Astronomy & Astrophysics manuscript no. c6h4 ©ESO 2021 August 6, 2021 Letter to the Editor 1 Discovery of benzyne, o-C6H4, in TMC-1 with the QUIJOTE line survey? J. Cernicharo1, M. Agúndez1, R. I. Kaiser2, C. Cabezas1, B. Tercero3; 4, N. Marcelino1, J. R. Pardo1, P. de Vicente3 1 Grupo de Astrofísica Molecular, Instituto de Física Fundamental (IFF-CSIC), C/ Serrano 121, 28006 Madrid, Spain e-mail: [email protected] 2 Department of Chemistry, University of Hawaii at Manoa, Honolulu, HI 96822, USA 3 Centro de Desarrollos Tecnológicos, Observatorio de Yebes (IGN), 19141 Yebes, Guadalajara, Spain 4 Observatorio Astronómico Nacional (OAN, IGN), Madrid, Spain Received; accepted ABSTRACT We report the detection, for the first time in space, of a new non-functionalised hydrocarbon cycle in the direction of TMC-1: o- 11 −2 C6H4 (ortho-benzyne). We derive a column density for this hydrocarbon cycle of (5.0±1.0)×10 cm . The abundance of this species is around 30 times lower than that of cyclopentadiene and indene. We compare the abundance of benzyne with that of other pure hydrocarbons, cycles or chains, and find that it could be formed from neutral-radical reactions such as C2H + CH2CHCCH and C + C5H5, and possibly through C4H + C2H4,C3H + CH2CCH2, and C3H2+C3H3. Hence, the rich content of hydrocarbon cycles observed in TMC-1 could arise through a bottom-up scenario involving reactions of a few radicals with the abundant hydrocarbons recently revealed by the QUIJOTE line survey. Key words. molecular data – line: identification – ISM: molecules – ISM: individual (TMC-1) – astrochemistry 1. Introduction nicharo et al., 2001) suggests a bottom-up approach in which the small hydrocarbons that formed during the AGB phase, such The recent discovery with the QUIJOTE1 line survey as C2H2 and C2H4, interact with the UV radiation produced by of abundant hydrocarbons such as the propargyl radical the star in its evolution to the white dwarf phase (Woods et al. , (CH2CCH), vinylacetylene (CH2CHCCH), and ethynylallene 2002; Cernicharo, 2004). Other hypotheses involve the process- (H2CCCHCCH) and cyclic hydrocarbons such as cyclopentadi- ing of dust grains around evolved stars, either through UV pho- ene (c-C5H6) and indene (c-C9H8) in the cold dark cloud TMC-1 tons (Pilleri et al., 2015) or by chemical processes (Martínez (Agúndez et al., 2021a; Cernicharo et al., 2021a,b,c; Burkhardt et al., 2020). Hence, it has been surprising to see that pure et al., 2021), as well as the detection of their cyano derivatives PAHs and their cyanide derivatives have been found in TMC- (McGuire et al., 2018, 2021) in the same source, points to a new 1, which is well protected against UV radiation (Cernicharo et and interesting chemistry that had been completely unforeseen al., 2021c; Burkhardt et al., 2021; McGuire et al., 2018, 2021). by the most sophisticated state-of-the-art chemical models. The It is unlikely that these molecules arise from a reservoir existing detection of indene, the first non-functionalised polycyclic aro- since the early stages of the cloud because these relatively small matic hydrocarbons (PAHs) detected in space, in the unexpected PAHs would not have survived the diffuse cloud stage. An in environment of a cold starless core is surprising and introduces situ formation mechanism for benzene, cyclopentadiene, indene, new challenges to our understanding of the place were these and naphthalene must involve abundant hydrocarbons contain- PAHs are formed in space. ing from two to five carbon atoms (Kaiser & Hansen , 2021). Many efforts have been devoted over the last 40 years to Moreover, some of these species should allow an easy cycli- understanding the chemical processes that lead to the forma- sation in bimolecular reactions to efficiently form the first aro- tion of these molecular species (see e.g. Jones et al. 2011; matic ring – benzene (c-C6H6), benzyne (c-C6H4), the phenyl Parker et al. 2012; Joblin & Cernicharo 2018). Circumstellar arXiv:2108.02308v1 [astro-ph.GA] 4 Aug 2021 radical (c-C6H5), or any other species – from which larger PAHs envelopes around carbon-rich asymptotic giant branch (AGB) can grow (Jones et al. , 2011; Parker et al., 2012; McCabe et stars have been suggested as the factories of PAHs (Cherchn- al., 2020; Doddipatla et al., 2021). The progargyl radical and eff et al., 1992). However, PAHs are only detected in well-UV- the closed-shell hydrocarbons methyl-, vinyl-, and ethynylallene illuminated regions of the interstellar medium. The detection of have been detected in TMC-1 with high abundances (Agúndez benzene in the carbon-rich protoplanetary nebula CRL 618 (Cer- et al., 2021a; Cernicharo et al., 2021b,c). Detecting all possible products of the reactions between these hydrocarbons and rad- ? Based on observations carried out with the Yebes 40m telescope icals is a mandatory step towards understand the