N. W. Broten, T. Oka, L. W. Avery, J. M. MacLeod, and H. W. Kroto,
The Detection of HC₉N in Interstellar Space
Astrophys. J. 223, L105–L107 (1978)
reported the first detection of this cyanopolyyne in two transitions (J = 18 – 17 and 25 – 24 near 10.5 and 14.5 GHz, respectively) in Heile's Cloud 2 employing the 46 m radio telecope of the Algonquin Radio Observatory before the molecules was detected in the laboratory. The spectroscopic parameters were estimated from the smaller cyanopolyynes. Heile's Cloud 2 is better known as TMC-1.

More recently, the molecule was also detected in six transitions in the circumstellar shell of the famous carbon star CW Leo:
M. B. Bell, L. W. Avery, J. M. MacLeod, and H. E. Matthews,
The Excitation Temperature of HC₉N in the Circumstellar Envelope of IRC +10216
Astrophys. J. 400, 551–555 (1992).

Truong-Bach, D. Graham, and Nguyen-Q-Rieu,
HC₉N from the Envelopes of IRC +10216 and CRL 2688
Astron. Astrophys. 277, 133–138 (1993);
detected one additional higher-J transition toward the former source and the same transition in a protoplanetary nebula.

J. R. Pardo, C. Bermúdez, C. Cabezas, M. Agúndez, J. D. Gallego, J. P. Fonfría, L. Velilla-Prieto, G. Quintana-Lacaci, B. Tercero, M. Guélin, and J. Cernicharo
reported on the
Detection of Vibrationally Excited HC₇N and HC₉N in IRC +10216
Astron. Astrophys. 640, Art. No. L13 (2020).
The authors carried out an unbiased molecular line survey with the Yebes 40 m dish between 31 and 50 GHz toward the circumstellar envelope of the famous C-rich AGB star CW Leonis. Numerous transitions of HC₉N in its lowest excited state v₁₉ = 1 were detected despite the low rotational temperature. The lines could be assigned unambiguously although no laboratory data were available for this excited state. The derived HC₅N to HC₇N ratio is extremely low, roughly 2.4. The derived HC₇N to HC₉N ratio is still very low, roughly 7.7.

Contributor(s): H. S. P. Müller; 02, 2006; 06, 2009; 08, 2020.