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

Abstract

We report the detection of fulvenallene (c-C5H4CCH2) in the direction of TMC-1 with the QUIJOTE1line survey. Thirty rotational transitions withKa = 0,1,2,3 andJ = 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) × 1012cm−2, which is only a factor of 4.4 below that of its potential precursor cyclopentadiene (c-C5H6), and 1.4–1.9 times higher than that of the ethynyl derivatives of cyclopentadiene. We searched for fulvene (c-C5H4CH2), a CH2derivative of cyclopentadiene, for which we derive a 3σupper limit to its column density of (3.5 ± 0.5) × 1012cm−2. Upper limits were also obtained for toluene (C6H5CH3) and styrene (C6H5C2H3), the methyl and vinyl derivatives of benzene. Fulvenallene and ethynyl cyclopentadiene are likely formed in the reaction between cyclopentadiene (c-C5H6) 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 C3H3+and C2H4produces a good agreement between model and observed abundances.