Volume 568, August 2014
|Number of page(s)||6|
|Section||Interstellar and circumstellar matter|
|Published online||08 August 2014|
First detection of [N II] 205 μm absorption in interstellar gas
1 Chalmers University of Technology, Department of Earth and Space Sciences, Onsala Space Observatory, 439 92 Onsala, Sweden
2 LERMA-LRA, UMR 8112 du CNRS, Observatoire de Paris, École Normale Supérieure, UPMC & UCP, 24 rue Lhomond, 75231 Paris Cedex 05, France
3 Tata Institute of Fundamental Research, Homi Bhabha Road, 400005 Mumbai, India
4 Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC), 28049 Cantoblanco, Madrid, Spain
5 Department of Physics & Astronomy, Siena College, Loudonville NY 12211, USA
6 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
7 Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406 Saint-Martin-d’ Hères, France
Received: 14 April 2014
Accepted: 11 June 2014
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: W 31C (G10.6−0.4), W 49N (G43.2−0.1), W 51 (G49.5−0.4), and G34.3+0.1. 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.
Key words: ISM: atoms / ISM: abundances / ISM: structure / line: formation / atomic processes / Galaxy: general
Appendix A is available in electronic form at http://www.aanda.org
© ESO, 2014
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