2014A&A...568A..37P

We present high resolution [NII] 205µm (³P₁-³P₀) spectra obtained with Herschel-HIFI towards a small sample of far-infrared bright star forming regions in the Galactic plane: W31C (G10.6-0.4), W49N (G43.2-0.1), W51 (G49.5-0.4), and G34.3+0.1. All sources display an emission line profile associated directly with the HII regions themselves. For the first time we also detect absorption of the [NII] 205µm line by extended low-density foreground material towards W31C and W49N over a wide range of velocities. We attribute this absorption to the warm ionised medium (WIM) and find N(N⁺)≃1.5x10¹⁷cm^–2 towards both sources. This is in agreement with recent Herschel-HIFI observations of [CII] 158µm, also observed in absorption in the same sight-lines, if ≃7-10% of all C⁺ ions exist in the WIM on average. Using an abundance ratio of [N]/[H] = 6.76x10^–5 in the gas phase we find that the mean electron and proton volume densities are ∼0.1-0.3cm^–3 assuming a WIM volume filling fraction of 0.1-0.4 with a corresponding line-of-sight filling fraction of 0.46-0.74. A low density and a high WIM filling fraction are also supported by RADEX modelling of the [NII] 205µm absorption and emission together with visible emission lines attributed mainly to the WIM. The detection of the 205µm line in absorption emphasises the importance of a high spectral resolution, and also offers a new tool for investigation of the WIM.