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, L37–L40 (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, L71–L74 (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, L19–L22 (2008).

Contributor(s): H. S. P. Müller; 5, 2007; 1, 2008