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Planck allows unbiased mapping of Galactic sub-millimetre and millimetreemission from the most diffuse regions to the densest parts of molecularclouds. We present an early analysis of the Taurus molecular complex, online-of-sight-averaged data and without component separation. The emissionspectrum measured by Planck and IRAS can be fitted pixel by pixel using asingle modified blackbody. Some systematic residuals are detected at 353 GHzand 143 GHz, with amplitudes around -7 % and +13 %, respectively, indicatingthat the measured spectra are likely more complex than a simple modifiedblackbody. Significant positive residuals are also detected in the molecularregions and in the 217 GHz and 100 GHz bands, mainly caused by to thecontribution of the J=2-1 and J=1-0 12CO and 13CO emission lines. We derivemaps of the dust temperature T, the dust spectral emissivity index beta, andthe dust optical depth at 250 microns tau. The temperature map illustrates thecooling of the dust particles in thermal equilibrium with the incidentradiation field, from 16-17 K in the diffuse regions to 13-14 K in the denseparts. The distribution of spectral indices is centred at 1.78, with a standarddeviation of 0.08 and a systematic error of 0.07. We detect a significantT-beta anti-correlation. The dust optical depth map reveals the spatialdistribution of the column density of the molecular complex from the densestmolecular regions to the faint diffuse regions. We use near-infrared extinctionand HI data at 21-cm to perform a quantitative analysis of the spatialvariations of the measured dust optical depth at 250 microns per hydrogen atomtau/NH. We report an increase of tau/NH by a factor of about 2 between theatomic phase and the molecular phase, which has a strong impact on theequilibrium temperature of the dust particles.

Planck Early Results: Thermal dust in Nearby Molecular Clouds