Discovery of fulvenallene in TMC-1 with the QUIJOTE line survey

• Published in: Astronomy and astrophysics (Berlin) 2022-07, Vol.663, p.L9
• Main Authors: Cernicharo, J., Fuentetaja, R., Agúndez, M., Kaiser, R. I., Cabezas, C., Marcelino, N., Tercero, B., Pardo, J. R., de Vicente, P.
• Format: Article
• Language: English
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361/202244399

We report the detection of fulvenallene ( c -C 5 H 4 CCH 2 ) in the direction of TMC-1 with the QUIJOTE 1 line survey. Thirty rotational transitions with K a  = 0,1,2,3 and J  = 9−15 were detected. The best rotational temperature fitting of the data is 9 K and a derived column density is (2.7 ± 0.3) × 10 12 cm −2 , which is only a factor of 4.4 below that of its potential precursor cyclopentadiene ( c -C 5 H 6 ), and 1.4–1.9 times higher than that of the ethynyl derivatives of cyclopentadiene. We searched for fulvene ( c -C 5 H 4 CH 2 ), a CH 2 derivative of cyclopentadiene, for which we derive a 3 σ upper limit to its column density of (3.5 ± 0.5) × 10 12 cm −2 . Upper limits were also obtained for toluene (C 6 H 5 CH 3 ) and styrene (C 6 H 5 C 2 H 3 ), the methyl and vinyl derivatives of benzene. Fulvenallene and ethynyl cyclopentadiene are likely formed in the reaction between cyclopentadiene ( c -C 5 H 6 ) and the ehtynyl radical (CCH). However, the bottom-up gas-phase synthesis of cycles in TMC-1 underestimates the abundance of cyclopentadiene by two orders of magnitude, which strengthens the need to study all possible chemical pathways to cyclisation in cold dark cloud environments, such as TMC-1. However, the inclusion of the reaction between C 3 H 3 + and C 2 H 4 produces a good agreement between model and observed abundances.