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Shortened abstract: Observations of the nearby Chamaeleon clouds in gammarays with the Fermi Large Area Telescope and in thermal dust emission withPlanck and IRAS have been used with the HI and CO radio data to (i) map the gascolumn densities in the different phases and at the dark neutral medium (DNM)transition between the HI-bright and CO-bright media; (ii) constrain theCO-to-$H_2$ conversion factor, $X_{CO}$; (iii) probe the dust properties pergas nucleon in each gas phase and spatially across the clouds. We haveseparated clouds in velocity in HI and CO emission and modelled the 0.4-100 GeVintensity, the dust optical depth at 353 GHz, the thermal radiance of the largegrains, and an estimate of the dust extinction empirically corrected for thestarlight intensity, $A_{VQ}$. The gamma-ray emissivity spectra confirm thatthe 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 comparablespatial distributions and twice as much mass as in the CO-bright clouds. Wegive constraints on the HI-DNM-CO transitions and CO-dark $H_2$ fractions forseparate clouds. The corrected extinction provides the best fit to the totalgas 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 withdecreasing dust temperature. We observe less variations for the specific powerof the grains, except for a decline by half in the CO cores. This combinedinformation suggests grain evolution. The gamma rays and dust radiance yieldconsistent $X_{CO}$ estimates near $0.7\times10^{20}$ cm$^{-2}$ (Kkm/s)$^{-1}$. The other dust tracers yield biased values because of the grainopacity rise in the CO clouds. These results also confirm a factor of 2difference between $X_{CO}$ estimates at pc and kpc scales.

Planck intermediate results. XXVIII. Interstellar gas and dust in the Chamaeleon clouds as seen by Fermi LAT and Planck