2014/06/26 by C. M. Persson, Maryvonne Gérin, M. Gerin +12 · 2 citations
Earth and Planetary Sciences · Physics and Astronomy · #Astrophysics and Star Formation Studies #Atmospheric Ozone and Climate #Galaxies: Formation, Evolution, Phenomena #Stellar, planetary, and galactic studies #astro-ph.GA
paper · pdf · doi:10.1051/0004-6361/201423997
published as A&A 568, A37 (2014) · 7 pages, 4 figures, accepted for publication in Astronomy & Astrophysics, 11 June 2014
arxiv created 2014/06/26 · openalex publication_date 2014/08/01 · arxiv updated 2014/09/05 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/22
We present high resolution [N II] 205 μm (3P1 − 3P0) spectra obtained with Herschel-HIFI towards a small sample of far-infrared bright star forming regions in the Galactic plane: All sources display an emission line profile associated directly with the H II regions themselves. For the first time we also detect absorption of the [N II] 205 μm line by extended low-density foreground material towards W 31C and W 49N over a wide range of velocities. We attribute this absorption to the warm ionised medium (WIM) and find N(N+) ≈ 1.5×1017 cm-2 towards both sources. This is in agreement with recent Herschel-HIFI observations of [C II] 158 μm, 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] = 6.76×10-5 in the gas phase we find that the mean electron and proton volume densities are ∼0.1–0.3 cm-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 [N II] 205 μm absorption and emission together with visible emission lines attributed mainly to the WIM. The detection of the 205 μm line in absorption emphasises the importance of a high spectral resolution, and also offers a new tool for investigation of the WIM.