $M_{s}$ increases from 2-7. Finally, spinning dust parameters are constrained by fitting our improved model to five-year {\it Wilkinson Microwave Anisotropy Probe} cross-correlation foreground spectra, for both the H$\alpha$-correlated and 100 $\mu$m-correlated emission spectra.Comment: 24 pages, 17 figures, relation to molecular rotation spectra added, accepted by Astrophysical Journa