The butadiynylide ion has been detected toward the carbon rich star
IRC +10216 in 5 rotational transitions from J =
9 8 to 15 14 which occur in
the 3 and 2 mm regions:
J. Cernicharo, M. Guélin, M. Agúndez, K. Kawaguchi, M. C. McCarthy, and P. Thaddeus,
Astronomical Detection of C4H, the Second Interstellar Anion
Astron. Astrophys. 467, L37L40 (2007).
Several additional transitions of C6H, detected and reported earlier, were also observed.
The molecule was detected tentatively toward a low-mass protostar:
N. Sakai, T. Sakai, and S. Yamamoto,
Tentative Detection of C4H toward the Low-Mass Protostar IRAS 04368+2557 in L1527
Astrophys. J. 673, L71L74 (2008);
this detection was confirmed in a rather extensive study of anions and related neutral species:
M. Agúndez, J. Cernicharo, M. Guélin, M. Gerin, M. C. McCarthy, and P. Thaddeus,
Search for Anions in Molecular Sources: C4H Detection in L1527
Astron. Astrophys. 478, L19L22 (2008).