chemistry of (projects 19A003, 20A014, 20D023, and 21A011). The 40m radio tele- PAHs in cold dark clouds. scope at Yebes Observatory is operated by the Spanish Geographic In- stitute (IGN; Ministerio de Transportes, Movilidad y Agenda Urbana). The name benzyne refers to three isomeric species with 1 Q-band Ultrasensitive Inspection Journey to the Obscure TMC-1 molecular formula c-C6H4, which are six-membered rings with Environment four hydrogen atoms. They can be considered as derivatives of Article number, page 1 of 7 A&A proofs: manuscript no. c6h4 benzene molecules due to the loss of two hydrogen atoms. The most stable isomer is 1,2-didehydrobenzene (ortho-benzyne), followed by 1,3-didehydrobenzene (meta-benzyne) and the 1,4- didehydrobenzene (para-benzyne), placed at 147 and 220 kJ mol−1 from the ortho-benzyne, respectively (Olsen , 1971). The molecular structure of ortho-benzyne (hereinafter referred to as benzyne or as o-C6H4) is interesting because it contains a for- mal triple bond (‘-yne’) within a six-membered ring (see Fig. 1). This peculiar structure confers a high reactivity, and thus it is a reactive intermediate in organic chemistry that plays an im- portant role in aromatic nucleophilic substitution reactions. In addition, it is a potential key species in the formation of PAHs and soot in pyrolysis experiments of organic molecules (Hirsch et al. , 2018). In this letter we report the discovery of the ortho isomer of benzyne, o-C6H4, and we discuss the relative abundances of pure hydrocarbons, chains and cycles, in the context of chemi- cal models that include new radical-neutral reactions involving the abundant hydrocarbons recently found in TMC-1 (Agúndez et al., 2021a; Cernicharo et al., 2021a,b,c). We also discuss the evolution of these molecules when the volume density increases Fig. 1. Scheme of the ortho, para, and meta isomers of benzyne. due to the gravitational collapse of the core to form protostars and their dusty planetary disks. The two different frequency throws allow unambiguously neg- ative features produced in the folding of the frequency switch- 2. Observations ing data to be identified. These data allowed the detection of the + + New receivers, built within the Nanocosmos project2 and in- acetyl cation, CH3CO (Cernicharo et al., 2021d), HC3S (Cer- stalled at the Yebes 40m radio telescope, were used for the nicharo et al., 2021e), the isomers of C4H3N (Marcelino et al., h m s 2021), l-H2C5 (Cabezas et al., 2021c), vinyl and allenyl acety- observations of TMC-1 (αJ2000 = 4 41 41:9 and δJ2000 = +25◦41027:000). A detailed description of the system is given by lene (Cernicharo et al., 2021a,b), ethynyl cyclopropenylidene, Tercero et al. (2021). The receiver consists of two cold high elec- cyclopentadiene, and indene (Cernicharo et al., 2021c). Several tron mobility transistor amplifiers that cover the 31.0-50.3 GHz sulphur-bearing species (NCS, HCCS, H2CCS, H2CCCS, C4S, band with horizontal and vertical polarisations. Receiver temper- HCSCN, and HCSCCH) have also been detected with these data atures in the runs achieved in 2020 vary from 22 K at 32 GHz to (Cernicharo et al., 2021f,g). In addition, some molecules typi- 42 K at 50 GHz. Some power adaptation in the down-conversion cal of hot cores and corinos (C2H3CHO, C2H3OH, HCOOCH3, chains have reduced the receiver temperatures in 2021 to 16 K at CH3OCH3, CH3CH2CN) and species typical of circumstellar en- 32 GHz and 25 K at 50 GHz. The back ends are 2 × 8 × 2:5 GHz velopes (C5S, HCCN, HC4N) were also detected (Agúndez et fast Fourier transform spectrometers with a spectral resolution of al., 2021b; Cernicharo et al., 2021a). Last but not least, a sig- 38.15 kHz, providing the whole coverage of the Q band in both nificant number of singly and doubly deuterated species, such as HDCCN (Cabezas et al., 2021a), CH2DC3N (Cabezas et al., polarisations. All observations were performed in the frequency 13 switching mode. The main beam efficiency varies from 0.6 at 32 2021b), and the D and C doubly substituted isotopologues of GHz to 0.43 at 50 GHz. The intensity scale used in this work, HC3N (Tercero et al. 2021, in preparation), have also been ob- ∗ served for the first time in space thanks to QUIJOTE. antenna temperature (TA), was calibrated using two absorbers at different temperatures and the atmospheric transmission model The total observing time on the source by May 2021 was 240 ATM (Cernicharo, 1985; Pardo et al., 2001). Calibration uncer- hours. QUIJOTE is a living line survey, and we expect a total tainties were adopted to be 10 %. All data were analysed using observing time on the source of 450 hours by the end of 2021. the GILDAS package3. The final goal of QUIJOTE, a sensitivity of 0.1 mK (lines of 0.5 The QUIJOTE line survey was performed in several sessions mK detected at the 5σ level), will require around 1000 hours of between 2019 and 2021.