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 QUIJOTE1 line survey. Thirty rotational transitions with Ka = 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) × 1012 cm−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 CH2 derivative of cyclopentadiene, for which we derive a 3σ upper limit to its column density of (3.5 ± 0.5) × 1012 cm−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 C2H4 produces a good agreement between model and observed abundances.

Discovery of two new interstellar molecules with QUIJOTE: HCCCHCCC and HCCCCS

“…The TMC-1 cloud is remarkably rich in hydrocarbons, such as long carbon chains, propargyl (Agúndez et al 2021), vinyl acetylene (Cernicharo et al 2021b), and allenyl diacetylene (Fuentetaja et al 2022). Furthermore, several cyclic molecules, such as indene, cyclopentadiene (Cernicharo et al 2021c), orthobenzyne (Cernicharo et al 2021a), and fulvenallene (Cernicharo et al 2022), have been discovered there.…”

The QUIJOTE line survey of TMC-1

“…The detection of cyclopentadiene and of indene [5], together with very abundant hydrocarbons such as CH 2 CCH, CH 2 CHCCH, CH 2 CCHCCH, CH 2 CCHC 4 H [2][3][4]9], indicates that many reactions have to be included in the chemical models. In particular reactions between cations and radicals, and cations with large hydrocarbons are missing so far in the best models of cold astrophysical environments [26]. The large abundances of hydrocarbons and PAHs in these unexpected environments require a sensitive survey such as QUIJOTE to unveil all the aspects of the chemical complexity of cold prestellar cores [26].…”

“…In particular reactions between cations and radicals, and cations with large hydrocarbons are missing so far in the best models of cold astrophysical environments [26]. The large abundances of hydrocarbons and PAHs in these unexpected environments require a sensitive survey such as QUIJOTE to unveil all the aspects of the chemical complexity of cold prestellar cores [26].…”

The reactivity of pyridine in cold interstellar environments: The reaction of pyridine with the CN radical

“…Amongst the molecules that have been detected recently, cyclic (aromatic or not) molecules in cold environments constitute a significant breakthrough. To date, several cyclic molecules containing five atoms or more in their structure have been detected, most of them in the Taurus Molecular Cloud (TMC-1) namely benzonitrile (c-C 6 H 5 -CN) (McGuire et al, 2018), cyano-cyclopentadiene (c-C 5 H 4 -CN) (McCarthy et al, 2020), cyano-napthalene, (McGuire et al, 2021), benzyne (Cernicharo et al, 2021c), cyclopentadiene (Cernicharo et al, 2021b), indene (Cernicharo et al, 2021b), or fulvenallene (Cernicharo, J. et al, 2022). From the last wave of detections numerous questions arise, for example, whether the molecules are formed from a bottom-up approach from simpler precursors or are produced from the fragmentation of, for example, graphene (Merino et al, 2014).…”