Planck early results. XXV. Thermal dust in nearby molecular clouds

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 the contribution of the J = 2 --> 1 and J = 1 --> 0 <SUP>12</SUP>CO and <SUP>13</SUP>CO emission lines. We derive maps of the dust temperature T, the dust spectral emissivity index beta, and the dust optical depth at 250 mum tau<SUB>250</SUB>. 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 mum per hydrogen atom tau<SUB>250</SUB>/N<SUB>H</SUB>. We report an increase of tau<SUB>250</SUB>/N<SUB>H</SUB> 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. Corresponding author A. Abergel, e-mail alain.abergel@ias.u-psud.fr