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,
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.,
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,
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,
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,
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.,
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,
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
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,
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
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
//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.

Contributor(s): H. S. P. Müller; 2003; 02, 2006; 03, 2007; 04, 2011; 05, 2020; 07, 2021.