First detection of [N II] 205 micrometer absorption in interstellar gas
| Autor: | Persson, C. M., Gerin, M., Mookerjea, B., Black, J. H., Olberg, M., Goicoechea, J. R., Hassel, G. E., Falgarone, E., Levrier, F., Menten, K. M., Pety, J. |
|---|---|
| Rok vydání: | 2014 |
| Předmět: | |
| Zdroj: | A&A 568, A37 (2014) |
| Druh dokumentu: | Working Paper |
| DOI: | 10.1051/0004-6361/201423997 |
| Popis: | We present high resolution [NII] 205 micrometer ^3P_1-^3P_0 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] 205 micrometer 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^+)\approx 1.5x10^17 cm^-2 towards both sources. This is in agreement with recent Herschel-HIFI observations of [CII] 158 micrometer, also observed in absorption in the same sight-lines, if \approx7-10 % 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 ~0.1-0.3 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 [NII] 205 micrometer absorption and emission together with visible emission lines attributed mainly to the WIM. The detection of the 205 micrometer line in absorption emphasises the importance of a high spectral resolution, and also offers a new tool for investigation of the WIM. Comment: 7 pages, 4 figures, accepted for publication in Astronomy & Astrophysics, 11 June 2014 |
| Databáze: | arXiv |
| Externí odkaz: |
|