Dust variations in the diffuse interstellar medium: constraints on Milky
  Way dust from Planck-HFI observations
  The Planck-HFI all-sky survey from 353 to 857GHz combined with the 100
microns IRAS show that the dust properties vary in the diffuse ISM at high
Galactic latitude (1e19<NH<2.5e20 H/cm2). Our aim is to explain these
variations with changes in the ISM properties and grain evolution. Our starting
point is the latest core-mantle dust model. It consists of small aromatic-rich
carbon grains, larger amorphous carbon grains with aliphatic-rich cores and
aromatic-rich mantles, and amorphous silicates with Fe/FeS nano-inclusions
covered by aromatic-rich carbon mantles. We explore whether variations in the
radiation field or in the gas density distribution in the diffuse ISM could
explain the observations. The dust properties are also varied in terms of
mantle thickness, Fe/FeS inclusions, carbon abundance, and size distribution.
Variations in the radiation field intensity and gas density distribution cannot
explain the observed variations but radiation fields harder than the standard
ISRF may participate in creating part of them. We further show that variations
in the grain mantle thickness coupled with changes in the grain size
distribution can reproduce most of the observations. We put a limit on the
mantle thickness of the silicates (~10-15nm), and find that aromatic-rich
mantles are needed for the carbon grains (at least 5-7.5nm thick). We also find
that changes in the carbon abundance in the grains could explain part of the
observed variations. Finally, we show that varying the composition of Fe/FeS
inclusions in the silicates cannot account for the variations. With small
variations in the dust properties, we are able to explain most of the
variations in the dust emission observed by Planck-HFI in the diffuse ISM. We
also find that the small realistic changes in the dust properties that we
consider almost perfectly match the anti-correlation and scatter in the
observed beta-T relation.
