===== On the Detection of Hydroxyl, OH, in Space =====
The OH radical was the first molecule whose detection by means of radio astronomy was reported in 1963.\\
S. Weinreb, A. H. Barrett, M. L. Meeks, and J. C. Henry\\
announced\\
**[[https://doi.org/10.1038/200829a0|Radio Observations of OH in the Interstellar Medium]]**\\
//Nature// **200**, 829–831 (1963).\\
They used the 25.6 m dish of the Millstone Hill Observatory of Lincoln Laboratory (MIT) to identify the two strongest hyperfine structure (HFS) components of the 2Π3/2 fundamental (//J// = 1.5 − 1.5) Λ-doubling transition at 1667.359 and 1665.402 MHz in absorption toward the supernova remnant Cassiopeia A, aka 3C 461 or GAL 111.7-02.1.\\
J. G. Bolton, K. J. van Damme, F. F. Gardner, and B. J. Robinson\\
carried out\\
**[[https://doi.org/10.1038/201279a0|Observation of OH Absorption Lines in the Radio Spectrum of the Galactic Centre]]**\\
//Nature// **201**, 279 (1964).\\
They employed the Parkes 64 m dish to observed the same lines toward Sagittarius A.\\
F. F. Gardner, B. J. Robinson, J. G. Bolton, and K. J. van Damme\\
reported on the\\
**[[https://doi.org/10.1103/PhysRevLett.13.3|Detection of the Interstellar OH Lines at 1612 and 1720 Mc/sec]]**\\
//Phys. Rev. Lett.// **15**, 3−5 (1964).\\
These weaker HFS components were measured with the same dish as above on the basis of improved spectroscopic parameters prior to their measurements in the laboratory.
H. Weaver, D. R. W. Williams, N. H. Dieter, and W. T. Lum\\
carried out\\
**[[https://doi.org/10.1038/208029a0|Observations of a Strong Unidentified Microwave Line and of Emission from the OH Molecule]]**\\
//Nature// **208**, 29−31 (1965).\\
They used the 25.9 m dish of the Hat Creek Observatory for OH observations toward several sources. OH was found in emission toward the high mass star-forming regions W49 and NGC 6334.\\
\\
B. Zuckerman, P. Palmer, H. Penfield, and A. E. Lilley\\
announced the\\
**[[https://doi.org/10.1086/180223|Detection of Microwave Radiation from the 2Π1/2, //J// = 1/2 State of OH]]**\\
//Astrophys. J.// **153**, L69−L76 (1968).\\
The NRAO 43 m dish was employed to detect the //F// = 1 − 0 HFS component toward the high mass star-forming regions W3 and W49.\\
\\
B. Zuckerman, J. L. Yen, C. A. Gottlieb, and P. Palmer\\
reported on\\
**[[https://doi.org/10.1086/151686|Observations of the 2Π3/2, //J// = 5/2 State of Interstellar OH]]**\\
//Astrophys. J.// **177**, 59−78 (1972).\\
Using the same dish, the strongest //F// = 3 − 3 HFS component toward the high mass star-forming regions W3(OH), W75N, W49, Sgr B2(OH), and NGC 6334 N, as well as toward the O-rich late-type star NML Cyg. The weaker //F// = 2 − 2 component was detected toward W3(OH) and NGC 6334 N.\\
\\
J. W. V. Storey, D. M. Watson, and C. H. Townes\\
published\\
**[[https://doi.org/10.1086/183472|Detection of Interstellar OH in the Far-Infrared]]**\\
//Astrophys. J.// **244**, L27−L30 (1981).\\
The Kuiper Airborne Observatory was employed to detect the fundamental rotational transition of OH (2Π3/2 //J// = 2.5 − 1.5) near 2.51 THz toward Sgr B2.\\
Some time later,\\
R. Genzel, D. M. Watson, M. K. Crawford, and C. H. Townes\\
investigated\\
**[[https://doi.org/10.1086/163574|The Neutral-Gas Disk around the Galactic Center]]**\\
//Astrophys. J.// **297**, 766−786 (1985).\\
The Kuiper Airborne Observatory was employed to observe also the low energy rotational transition of OH (2Π1/2 //J// = 1.5 − 0.5) near 1.836 THz.\\
\\
K. E. S. Ford, D. A. Neufelf, P. F. Goldsmith, and G. J. Melnick\\
announced the\\
**[[https://doi.org/10.1086/374552|Detection of OH toward the Extreme Carbon Star IRC +10216]]**\\
//Astrophys. J.// **589**, 430−438 (2003).\\
The 305 m Arecibo dish was used to detect the two strongest HFS components of fundamental Λ-doubling transition near 1665 and 1667 MHz.\\
\\
J. R. Goicoechea, J. Cernicharo, M. R. Lerate, F. Daniel, M. J. Barlow, B. M. Swinyard, T. L. Lim, S. Viti, and J. Yates\\
published on\\
**[[https://doi.org/10.1086/503799|Far-Infrared Excited Hydroxyl Lines from Orion KL Outflows]]**\\
//Astrophys. J.// **641**, L49−L52 (2006).\\
Several rotational transitions of OH in both spin-ladders as well as between them were detected with ISO-LWS.\\
\\
T. Khouri, L. Velilla-Prieto, E. De Beck, W. H. T. Vlemmings, H. Olofsson, B. Lankhaar, J. H. Black, and A. Baudry\\
described the\\
**[[https://doi.org/10.1051/0004-6361/201935049|Detection of Highly Excited OH towards AGB Stars — A New Probe of Shocked Gas in the Extended Atmospheres]]**\\
//Astron. Astrophys.// **623**, Art. No. L1 (2019).\\
Employing ALMA, Λ-doubling transitions in //v// = 0 up to //N// = 18 were detected toward W Hya and R Dor; transitions in //v// = 1 up to //N// = 17 were detected toward W Hya.\\
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F. F. Gardner, R. X. McGee, and M. W. Sinclair\\
detected\\
**[[https://ui.adsabs.harvard.edu/abs/1970ApL.....5...67G/abstract|18OH in Sagittarius]]**\\
//Astrophys. Lett.// **5**, 67−71 (1970).\\
The two strongest HFS components of the fundamental Λ-doubling transition were observed with the Parkes 64 m dish toward Sgr A and Sgr B2. The 16O/18O rotio was ~314 and ~203, respectively, somewhat lower than ~500 terrestrial.\\
\\
F.F. Gardner and J.B. Whiteoak\\
published on\\
**[[https://doi.org/10.1093/mnras/176.1.57P|The Detection of Interstellar 17OH]]**\\
//Mon. Not. R. Astron. Soc.// **176**, 57P−60P (1976).\\
Similar observations led to the detection in Sgr A. The 18O/17O rotio was 4.2 ± 0.4, again slightly smaller than ~5.5 terrestrial.\\
\\
B. Parise, F. Du, F.-C. Liu, A. Belloche, H. Wiesemeyer, R. Güsten, K. M. Menten, H.-W. Hübers, and B. Klein\\
announced the\\
**[[https://doi.org/10.1051/0004-6361/201218784|Detection of OD towards the Low-Mass Protostar IRAS 16293−2422]]**\\
//Astron. Astrophys.// **542**, Art. No. L5 (2012).\\
The GREAT receiver on board SOFIA was employed for the observations. A relatively high ratio of OD to HDO was found.\\
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Contributor(s): H. S. P. Müller; 2026; 07.
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