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| Methylidynium, CH<sup>+</sup>, was one of the first molecules to be identified in space. Its electronic //A// <sup>1</sup>Π – //X// <sup>1</sup>Σ 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:\\ | Methylidynium, CH<sup>+</sup>, was one of the first molecules to be identified in space. Its electronic //A// <sup>1</sup>Π – //X// <sup>1</sup>Σ 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,\\ | A. E. Douglas and G.Herzberg,\\ |
| **[[http://cdsads.u-strasbg.fr/abs/1941ApJ....94..381D|Note on CH<sup>+</sup> in Interstellar Space and in the Laboratory.]]**\\ | **[[https://ui.adsabs.harvard.edu/abs/1941ApJ....94..381D|Note on CH<sup>+</sup> in Interstellar Space and in the Laboratory.]]**\\ |
| //Astrophys. J.// **94**, 381 (1941).\\ | //Astrophys. J.// **94**, 381 (1941).\\ |
| The first two lines were detected toward ξ Per, χ<sup>2</sup> Ori, and 55 Cyg, the very first one also toward χ Aur by\\ | The first two lines were detected toward ξ Per, χ<sup>2</sup> Ori, and 55 Cyg, the very first one also toward χ Aur by\\ |
| T. Dunham, Jr.,\\ | T. Dunham, Jr.,\\ |
| **[[http://cdsads.u-strasbg.fr/abs/1937PASP...49...26D|Interstellar Neutral Potassium and Neutral Calcium]]**\\ | **[[https://ui.adsabs.harvard.edu/abs/1937PASP...49...26D|Interstellar Neutral Potassium and Neutral Calcium]]**\\ |
| //Publ. Astron. Soc. Pac.// **49**, 26–28 (1937).\\ | //Publ. Astron. Soc. Pac.// **49**, 26–28 (1937).\\ |
| As mentioned there, these lines were also observed by:\\ | As mentioned there, these lines were also observed by:\\ |
| W. S. Adams,\\ | W. S. Adams,\\ |
| **[[cdsads.u-strasbg.fr/abs/1941ApJ....93...11A|Some Results with the COUDÉ Spectrograph of the Mount Wilson Observatory]]**\\ | **[[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);\\ | //Astrophys. J.// **93**, 11–23 (1941);\\ |
| He reportedly also observed the third line. | He reportedly also observed the third line.\\ |
| |
| The detection of its pure rotational spectrum toward in the planetary nebula NGC 7027 was reported by\\ | 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,\\ | J. Cernicharo, X.-W. Liu, E. González-Alfonso, P. Cox, M. J. Barlow, T. Lim, and B. M. Swinyard,\\ |
| **[[http://dx.doi.org/10.1086/310729|Discovery of Far-Infrared Pure Rotational Transitions of CH<sup>+</sup> in NGC 7027]]**\\ | **[[https://doi.org/10.1086/310729|Discovery of Far-Infrared Pure Rotational Transitions of CH<sup>+</sup> in NGC 7027]]**\\ |
| //Astrophys. J.// **483**, L65–L68 (1997). | //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 <sup>12</sup>CH<sup>+</sup> is hampered by a terrestrial O<sub>2</sub> line, <sup>13</sup>CH<sup>+</sup> was observed instead:\\ | 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 <sup>12</sup>CH<sup>+</sup> is hampered by a terrestrial O<sub>2</sub> line, <sup>13</sup>CH<sup>+</sup> was observed instead:\\ |
| E. Falgarone, T. G. Phillips, and J. C. Pearson,\\ | E. Falgarone, T. G. Phillips, and J. C. Pearson,\\ |
| **[[http://dx.doi.org/10.1086/499047|First Detection of <sup>13</sup>CH<sup>+</sup> (//J// = 1 – 0)]]**\\ | **[[https://doi.org/10.1086/499047|First Detection of <sup>13</sup>CH<sup>+</sup> (//J// = 1 – 0)]]**\\ |
| //Astrophys. J.// **634**, L149–L152 (2005).\\ | //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. | 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 <sup>12</sup>CH<sup>+</sup> has been detected with the //Herschel// satellite in emission toward the Orion Bar by\\ | The //J// = 1 – 0 transition of <sup>12</sup>CH<sup>+</sup> has been detected with the //Herschel// satellite in emission toward the Orion Bar by\\ |
| E. Habart, E. Dartois, A. Abergel, et al.,\\ | E. Habart, E. Dartois, A. Abergel, et al.,\\ |
| **[[http://dx.doi.org/10.1051/0004-6361/201014654|SPIRE Spectroscopy of the Prototypical Orion Bar Photodissociation Region]]**\\ | **[[https://doi.org/10.1051/0004-6361/201014654|SPIRE Spectroscopy of the Prototypical Orion Bar Photodissociation Region]]**\\ |
| //Astron. Astrophys.// **518**, Art. No. L116 (2010);\\ | //Astron. Astrophys.// **518**, Art. No. L116 (2010);\\ |
| and by\\ | and by\\ |
| D. A. Naylor, E. Dartois, E. Habart,\\ | D. A. Naylor, E. Dartois, E. Habart,\\ |
| **[[http://dx.doi.org/10.1051/0004-6361/201014656|First Detection of the Methylidyne Cation (CH<sup>+</sup>) Fundamental Rotational Line with the //Herschel///SPIRE FTS]]**\\ | **[[https://doi.org/10.1051/0004-6361/201014656|First Detection of the Methylidyne Cation (CH<sup>+</sup>) Fundamental Rotational Line with the //Herschel///SPIRE FTS]]**\\ |
| //Astron. Astrophys.// **518**, Art. No. L117 (2010).\\ | //Astron. Astrophys.// **518**, Art. No. L117 (2010).\\ |
| The latter work also describes the observation of this transition in absorption toward two compact HII regions.\\ | 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\\ | E. Falgarone, V. Ossenkopf, M. Gerin, et al. observed\\ |
| **[[http://dx.doi.org/10.1051/0004-6361/201014671|Strong CH<sup>+</sup> //J// = 1 – 0 Emission and Absorption in DR21]]**\\ | **[[https://doi.org/10.1051/0004-6361/201014671|Strong CH<sup>+</sup> //J// = 1 – 0 Emission and Absorption in DR21]]**\\ |
| //Astron. Astrophys.// **518**, Art. No. L118 (2010).\\ | //Astron. Astrophys.// **518**, Art. No. L118 (2010).\\ |
| These authors were the first to use the correct rest frequency of 835137.5 MHz. | These authors were the first to use the correct rest frequency of 835137.5 MHz.\\ |
| |
| Higher-//J// transitions of CH<sup>+</sup> were also detected with the PACS instrument on board of the //Herschel// satellite in the disc around a Herbig Be star by:\\ | Higher-//J// transitions of CH<sup>+</sup> 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,\\ | 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,\\ |
| **[[http://dx.doi.org/10.1051/0004-6361/201116678|Detection of CH<sup>+</sup> Emission from the Disc around HD 100546]]**\\ | **[[https://doi.org/10.1051/0004-6361/201116678|Detection of CH<sup>+</sup> Emission from the Disc around HD 100546]]**\\ |
| //Astron. Astrophys.// **530**, Art. No. L2 (2011). | //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<sup>+</sup>) 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<sup>+</sup> 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<sup>+</sup> 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 | |
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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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