===== On the Detection of H3+ and its Isotopologs in the Interstellar Medium =====
H3+ and D3+ do not have a pure rotational spectrum because of their cyclic (triangular) symmetry, but H3+ can been detected in the ISM by its rovibrational spectrum in the infrared region.\\
T. R. Geballe and T. Oka\\
used the United Kingdom InfraRed Telescope (UKIRT) for the\\
**[[https://doi.org/10.1038/384334a0|Detection of H3+ in Interstellar Space]]**\\
//Nature// **384**, 334–335 (1996).\\
They detected the lowest //para// transitions R(1,1)+ and R(1,1)– and the lowest //ortho// transition R(1,0) of the degenerate //ν//2 band near 2700 cm–1 in absorption in the dense molecular clouds RAFGL (or CRL) 2136 and W33A which both surround deeply embedded young stellar objects. CRL 2136 is also known as IRAS 18196-1331 or OH 17.6+0.2; W33A is also known as IRAS 18117-1753.
H3+ was also observed toward many other dense molecular clouds. Moreover, shortly thereafter,\\
B. J. McCall, T. R. Geballe, K. H. Hinkle, and T. Oka\\
reported on the\\
**[[https://doi.org/10.1126/science.279.5358.1910|Detection of H3+ in the Diffuse Interstellar Medium Toward Cygnus OB2 No. 12]]**\\
//Science// **279**, 1910–1913 (1998).\\
UKIRT was again employed to detect the same three rovibrational transitions as above. The source is also known as VI Cygni 12 or IRC +40430 or IRAS 20308+4104. The authors found surprisingly high column densities of the molecular cation. Similar findings were made in other diffuse molecular clouds. An explanation was found by\\
B. J. McCall, A. J. Huneycutt, R. J. Saykally, T. R. Geballe, N. Djuric, G. H. Dunn, J. Semaniak, O. Novotny, A. Al-Khalili, A. Ehlerding, F. Hellberg, S. Kalhori, A. Neau, R. Thomas, F. Österdahl, and M. Larsson,\\
who derived\\
**[[https://doi.org/10.1038/nature01498|An Enhanced Cosmic-ray Flux towards ζ Persei Inferred from a Laboratory Study of the H3+–e– Recombination Rate]]**\\
//Nature// **422**, 500–502 (2003).
Hot H3+ was detected in the Central Molecular Zone (CMZ), a region of radius ∼150 pc at the Galactic center both in diffuse molecular clouds via the metastable R(3,3)l transition at 2829.9 cm–1 by\\
M. Goto, B. J. McCall, T. R. Geballe, T. Usuda, N. Kobayashi, H. Terada, and T. Oka,\\
**[[https://ui.adsabs.harvard.edu/abs/2002PASJ...54..951G|Absorption Line Survey of H3+ toward the Galactic Center Sources I. GCS 3–2 and GC IRS3]]**\\
//Publ. Astron. Soc. Japan// **44**, 951–961 (2002)\\
using the Subaru IRCS; and also in dense molecular clouds via the metastable R(2,2)l transition at 2762.1 cm–1 in addition to R(3,3)l by\\
M. Goto, T. Usuda, T. Nagata, T. R. Geballe, B. J. McCall, N. Indriolo, H. Suto, T. Henning, C. P. Morong, and T. Oka,\\
**[[https://doi.org/10.1086/591657|Absorption Line Survey of H3+ toward the Galactic Center Sources. II. Eight Infrared Sources within 30 pc of the Galactic Center]]**\\
//Astrophys. J.// **688**, 306–319 (2008).\\
As it turned out, some velocity components toward GC IRS3 include dense molecular clouds because of the presence of this high density indicating transition, whereas it is absent in other velocity components. Subaru IRCS and UKIRT were employed in this study.
