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First detection of [NII] 205 μm absorption in interstellar gas. Herschel-HIFI observations towards W31C, W49N, W51, and G34.3+0.1.

Carina M. Persson (Institutionen för rymd- och geovetenskap, Radioastronomi och astrofysik) ; Maryvonne Gerin ; Bhaswati Mookerjea ; John H. Black (Institutionen för rymd- och geovetenskap, Radioastronomi och astrofysik) ; Michael Olberg (Institutionen för rymd- och geovetenskap, Onsala rymdobservatorium) ; Javier Goicoechea ; George Hassel ; Edith Falgarone ; F. Levrier ; K.M. Menten ; Jerome Pety
Astronomy and Astrophysics (0004-6361). Vol. 568 (2014), p. 37.
[Artikel, refereegranskad vetenskaplig]

We present high resolution [N II] 205 μm (3^P_1 − 3^P_0) 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+) ≈ 1.5×10^17 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.

Nyckelord: ISM: atoms; ISM: abundances; ISM: structure; line formation; atomic processes; Galaxy: general

Denna post skapades 2014-08-11. Senast ändrad 2015-05-13.
CPL Pubid: 201133


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