Planck 2018 results. XI. Polarized dust foregrounds
  The study of polarized dust emission has become entwined with the analysis of
the cosmic microwave background (CMB) polarization. We use new Planck maps to
characterize Galactic dust emission as a foreground to the CMB polarization. We
present Planck EE, BB, and TE power spectra of dust polarization at 353 GHz for
six nested sky regions covering from 24 to 71 % of the sky. We present
power-law fits to the angular power spectra, yielding evidence for
statistically significant variations of the exponents over sky regions and a
difference between the values for the EE and BB spectra. The TE correlation and
E/B power asymmetry extend to low multipoles that were not included in earlier
Planck polarization papers. We also report evidence for a positive TB dust
signal. Combining data from Planck and WMAP, we determine the amplitudes and
spectral energy distributions (SEDs) of polarized foregrounds, including the
correlation between dust and synchrotron polarized emission, for the six sky
regions as a function of multipole. This quantifies the challenge of the
component separation procedure required for detecting the reionization and
recombination peaks of primordial CMB B modes. The SED of polarized dust
emission is fit well by a single-temperature modified blackbody emission law
from 353 GHz to below 70 GHz. For a dust temperature of 19.6 K, the mean
spectral index for dust polarization is $\beta_{\rm d}^{P} = 1.53\pm0.02 $. By
fitting multi-frequency cross-spectra, we examine the correlation of the dust
polarization maps across frequency. We find no evidence for decorrelation. If
the Planck limit for the largest sky region applies to the smaller sky regions
observed by sub-orbital experiments, then decorrelation might not be a problem
for CMB experiments aiming at a primordial B-mode detection limit on the
tensor-to-scalar ratio $r\simeq0.01$ at the recombination peak.
