===== On the Detection of Propadienediylidene, C3, in Space =====
The pure carbon chain molecule propadienediylidene, C3, has been detected in the circumstellar envelope of the famous carbon star CW Leo, aka IRC +10216, by\\
K. H. Hinkle, J. J. Keady, and P. F. Bernath,\\
**[[https://doi.org/10.1126/science.241.4871.1319|Detection of C3 in the Circumstellar Shell of IRC +10216]]**,\\
//Science// **241**, 1319–1322 (1988)\\
at about the same time as the corresponding laboratory spectrum has been recorded. 15 //P// and //R// branch transitions of the ν3 band were detected around 2040 cm–1 of which 12 appear to be unblended.
Evidence for the presence of C3 was obtained from the observation of a weak absorption feature at 1968.594 GHz with the KAO toward Sagittarius B2 which was tentatively assigned to //R//(2) of the low-lying ν2 mode by\\
A. van Orden, J. D. Cruzan, R. A. Provencal, T. F. Giesen, R. J. Saykally, R. T. Boreiko, and A. L. Betz,\\
**[[https://ui.adsabs.harvard.edu/abs/1995ASPC...73...67V|A Search for the C3 Carbon Cluster in the Interstellar Medium]]**,\\
//ASP Conf. Ser.// **73**, 67–70 (1995)\\
This detection was confirmed by\\
J. Cernicharo, J. R. Goicoechea, and E. Caux,\\
**[[https://doi.org/10.1086/312668|Far-infrared Detection of C3 in Sagittarius B2 and IRC +10216]]**,\\
//Astrophys. J.// **534**, L199–L202 (2000).\\
The authors identified 9 transitions detected with ISO toward Sgr B2 along with the //R//(2) and //R//(4) toward CW Leo.
C3 has also been observed in the diffuse interstellar medium toward several stars by recording its //A// 1Πu – //X// 1Σ+g, //v// = 0 – 0 electronic spectrum around 405.2 nm.\\
J. P. Maier, N. M. Lakin, G. A. H. Walker, and D. A. Bohlender,\\
**[[https://doi.org/10.1086/320668|Detection of C3 in Diffuse Interstellar Clouds]]**,\\
//Astrophys. J.// **553**, 267–273 (2001),\\
studied sightlines toward 20 Aql, ζ Oph, and ζ Per. Two components were detected, a warm one around 60 K and a hot one around 250 K.
T. F. Giesen, B. Mookerjea, G. W. Fuchs, A. A. Breier, D. Witsch, R. Simon, and J. Stutzki\\
reported on the\\
**[[https://doi.org/10.1051/0004-6361/201936538|First Detection of the Carbon Chain Molecules 13CCC and C13CC towards SgrB2(M)]]**,\\
//Astron. Astrophys.// **633** Art. No. A120 (2020).\\
The average 12C/13C ratio per C atom is about 20.5, in the expected range. However, C13CC is slightly overabundant with a ratio of 14.8 ± 2.7 whereas 13CCC is underabundant with a ratio of 12.7 ± 2.8; the latter value corresponds to a 12C/13C ratio per C atom of 25.4 ± 5.6 because of two equivalent terminal C atoms.\\
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Contributor(s): H. S. P. Müller; 08, 2010; 02, 2020
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