摘要：Aims.We have explored the capabilities of dust extinction andγrays to probe the properties of the interstellar medium in the nearby anti-centre region. In particular, we aim at quantifying the variations of the dust properties per gas nucleon across the different gas phases and different clouds. The comparison of dust extinction and emission properties with other physical quantities of large grains (emission spectral indexβ, dust colour temperatureTdust, total-to-selective extinction factorRV) helps the theoretical modelling of grains as they evolve from diffuse to dense cloud environments.Methods.We have jointly modelled theγ-ray intensity, recorded between 0.4 and 100 GeV with theFermiLarge Area Telescope (LAT), and the stellar reddening,E(B−V), inferred from Pan-STARRS and 2MASS photometry, as a combination of HI-bright, CO-bright, and ionised gas components. The complementary information from dust reddening andγrays is used to reveal the gas not seen, or poorly traced, by HI, free-free, and12CO emissions, namely (i) the opaqueHIand diffuse H2present in the dark neutral medium (DNM) at the atomic-molecular transition, and (ii) the dense H2to be added where12CO lines saturate (COsat). We compare the total gas column densities,NH, derived from theγrays and stellar reddening with those inferred from a similar, previously published analysis ofγrays and of the optical depth of the thermal dust emission,τ353, at 353 GHz. We can therefore compare environmental variations in specific dust reddening,E(B−V)∕NH, and in dust emission opacity (dust optical depth per gas nucleon),τ353∕NH.Results.The gas column densities obtained when combiningγrays with either dust reddening or dust emission compare reasonably well in the atomic and DNM gas phases and over most of the CO-bright phase, but we find localised differences in the dense media (COsatcomponent) due to differences in the two dust tracers. Over the whole anti-centre region, we find an averageE(B−V)∕NHratio of (2.02 ± 0.48) ×10−22mag cm2, with maximum local variations of about ± 30% at variance with the two to six fold coincident increase seen in emission opacity as the gas column density increases. We show how the specific reddening and opacity vary with the colour temperature and spectral index of the thermal emission of the large grains. Additionally, we find a better agreement between theXCO=N(H2)/WCOconversion factors derived with dust reddening or withγrays than with those inferred from dust emission, especially towards clouds with largeτ353optical depths. The comparison confirms that the highXCOvalues found with dust emission are biased by the significant rise in emission opacity inside molecular clouds.Conclusions.In the diffuse medium, we find only small variations in specific reddening,E(B−V)∕NH, compatible with the dispersion in theRVfactor reported by other studies. This implies a relatively uniform dust-to-gas mass ratio in the diffuse parts of the anti-centre clouds. The small amplitude of theE(B−V)∕NHvariations with increasingNHcolumn density confirms that the large opacityτ353∕NHrise seen towards dense CO clouds is primarily due to changes in dust emissivity. The environmental changes are qualitatively compatible with model predictions based on mantle accretion on the grains and the formation of grain aggregates.
关键词：Key wordsendust, extinctioncosmic raysISM: cloudssolar neighborhoodgamma rays: ISM
