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,
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,
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,
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 ¹Π_u – 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,
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
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.

Contributor(s): H. S. P. Müller; 08, 2010; 02, 2020