Modelling the spinning dust emission from dense interstellar clouds
  Electric dipole emission arising from PAHs is often invoked to explain the
anomalous microwave emission (AME). This assignation is based on an observed
tight correlation between the mid-IR emission of PAHs and the AME; and a good
agreement between models of spinning dust and the broadband AME spectrum. So
far often detected at large scale in the diffuse interstellar medium, the AME
has recently been studied in detail in well-known dense molecular clouds with
the help of Planck data. While much attention has been given to the physics of
spinning dust emission, the impact of varying local physical conditions has not
yet been considered in detail. Our aim is to study the emerging spinning dust
emission from interstellar clouds with realistic physical conditions and
radiative transfer. We use the DustEM code from Compiegne et al. to describe
the extinction and IR emission of all dust populations. The spinning dust
emission is obtained with SpDust, as described by Silsbee et al., that we have
coupled to DustEM. We carry out full radiative transfer simulations and
carefully estimate the local gas state as a function of position within
interstellar clouds. We show that the spinning dust emission is sensitive to
the abundances of the major ions and we propose a simple scheme to estimate
these abundances. We also investigate the effect of changing the cosmic-ray
rate. In dense media, where radiative transfer is mandatory, we show that the
relationship between the spinning and mid-IR emissivities of PAHs is no longer
linear and that the spinning dust emission may actually be strong at the centre
of clouds where the mid-IR PAH emission is weak. These results provide new ways
to trace grain growth from diffuse to dense medium and will be useful for the
analysis of AME at the scale of interstellar clouds.
