First detection of [N II] 205 μm absorption in interstellar gas

Herschel-HIFI observations towards W 31C, W 49N, W 51, and G34.3+0.1^⋆,⋆⋆

¹ Chalmers University of Technology, Department of Earth and Space Sciences, Onsala Space Observatory, 439 92 Onsala, Sweden
e-mail: carina.persson@chalmers.se
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

Abstract

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.