First detection of [N II] 205 micrometer absorption in interstellar gas
arXiv:1406.3052 · doi:10.1051/0004-6361/201423997
Abstract
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.
arXiv comment7 pages, 4 figures, accepted for publication in Astronomy & Astrophysics, 11 June 2014
References in corpus (19)
- The chemical composition of the Sun
- A computer program for fast non-LTE analysis of interstellar line spectra
- A KInetic Database for Astrochemistry (KIDA)
- The warm ionized medium in spiral galaxies
- Interstellar Carbon in Translucent Sightlines
- Radiative recombination data for modelling dynamic finite-density plasmas
- Interstellar OH+, H2O+ and H3O+ along the sight-line to G10.6-0.4
- Trigonometric Parallax of W51 Main/South
- Herschel/HIFI observations of interstellar OH+ and H2O+ towards W49N: a probe of diffuse clouds with a small molecular fraction
- Parallaxes for W49N and G048.60+0.02: Distant Star Forming Regions in the Perseus Spiral Arm
- Detection of the 205 um [NII] Line from the Carina Nebula
- Molecular absorption lines toward star-forming regions : a comparative study of HCO+, HNC, HCN, and CN
- Water absorption in Galactic translucent clouds: conditions and history of the gas derived from Herschel/HIFI PRISMAS observations
- A Herschel Survey of the [N II] 205 micron Line in Local Luminous Infrared Galaxies --- The [N II] 205 micron Emission as a Star Formation Rate Indicator
- Ionized nitrogen at high redshift
- Excitation and Abundance of C3 in star forming cores:Herschel/HIFI observations of the sight-lines to W31C and W49N
- Detection of [N II] 5755 Emission from Low Density Ionized Interstellar Gas
- Is There Enhanced Depletion of Gas-Phase Nitrogen in Moderately Reddened Lines of Sight?
- Molecular cloud chemistry and the importance of dielectronic recombination