Silacyclopropynylidene, SiC₂, is a highly unsaturated ring molecule with a ¹A₁ ground electronic state. It is highly reactive, but not a radical. Based on a previously reported analysis of the origin band of its A – X electronic spectrum, 9 unidentified emission features from various observations of the circumstellar envelope (CSE) of the carbon-rich asymptotic giant branch star CW Leo, also known as IRC +10216, were assigned to this molecule by
P. Thaddeus, S. E. Cummins, and R. A. Linke,
Identification of the SiCC Radical toward IC +10216 – The First Molecular Ring in an Astronomical Source
Astrophys. J. 283, L45–L48 (1984).

A very large number of SiC₂ emission features to beyond 1100 GHz were observed by
H. S. P. Müller, J. Cernicharo, M. Agúndez, L. Decin, P. Encrenaz, J. C. Pearson, D. Teyssier und L. B. F. M. Waters,
Spectroscopic Parameters for Silacyclopropynylidene, SiC₂, from Extensive Astronomical Observations toward CW Leo (IRC +10216) with the //Herschel// Satellite
J. Mol. Spectrosc. 271 (2012) 50 – 55;
permitting the spectroscopic parameters to be improved considerably.

Using the structural and spectroscopic parameters of the main isotopolog, the three J = 4 – 3 transitions of ²⁹SiC₂ and ³⁰SiC₂ were found soon thereafter in the CSE of CW Leo by
J. Cernicharo, C. Kahane, J. Gómez-González, and M. Guélin,
Detection of ²⁹SiC₂ and ³⁰SiC₂ toward IRC +10216
Astron. Astrophys. 167, L9–L12 (1986).

Even though silacyclopropynylidene has been observed most prominently toward the CSE of CW Leo, it has been detected in the envelopes of other C-rich AGB stars, such as II Lup by
L.-Å. Nyman, H. Olofsson, L. E. B. Johansson, R. S. Booth, U. Carlström, and R. Wolstencroft,
A Molecular Radio Line Survey of the Carbon Star IRAS 15194-5115
Astron. Astrophys. 269, 377–389 (1993).
Other sources, in which SiC₂ has been detected, are RW LMi (IRC +30219, IRAS 10131+3049, CIT 6), IRAS 15082-4808 (CRL 4211), and LL Peg (CRL 3068, IRAS 23166+1655).

P. D. Gensheimer and L. E. Snyder
reported on
A Search for Vibrationally Excited SiC₂ //v//₃ = 1 toward IRC +10216
Astrophys. J. 490, 819–822 (1997);
was deemed tentative by the authors because of different line widths, but confirmed later.

Several transitions of SiC₂ v₃ = 2 were identified by
N. A. Patel, K. H. Young, C. A. Gottlieb, P. Thaddeus, R. W. Wilson, K. M. Menten, M. J. Reid, M. C. McCarthy, J. Cernicharo, J. H. He, S. Brünken, D.-V. Trung, and E. Keto,
An Interferometric Spectral-line Survey of IRC +10216 in the 345 GHz Band
Astrophys. J. Suppl. Ser. 193, Art. No. 17 (2011).

S. Massalkhi, I. Jiménez-Serra, J. Martín-Pintado, V. M. Rivilla, L. Colzi, S. Zeng, S. Martín, B. Tercero, P. de Vicente, and M. A. Requena-Torres
announced
The First Detection of SiC₂ in the Interstellar Medium
Astron. Astrophys. 678, Art. No. A45 (2023).
The molecule was detected in the course of a molecular line survey of the Galactic Center molecular cloud G+0.693–0.027 employing the Yebes 40 m and IRAM 30 m radio telecopes. The reported 6 a-type transitions, of which 4 were deemed to be unblended. The authors also identified SiS, ²⁹SiO, ³⁰SiO, Si¹⁸O, and SiO. The last molecule was optically thick, and its abundance was estimated from that of Si¹⁸O.

Contributor(s): H. S. P. Müller; 03, 2011; 06, 2013; 01, 2024