Planck intermediate results. XXVIII. Interstellar gas and dust in the
  Chamaeleon clouds as seen by Fermi LAT and Planck
  Shortened abstract: Observations of the nearby Chamaeleon clouds in gamma
rays with the Fermi Large Area Telescope and in thermal dust emission with
Planck and IRAS have been used with the HI and CO radio data to (i) map the gas
column densities in the different phases and at the dark neutral medium (DNM)
transition between the HI-bright and CO-bright media; (ii) constrain the
CO-to-$H_2$ conversion factor, $X_{CO}$; (iii) probe the dust properties per
gas nucleon in each gas phase and spatially across the clouds. We have
separated clouds in velocity in HI and CO emission and modelled the 0.4-100 GeV
intensity, the dust optical depth at 353 GHz, the thermal radiance of the large
grains, and an estimate of the dust extinction empirically corrected for the
starlight intensity, $A_{VQ}$. The gamma-ray emissivity spectra confirm that
the GeV-TeV cosmic rays uniformly permeate all gas phases up to the CO cores.
The dust and cosmic rays reveal large amounts of DNM gas, with comparable
spatial distributions and twice as much mass as in the CO-bright clouds. We
give constraints on the HI-DNM-CO transitions and CO-dark $H_2$ fractions for
separate clouds. The corrected extinction provides the best fit to the total
gas traced by the gamma rays, but we find evidence for a rise in $A_{VQ}/N_H$
and a steep rise in opacity, with increasing $N_H$ and $H_2$ fraction, and with
decreasing dust temperature. We observe less variations for the specific power
of the grains, except for a decline by half in the CO cores. This combined
information suggests grain evolution. The gamma rays and dust radiance yield
consistent $X_{CO}$ estimates near $0.7\times10^{20}$ cm$^{-2}$ (K
km/s)$^{-1}$. The other dust tracers yield biased values because of the grain
opacity rise in the CO clouds. These results also confirm a factor of 2
difference between $X_{CO}$ estimates at pc and kpc scales.