==== On the Detection of H2D+ ====
H2D+ as well as HD2+ have sizeable dipole moments because their centers of mass are different from the respective centers of charge.\\
R. Stark, F. F. S. van der Tak, and E. van Dishoek\\
used the JCMT 15 m radio telescope for the\\
**[[https://doi.org/10.1086/312182|Detection of Interstellar H2D+ Emission]]**\\
//Astrophys. J.// **521**, L67–L70 (1999).\\
The 11,0 – 11,1 //ortho// transition at 372421.385 MHz was detected toward the young stellar object NGC 1333 IRAS 4A.\\
P. Caselli, F. F. S. van der Tak, C. Ceccarelli, and A. Bacmann\\
used the CSO 14.4 m dish to detect the same line and concluded\\
**[[https://doi.org/10.1051/0004-6361:20030526|Abundant H2D+ in the Pre-stellar Core L1544]]**\\
//Astron. Astrophys.// **403**, L37–L41 (2003).\\
Several further reports appeared.
There is a second rotational ground state transition of //ortho//-H2D+, the 21,2 – 11,1 transition at 2363.243 GHz. It was detected in absorption with ISO by\\
J. Cernicharo, E. Polehampton, and J. R. Goicoechea:\\
**[[https://doi.org/10.1086/512857|Far-Infrared Detection of H2D+ toward Sgr B2]]**\\
//Astrophys. J.// **657**, L21–L24 (2007).
S. Brünken, O. Sipilä, E. T. Chambers, J. Harju, P. Caselli, O. Asvany, C. E. Honingh, T. Kamiński, K. M. Menten, J. Stutzki, and S. Schlemmer,\\
used the GREAT instrument on board the SOFIA aircraft observatory to detect the 10,1 – 00,0 ground state transition of //para//-H2D+ at 1370.085 GHz in absorption toward IRAS 16293-2422 and to deduce\\
**[[https://doi.org/10.1038/nature13924|H2D+ observations give an age of at least one million years for a cloud core forming Sun-like stars]]**\\
//Nature// **516**, 219–221 (2014).\\
See also the **[[https://www.mpifr-bonn.mpg.de/pressreleases/2014/11|MPIfR press release]]** (in English; **[[https://www.mpifr-bonn.mpg.de/pressemeldungen/2014/11|German version]]** also available) or the **[[https://portal.uni-koeln.de/universitaet/aktuell/presseinformationen/detail/das-alter-von-stellaren-kinderstuben|Universität zu Köln press release]]** (in German).
==== On the Detection of HD2+ ====
C. Vastel, T. G. Phillips, and H. Yoshida\\
used the CSO 10.4 m radio telescope for the\\
**[[https://doi.org/10.1086/421265|Detection of D2H+ in the Dense Interstellar Medium]]**\\
//Astrophys. J.// **606**, L127–L130 (2004).\\
The 11,0 – 10,1 //para// transition at 691660.483 MHz was barely detected in emission toward the prestellar core IRAS 16293-2422E.\\
B. Parise, A. Belloche, F. Du, R. Güsten, and K. M. Menten\\
observed the same line with the APEX 12 m radio telescope in the H-MM1 prestellar core located in the Ophiuchus L1688 cloud and found\\
**[[https://doi.org/10.1051/0004-6361/201015475|Extended Emission of HD2+ in a Pre-stellar Core]]**\\
//Astron. Astrophys.// **526**, Art. No. A31 (2011).
J. Harju, O. Sipilä, S. Brünken, S. Schlemmer, P. Caselli, M. Juvela, K. M. Menten, J. Stutzki, O. Asvany, T. Kamiński, Y. Okada, and R. Higgins\\
used the GREAT instrument on board the SOFIA aircraft observatory to detect the 11,1 – 00,0 ground state transition of //ortho//-H2D+ at 1476.606 GHz in absorption toward IRAS 16293-2422\\
**[[https://doi.org/10.3847/1538-4357/aa6c69|Detection of Interstellar //Ortho//-D2H+ with SOFIA]]**\\
//Astrophys. J.// **840**, Art. No. 63 (2017).
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Contributor(s): H. S. P. Müller; 4, 2004; 01, 2014; 12, 2015; 05, 2017
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