  Planck allows unbiased mapping of Galactic sub-millimetre and millimetre
emission from the most diffuse regions to the densest parts of molecular
clouds. We present an early analysis of the Taurus molecular complex, on
line-of-sight-averaged data and without component separation. The emission
spectrum measured by Planck and IRAS can be fitted pixel by pixel using a
single modified blackbody. Some systematic residuals are detected at 353 GHz
and 143 GHz, with amplitudes around -7 % and +13 %, respectively, indicating
that the measured spectra are likely more complex than a simple modified
blackbody. Significant positive residuals are also detected in the molecular
regions and in the 217 GHz and 100 GHz bands, mainly caused by to the
contribution of the J=2-1 and J=1-0 12CO and 13CO emission lines. We derive
maps of the dust temperature T, the dust spectral emissivity index beta, and
the dust optical depth at 250 microns tau. The temperature map illustrates the
cooling of the dust particles in thermal equilibrium with the incident
radiation field, from 16-17 K in the diffuse regions to 13-14 K in the dense
parts. The distribution of spectral indices is centred at 1.78, with a standard
deviation of 0.08 and a systematic error of 0.07. We detect a significant
T-beta anti-correlation. The dust optical depth map reveals the spatial
distribution of the column density of the molecular complex from the densest
molecular regions to the faint diffuse regions. We use near-infrared extinction
and HI data at 21-cm to perform a quantitative analysis of the spatial
variations of the measured dust optical depth at 250 microns per hydrogen atom
tau/NH. We report an increase of tau/NH by a factor of about 2 between the
atomic phase and the molecular phase, which has a strong impact on the
equilibrium temperature of the dust particles.
