Improving the model of emission from spinning dust: effects of grain
  wobbling and transient spin-up
  Observations continue to support the interpretation of the anomalous
microwave foreground as electric dipole radiation from spinning dust grains as
proposed by Draine and Lazarian (1998ab). In this paper we present a refinement
of the original model by improving the treatment of a number of physical
effects. First, we consider a disk-like grain rotating with angular velocity at
an arbitrary angle with respect to the grain symmetry axis and derive the
rotational damping and excitation coefficients arising from infrared emission,
plasma-grain interactions and electric dipole emission. The angular velocity
distribution and the electric dipole emission spectrum for grains is calculated
using the Langevin equation, for cases both with and without fast internal
relaxation. Our results show that, the peak emissivity of spinning dust,
compared to earlier studies, increases by a factor of ~2 for the Warm Neutral
Medium (WNM), the Warm Ionized Medium (WIM), the Cold Neutral Medium (CNM) and
the Photodissociation Region (PDR), and by a factor ~4 for Reflection Nebulae
(RN). The frequency at the emission peak also increases by factors ~1.4 to ~2
for these media. The increased emission and peak frequency result from the
non-sphericity of grain shape and from the anisotropy in damping and excitation
along directions parallel and perpendicular to the grain symmetry axis. Second,
we provide a detailed numerical study including transient spin-up of grains by
single-ion collisions. The impulses broaden the emission spectrum and increase
the peak emissivity for the CNM, WNM and WIM. In addition, we present an
improved treatment of rotational excitation and damping by infrared emission.
