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, 13191322 (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 cm1 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, 6770 (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, L199L202 (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
v = 0 0 electronic spectrum around
J. P. Maier, N. M. Lakin, G. A. H. Walker, and D. A. Bohlender,
Detection of C3 in Diffuse Interstellar Clouds,
Astrophys. J. 553, 267273 (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.