Planck 2013 results. XII. Component separation
  Planck has produced detailed all-sky observations over nine frequency bands
between 30 and 857 GHz. These observations allow robust reconstruction of the
primordial cosmic microwave background (CMB) temperature fluctuations over
nearly the full sky, as well as new constraints on Galactic foregrounds. This
paper describes the component separation framework adopted by Planck. We test
four foreground-cleaned CMB maps derived using qualitatively different
component separation algorithms. The quality of our reconstructions is
evaluated through detailed simulations and internal comparisons, and shown
through various tests to be internally consistent and robust for CMB power
spectrum and cosmological parameter estimation up to l = 2000. The parameter
constraints on LambdaCDM cosmologies derived from these maps are consistent
with those presented in the cross-spectrum based Planck likelihood analysis. We
choose two of the CMB maps for specific scientific goals. We also present maps
and frequency spectra of the Galactic low-frequency, CO, and thermal dust
emission. The component maps are found to provide a faithful representation of
the sky, as evaluated by simulations. For the low-frequency component, the
spectral index varies widely over the sky, ranging from about beta = -4 to -2.
Considering both morphology and prior knowledge of the low frequency
components, the index map allows us to associate a steep spectral index (beta <
-3.2) with strong anomalous microwave emission, corresponding to a spinning
dust spectrum peaking below 20 GHz, a flat index of beta > -2.3 with strong
free-free emission, and intermediate values with synchrotron emission.
