===== On the Detection of Methylidynium, CH+, in Space =====
Methylidynium, CH+, was one of the first molecules to be identified in space. Its electronic //A// 1Π – //X// 1Σ spectrum was detected – in the lowest //R//(0) transition of the three //v// = 0 – 0, 1 – 0, and 2 – 0 vibrational bands at 423.257, 395.771, and 374.530 nm, respectively:\\
A. E. Douglas and G.Herzberg,\\
**[[https://ui.adsabs.harvard.edu/abs/1941ApJ....94..381D|Note on CH+ in Interstellar Space and in the Laboratory.]]**\\
//Astrophys. J.// **94**, 381 (1941).\\
The first two lines were detected toward ξ Per, χ2 Ori, and 55 Cyg, the very first one also toward χ Aur by\\
T. Dunham, Jr.,\\
**[[https://ui.adsabs.harvard.edu/abs/1937PASP...49...26D|Interstellar Neutral Potassium and Neutral Calcium]]**\\
//Publ. Astron. Soc. Pac.// **49**, 26–28 (1937).\\
As mentioned there, these lines were also observed by:\\
W. S. Adams,\\
**[[https://ui.adsabs.harvard.edu/abs/1941ApJ....93...11A|Some Results with the COUDÉ Spectrograph of the Mount Wilson Observatory]]**\\
//Astrophys. J.// **93**, 11–23 (1941);\\
He reportedly also observed the third line.\\
The detection of its pure rotational spectrum toward in the planetary nebula NGC 7027 was reported by\\
J. Cernicharo, X.-W. Liu, E. González-Alfonso, P. Cox, M. J. Barlow, T. Lim, and B. M. Swinyard,\\
**[[https://doi.org/10.1086/310729|Discovery of Far-Infrared Pure Rotational Transitions of CH+ in NGC 7027]]**\\
//Astrophys. J.// **483**, L65–L68 (1997).\\
More recently, interstellar methylidynium has been observed for the first time in high resolution and in absorption towards the star-forming region G10.6-0.4. Actually, as observation of the //J// = 1 – 0 transition of 12CH+ is hampered by a terrestrial O2 line, 13CH+ was observed instead:\\
E. Falgarone, T. G. Phillips, and J. C. Pearson,\\
**[[https://doi.org/10.1086/499047|First Detection of 13CH+ (//J// = 1 – 0)]]**\\
//Astrophys. J.// **634**, L149–L152 (2005).\\
Please note that the inferred rest-frequency is incorrect. Two alternative rest frequencies were proposed of which the higher one is quite appropriate.\\
The //J// = 1 – 0 transition of 12CH+ has been detected with the //Herschel// satellite in emission toward the Orion Bar by\\
E. Habart, E. Dartois, A. Abergel, et al.,\\
**[[https://doi.org/10.1051/0004-6361/201014654|SPIRE Spectroscopy of the Prototypical Orion Bar Photodissociation Region]]**\\
//Astron. Astrophys.// **518**, Art. No. L116 (2010);\\
and by\\
D. A. Naylor, E. Dartois, E. Habart,\\
**[[https://doi.org/10.1051/0004-6361/201014656|First Detection of the Methylidyne Cation (CH+) Fundamental Rotational Line with the //Herschel///SPIRE FTS]]**\\
//Astron. Astrophys.// **518**, Art. No. L117 (2010).\\
The latter work also describes the observation of this transition in absorption toward two compact HII regions.\\
E. Falgarone, V. Ossenkopf, M. Gerin, et al. observed\\
**[[https://doi.org/10.1051/0004-6361/201014671|Strong CH+ //J// = 1 – 0 Emission and Absorption in DR21]]**\\
//Astron. Astrophys.// **518**, Art. No. L118 (2010).\\
These authors were the first to use the correct rest frequency of 835137.5 MHz.\\
Higher-//J// transitions of CH+ were also detected with the PACS instrument on board of the //Herschel// satellite in the disc around a Herbig Be star by:\\
W-F. Thi, F. Menard, G. Meeus, C. Martin-Zaidi, P. Woitke, E. Tatulli, M. Benisty, I. Kamp, I. Pascucci, C. Pinte, C. A. Grady, S. Brittain, G.J. White, C. D. Howard, G. Sandell, C. Eiroa,\\
**[[https://doi.org/10.1051/0004-6361/201116678|Detection of CH+ Emission from the Disc around HD 100546]]**\\
//Astron. Astrophys.// **530**, Art. No. L2 (2011).\\
D. A. Neufeld, M. Goto, T. R. Geballe, R. Güsten, K. M. Menten, and H. Wiesemeyer\\
reported on the\\
**[[https://doi.org/10.3847/1538-4357/ab7191|Detection of Vibrational Emissions from the Helium Hydride Ion (HeH+) in the Planetary Nebula NGC 7027]]**\\
//Astrophys. J.// **894**, Art. No. 37 (2020).\\
They detected in addition the //v// = 1 − 0, //P//(6) to //R//(3) transitions of CH+ around 3.5 μm using the iSHELL spectrograph at the Infrared Telescope Facility on Mauna Kea. The //P//(6) line, however, is severely blended with a hydrogen recombination line. A separate account is planned on this subject.\\
T. Möller, P. Schilke, A. Schmiedeke, E. A. Bergin, D. C. Lis, Á. Sánchez-Monge, A. Schwörer, and C. Comito\\
described\\
**[[https://doi.org/10.1051/0004-6361/202040203|//Herschel// observations of EXtraOrdinary Sources: Full //Herschel///HIFI Molecular Line Survey of Sagittarius B2(M)]]**\\
//Astron. Astrophys.// **651**, Art. No. A9 (2021).\\
They observed clearly the //J// = 2 − 1 transition of CD+ near 906.8 GHz mostly in absorption. The //J// = 4 − 2 transition near 1811.2 GHz is seen weakly in absorption.\\
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Contributor(s): H. S. P. Müller; 2003; 02, 2006; 03, 2007; 04, 2011; 05, 2020; 07, 2021.
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