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The characterization of the Galactic foregrounds has been shown to be themain obstacle in the challenging quest to detect primordial B-modes in thepolarized microwave sky. We make use of the Planck-HFI 2015 data release athigh frequencies to place new constraints on the properties of the polarizedthermal dust emission at high Galactic latitudes. Here, we specifically studythe spatial variability of the dust polarized spectral energy distribution, andits potential impact on the determination of the tensor-to-scalar ratio. We usethe correlation ratio of the $C_\ell^{BB}$ angular power spectra between the217- and 353-GHz channels as a tracer of these potential variations, computedon different high Galactic latitude regions, ranging from 80% to 20% of thesky. The new insight from Planck data is a departure of the correlation ratiofrom unity that cannot be attributed to a spurious decorrelation due to thecosmic microwave background, instrumental noise, or instrumental systematics.The effect is marginally detected on each region, but the statisticalcombination of all the regions gives more than 99% confidence for thisvariation in polarized dust properties. In addition, we show that thedecorrelation increases when there is a decrease in the mean column density ofthe region of the sky being considered, and we propose a simple power-lawempirical model for this dependence, which matches what is seen in the Planckdata. We explore the effect that this measured decorrelation has on simulationsof the BICEP2-Keck Array/Planck analysis and show that the 2015 constraintsfrom those data still allow a decorrelation between the dust at 150 and 353GHzof the order of the one we measure. Finally we show that either spatialvariation of the dust SED or of the dust polarization angle could producedecorrelations between 217- and 353-GHz data similar to those we observe in thedata.

astronomy and astrophysics

Planck intermediate results. L. Evidence for spatial variation of the  polarized thermal dust spectral energy distribution and implications for CMB  $B$-mode analysis