The N = 1 – 0, J = 0 – 1, fine structure component of oxidaniumylidene or hydroxylium, OH⁺, near 909 GHz has recently been detected in absorption from ground with the APEX 12 m telescope toward the evolved massive star-forming region Sagittarius B2(M):
F. Wyrowski, K. M. Menten, R. Güsten, and A. Belloche
First Interstellar Detection of OH⁺
Astron. Astrophys. 518, (2010), Art. No. A26.
Interestingly, this is the weakest of the three fine structure components.

More recently, the strongest fine structure component near 972 GHz has been detected with the Herschel satellite toward several other sources, in particular toward W31C (G10.6–0.4), also in absorption:
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.
Model calculations were carried out indicating that OH⁺ and H₂O⁺ are formed in the mostly atomic diffuse ISM in these sources.

F. F. S. van der Tak, Z. Nagy, V. Ossenkopf, Z. Makai, J. H. Black, A. Faure, M. Gerin, and E. A. Bergin
report on
Spatially Extended OH⁺ Emission from the Orion Bar and Ridge
Astron. Astrophys. 560, (2013) Art. No. A95.
OH⁺ in emission was also detected toward the Crab Nebula supernova remnant, also known as M 1, by:
M. J. Barlow, B. M. Swinyard, P. J. Owen, J. Cernicharo, H. L. Gomez, R. J. Ivison, O. Krause, T. L. Lim, M. Matsuura, S. Miller, G. Olofsson, and E. T. Polehampton,
Detection of a Noble Gas Molecular Ion, ³⁶ArH⁺, in the Crab Nebula
Science 342, (2013) 1343.
Both observations were made with the Herschel satellite.

An unspecified electronic transition at 385.3769 nm or 27903.58 cm^–1 was detected in diffuse clouds toward several stars at ESO Paranal observatory by:
J. Krelowski, Y. Beletsky, and G. A. Galazutdinov,
Hydroxyl Cation in Translucent Interstellar Clouds
Astrophys. J. 719, (2010) L20.
This is the R(0) origin transition of the v = 0 – 0 vibrational band belonging to the A – X electronic transition which had been investigated by:
A. J. Merer, D. N. Malm, R. W. Martin, M. Horani, and J. Rostas,
The Ultraviolet Emission Spectra of OH⁺ and OD⁺. Rotational Structure and Perturbations in the //A// ³Π_//i// – //X// ³Σ^– Transition
Can. J. Phys. 53, (1975) 251.

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Contributor(s): H. S. P. Müller; 07 & 08, 2010; 12, 2013.

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