Polarization fraction P∕I in emission for models D (solid) and A (dot-dashed), for various ISRF intensities G₀. The spectral shape of the radiation was not changed, just the scaling factor G₀, as indicated.Data points combine Planck (λ ≥ 850 μm, Planck Collaboration Int. XXII 2015) and BLASTPol (λ ≤ 850 μm, Ashton et al. 2017) observations, normalized at 850 μm. We cut our spectrum at 5 cm because of the rising contribution to I_ν(λ) of the thermal emission of PAH and other components not modeled here (synchrotron emission, anomalous microwave emission, see Planck Collaboration Int. XXII 2015).