N. W. Broten, T. Oka, L. W. Avery, J. M. MacLeod, and H. W. Kroto,
The Detection of HC<sub>9</sub>N 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 HC<sub>9</sub>N in the Circumstellar Envelope of IRC +10216
Astrophys. J. 400, 551–555 (1992).

Truong-Bach, D. Graham, and Nguyen-Q-Rieu,
HC<sub>9</sub>N 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 HC<sub>7</sub>N and HC<sub>9</sub>N 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.

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