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

Abstract

We report on the discovery of two new molecules, HCCCHCCC and HCCCCS, towards the starless core TMC-1 in the Taurus region from the QUIJOTE line survey in the 31.1–50.2 GHz frequency range. We identify a total of twenty-nine lines of HCCCHCCC and six rotational transitions of HCCCCS. The rotational quantum numbers range from Ju = 10 up to 15 and Ka ≤ 2 for HCCCHCCC and Ju = 21/2 up to 31/2 for HCCCCS. We derived a column density for HCCCHCCC of N = (1.3±0.2) × 1011 cm−2 with a rotational temperature of 6±1 K, while for HCCCCS we derived N = (9.5±0.8) × 1010 cm−2 and Trot = 10±1 K. The abundance of HCCCHCCC is higher than that of its recently discovered isomer, l-H2C6. If we compare HCCCCS with its related molecules, HCS and HCCS, we obtain abundance ratios HCS/HCCCCS = 58 and HCCS/HCCCCS = 7.2. We investigated the formation of these two molecules using chemical modelling calculations. The observed abundances can be accounted for by assuming standard gas-phase formation routes involving neutral-neutral reactions and ion-neutral reactions.

Elucidating the Formation of Ethynylbutatrienylidene (HCCCHCCC; X1A′) in the Taurus Molecular Cloud (TMC-1) via the Gas-phase Reaction of Tricarbon (C3) with the Propargyl Radical (C3H3)

“…The calculated abundance of ethynylbutatrienylidene (HCCCHCCC) is shown in Figure 5 (top) together with the reactants involved in Reactions (3)-( 5). It can be clearly seen that the modeled peak abundance of ethynylbutatrienylidene (HCCCHCCC) of 1.2 × 10 −11 , reached at a time of a few 10 5 yr, agrees very well with the fractional abundance of 1.3 ± 0.2 × 10 −11 derived in TMC-1 by Fuentetaja et al (2022). The main reaction of formation of ethynylbutatrienylidene (HCCCHCCC) is Reaction (5) at a level of at least 99% (Figure 5, bottom), which is very efficient in the chemical model due to the very large abundances calculated for tricarbon (C 3 ), which is microwave inactive due to the lack of a permanent dipole moment, and the propargyl radical (C 3 H 3 ), which is well constrained by observations.…”

“…Note that propynylidyne (l-HCCC; Thaddeus et al 1985) Here we reveal through a combined computational and astrochemical modeling approach that the gas-phase formation of the recently detected ethynylbutatrienylidene (HCCCHCCC) can be quantitatively explained via the reaction of tricarbon (C 3 ) with the resonantly stabilized propargyl radical (C 3 H 3 ). The observed fractional abundance of 1.3 ± 0.2 × 10 −11 with respect to molecular hydrogen (Fuentetaja et al 2022) is nicely replicated for TMC-1 models with a simulated peak abundance of close to 1.2 × 10 −11 after a few 10 5 yr. Reaction (4) contributes 4 orders of magnitude less to the fractional abundances of ethynylbutatrienylidene (HCCCHCCC) compared to Reaction (5), whereas Reaction (3) forms triacetylene (HCCCCCCH) plus atomic hydrogen, but no ethynylbutatrienylidene (HCCCHCCC).…”

“…The recent detection of the C s symmetric ethynylbutatrienylidene (HCCCHCCC; X 1 A′) molecule (Fuentetaja et al 2022) -an isomer of triacetylene (HCCCCCCH; X 1 Σ g + ) and the C 2v symmetric hexapentaenylidene (H 2 CCCCCC) (Langer et al 1997)-in TMC-1 (Figure 2) represents a particular challenge since anticipated bimolecular neutral-neutral reactions of the ethynyl radical (C 2 H) with diacetylene (HCCCCH) (Reaction (1)) and of the butadiynyl radical (C 4 H) with acetylene (C 2 H 2 ) (Reaction (2)) only lead to the microwave inactive triacetylene (HCCCCCCH) isomer (Vakhtin et al 2001;Landera et al 2008;Gu et al 2009;Berteloite et al 2010). Therefore, bimolecular and exoergic reactions in the C2-C4 system cannot form the recently detected ethynylbutatrienylidene (HCCCHCCC) molecule…”

Linking the dust and chemical evolution: Taurus and Perseus

“…This cloud is in an environment less exposed to external radiation fields compared to, for example, photodissociation regions (PDRs; Xu et al 2016), leading to a high richness in molecular complexity (Agúndez & Wakelam 2013;Gratier et al 2016;Fehér et al 2016) with a plethora of hydrocarbon chains and cycles being currently detected (see, e.g., QUIJOTE Large Program, Cernicharo et al 2021aCernicharo et al ,b, 2022Cabezas et al 2022;Fuentetaja et al 2022). It features a differentiated chemistry between its northern and southern areas that may be linked to different evolutionary stages (Sipilä & Caselli 2018;Navarro-Almaida et al 2021).…”

Discovery of CH3CHCO in TMC-1 with the QUIJOTE line survey

“…2022a), and ethynylbutatrienylidene (Fuentetaja et al 2022b) have also been discovered toward TMC-1. Many of these species were not expected because they did not show a high abundance in chemical models.…”