Planck Early Results: Thermal dust in Nearby Molecular Clouds

Planck allows unbiased mapping of the sub-millimeter and millimeter 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 black-body . We derive maps of the temperature, spectral index, and optical depth. The distribution of spectral indices is narrow, centered at 1.78, with a stand ard deviation of 0.08 and a systematic error of 0.07. 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 black-body. Significant positive res iduals are also detected in the molecular regions and in the 217 GHz and 100 GHz bands, mainly due to the contribution of the J = 2 ! 1 and J = 1 ! 0 12 CO and 13 CO emission lines. The temperature map illustrates the cooling of the dust particles in thermal equilibrium with the inc ident radiation field, from 16‐17 K in the diffuse regions to 13‐14 K in the dense parts. The optical depth map reveals the spatial distribution of the column density of t he 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 mea sured optical depth per hydrogen atom�= NH. The derived map of�= NH shows where and on what angular scale the transition occurs between the diffuse (�1�10 −25 cm 2 ) and dense regions (�2�10 −25 cm 2 ). We find a systematic and sharp increase of�= NH in the outer parts of the molecular phase where Av > 1, by at least a factor of 2. Such variations of �= NH have a strong impact on the equilibrium temperature of the dust particles.