===== Reports on Hydrogen Cyanide in the ISM/CSM Observed by Rotational Spectroscopy ===== HCN is abundant in all kinds of environments, from dark clouds to star-forming regions and circumstellar envelopes. The Detection of Interstellar HCN was reported in detail by\\ L. E. Snyder and D. Buhl,\\ **[[https://ui.adsabs.harvard.edu/abs/1971ApJ...163L..47S|Observations of Radio Emission from Interstellar Hydrogen Cyanide]]**\\ //Astrophys. J.// **163**, L47–L52 (1971).\\ The detection of H13CN was also reported. **Note:** Often one finds confusion about the two stretching modes of HCN. According to the generally accepted recommendation, we designate //v//1 = 1 as the **H-C** stretching state at 3311.5 cm–1 or 4764 K, and //v//3 = 1 as the **C-N** stretching state at 2096.8 cm–1 or 3017 K. Rather frequently, the numbering of the two stretching states is exchanged. And on occasion people use the wrong state designations.\\ States are designated according to the number of quanta excited: (//v//1//v//2//v//3). L. M. Ziurys and B. E. Turner,\\ **[[https://ui.adsabs.harvard.edu/abs/1986ApJ...300L..19Z|Detection of Interstellar Vibrationally Excited HCN]]**\\ //Astrophys. J.// **300**, L19–L23 (1986),\\ reported on the detection of HCN transitions within (010) and (020). Emission lines of both states were seen in the circumstellar envelope of CW Leo, aka IRC +10216. Lines of (010) were also detected toward Orion KL. T. D. Groesbeck, T. G. Philips, and G. A. Blake,\\ **[[https://doi.org/10.1086/192076|The Molecular Emission-Line Spectrum of IRC +10216 Between 330 and 358 GHz]]**\\ //Astrophys. J. Suppl. Ser.// **94**, L147–L162 (1994),\\ report on the detection of HCN transitions within (001) and (100) as well as (010) of H13CN. The observation of even higher states, up to (120), was reported by\\ L. W. Avery //et al.//,\\ **[[https://ui.adsabs.harvard.edu/abs/1994ApJ...426..737A|Submillimeter molecular line observations of IRC +10216: Searches for MgH, SiH2, and HCO+, and detection of hot HCN]]**\\ //Astrophys. J.// **426**, 737–741 (1994).\\ In this paper, the stretching states are designated as indicated above. However, the authors claimed they were using the traditional labelling. A plethora of excited vibrational states, several still higher than those described above (up to almost 11000 K), were detected, again toward CW Leo by\\ J. Cernicharo, M. Agúndez, C. Kahane, M. Guélin, J. R. Goicoechea, N. Marcelino, E. De Beck, and L. Decin,\\ **[[https://doi.org/10.1051/0004-6361/201116717|Probing the Dust Formation Region in IRC +10216 with the High Vibrational States of Hydrogen Cyanide]]**\\ //Astron. Astrophys.// **529**, Art. No. L3 (2011). Maser emission within (010) and (020) was reported by\\ S. Guilloteau, A. Omont, and R. Lucas,\\ **[[https://ui.adsabs.harvard.edu/abs/1987A%26A...176L..24G|A New Strong Maser – HCN]]**\\ //Astron. Astrophys.// **176**, L24–L26 (1987); and\\ R. Lucas and J. Cernicharo,\\ **[[https://ui.adsabs.harvard.edu/abs/1989A%26A...218L..20L|Discovery of Strong Maser Emission from HCN in IRC+10216]]**\\ //Astron. Astrophys.// **218**, L20–L22 (1989). Maser emission involving the Coriolis-coupled (040) and (001) states was reported by\\ P.Schilke, D. M. Mehringer, and K. M. Menten,\\ **[[https://doi.org/10.1086/312416|A Submillimeter HCN Laser in IRC +10216]]**\\ //Astrophys. J.// **528**, L37–L40 (2000); and\\ P.Schilke, D. M. Mehringer, and K. M. Menten\\ **[[https://doi.org/10.1086/345099|Detection of a Second, Strong Submillimeter HCN Laser Line toward Carbon Stars]]**\\ //Astrophys. J.// **583**, 446–450 (2003). The detection of direct-//l//-type transitions (Δ//J// = 0) within (010) paved the way for observing vibrationally excited HCN at centimeter wavelengths !\\ S. Thorwirth, F. Wyrowski, P. Schilke, K. M. Menten, S. Brünken, H. S. P. Müller, and G. Winnewisser,\\ **[[https://doi.org/10.1086/367629|Detection of HCN Direct-//l//-Type Transitions Probing Hot Molecular Gas in the Proto-Planetary Nebula CRL 618]]**\\ //Astrophys. J.// **586**, 338–343 (2003). Early observations of DCN were reported by\\ K. B. Jefferts, A. A.Penzias, and R. W. Wilson,\\ **[[https://ui.adsabs.harvard.edu/abs/1973ApJ...179L..57J|Deuterium in the Orion Nebula]]**\\ //Astrophys. J.// **179**, L57–L59 (1973); and by\\ R. W. Wilson, A. A.Penzias, K. B. Jefferts, and P. M. Solomon,\\ **[[https://ui.adsabs.harvard.edu/abs/1973ApJ...179L.107W|Interstellar Deuterium: the Hyperfine Structure of DCN]]**\\ //Astrophys. J.// **179**, L107–L110 (1973).\\ More recently, DCN was observed even in the photodissociation region Orion Bar:\\ S. Leurini, R. Rolffs, S. Thorwirth, B. Parise, P. Schilke, C. Comito, F. Wyrowski, R. Güsten, P. Bergman, K. M. Menten and L.-Å. Nyman,\\ **[[https://doi.org/10.1051/0004-6361:20065555|APEX 1 mm Line Survey of the Orion Bar]]**\\ //Astron. Astrophys.// **454**, L47–L50 (2006). Early observations of HC15N were reported by\\ R. A. Linke, P. F. Goldsmith, P. G. Wannier, R. W. Wilson, and A. A.Penzias,\\ **[[https://ui.adsabs.harvard.edu/abs/1977ApJ...214...50L|Isotopic Abundance Variations in Interstellar HCN]]**\\ //Astrophys. J.// **214**, 50–59 (1977). Transitions pertaining to (010) of H13CN had been seen earlier, see above; transitions up to (030) were reported by\\ J. Cernicharo, M. Agúndez, C. Kahane, M. Guélin, J. R. Goicoechea, N. Marcelino, E. De Beck, and L. Decin,\\ **[[https://doi.org/10.1051/0004-6361/201116717|Probing the Dust Formation Region in IRC +10216 with the High Vibrational States of Hydrogen Cyanide]]**\\ //Astron. Astrophys.// **529**, Art. No. L3 (2011). Rovibrational transitions have been also detected:\\ S. T. Ridgway, D. F. Carbon, and D. N. B. Hall,\\ **[[https://ui.adsabs.harvard.edu/abs/1978ApJ...225..138R|Polyatomic Species Contributing to the Carbon-star 3 micron Band]]**\\ //Astrophys. J.// **225**, 138–147 (1978). ---- Contributor(s): H. S. P. Müller; 04, 2004; 03, 2007; 06, 2011 ----