The J = 1.5 – 0.5 fine structure component of the 1₁₁ –0₀₀ transition of ortho-oxidaniumyl, o-H₂O⁺, near 1115 GHz has recently been detected in absorption with the Herschel satellite toward the star-forming regions DR21, Sagittarius B2(M), and NGC 6334:
V. Ossenkopf, H. S. P. Müller, D. C. Lis, et al.
Detection of Interstellar Oxidaniumyl: Abundant H₂O⁺ towards the Star-forming Regions DR21, Sgr B2, and NGC6334
Astron. Astrophys. 518, (2010) Art. No. L111.
While the spectra toward DR21 and NGC 6334 show partially resolved hyperfine structure, the spectrum toward Sgr B2(M) is rather complex because of additional absorption features caused by diffuse spiral arm clouds.

It is worthwhile mentioning that the same line was also observed in absorption toward several other sources. e.g. toward W31C (G10.6–0.4):
M. Gerin, M. De Luca, J. Black, et al.
Interstellar OH⁺, H₂O⁺ and H₃O⁺ along the Sight-line to G10.6–0.4
Astron. Astrophys. 518, (2010) Art. No. L110.
This article presents also model calculation which indicate that in these sources OH⁺ and H₂O⁺ are formed in the mostly atomic diffuse ISM.

All four fine structure components of the 1₁₀ – 1₀₁ transition above 600 GHz have been detected in absorption with the Herschel satellite toward Sgr B2(M):
P. Schilke, C. Comito, H. S. P. Müller, et al.,
Herschel Observations of //ortho//- and //para//-Oxidaniumyl (H₂O⁺) in Spiral Arm Clouds toward Sagittarius B2(M)
Astron. Astrophys. 521, (2010) Art. No. L11.
The authors investigated the ortho-to-para ratio of the radical cation.
Considering these as well as additional data, including observations of the 1₁₀ – 1₀₁ transition around 1630 GHz,
P. Schilke, D. C. Lis, E. A. Bergin, R. Higgins, and C. Comito
reported on the
//Ortho/////Para// Ratio of H₂O⁺ Toward Sagittarius B2(M) Revisited
J. Phys. Chem. A 117, (2013) 9766.
The derived values are now all compatible with the high temperature ratio of 3.

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Contributor(s): H. S. P. Müller; 07, 2010; 12, 2013

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