Discovery of allenyl acetylene, H₂CCCHCCH, in TMC-1

Abstract

We present the discovery in TMC-1 of allenyl acetylene, H2CCCHCCH, through the observation of nineteen lines with a signal-to-noise ratio ∼4–15. For this species, we derived a rotational temperature of 7 ± 1 K and a column density of 1.2 ± 0.2 × 1013 cm−2. The other well known isomer of this molecule, methyl diacetylene (CH3C4H), has also been observed and we derived a similar rotational temperature, Tr = 7.0 ± 0.3 K, and a column density for its two states (A and E) of 6.5 ± 0.3 × 1012 cm−2. Hence, allenyl acetylene and methyl diacetylene have a similar abundance. Remarkably, their abundances are close to that of vinyl acetylene (CH2CHCCH). We also searched for the other isomer of C5H4, HCCCH2CCH (1.4-Pentadiyne), but only a 3σ upper limit of 2.5 × 1012 cm−2 to the column density can be established. These results have been compared to state-of-the-art chemical models for TMC-1, indicating the important role of these hydrocarbons in its chemistry. The rotational parameters of allenyl acetylene have been improved by fitting the existing laboratory data together with the frequencies of the transitions observed in TMC-1.

Detection of deuterated methylcyanoacetylene, CH2DC3N, in TMC-1

“…Even though the equilibrium rotational constants differ slightly from the ground state constants, we can assume similar discrepancies for CH 3 C 3 N and CH 2 DC 3 N, and thus that the estimated constants for CH 2 DC 3 N are essentially unaffected. Methyldiacetylene (CH 3 C 4 H) is 7.5 times more abundant than, and lines have similar intensities to, CH 3 C 3 N (Marcelino et al 2021;Cernicharo et al 2021c). Hence, it is straightforward to think that spectral signatures of the deuterated species of CH 3 C 4 H could also be detected in our line survey.…”

“…We only consider gas-phase mechanisms that allow quantitative predictions based on some experimental measurements and theoretical studies. The chemistry of the different C 4 H 3 N and C 5 H 4 isomers has been discussed recently in Cernicharo et al (2021c) and Marcelino et al (2021), respectively, in relation with their detection in TMC-1. These two chemical families are tightly linked to the chemistry of methylacetylene, CH 3 CCH, and its isomer allene , CH 2 CCH 2 : Considering the deuterated analogues of these reactions introduces diverse questions, for example whether the CN reactions proceed without changing the methyl radical or lead to some scrambling of the hydrogen atoms in a quasi-stationary intermediate followed by different reaction channels.…”

TMC-1, the starless core sulfur factory: Discovery of NCS, HCCS, H2CCS, H2CCCS, and C4S and detection of C5S

“…For C 4 S and C 5 S, we observed several individual lines and their detection is solid. In order to provide additional support for it, we stacked the data following the procedure used recently to detect CH 3 CH 2 CCH in TMC-1 (Cernicharo et al 2021c). The data were normalised to the expected intensity of each transition computed under local thermodynamic equilibrium (LTE) for the rotational temperature derived from the analysis of the individual lines.…”

Detection of interstellar H2CCCHC3N

“…Because the allenyl acetylene was detected with observations at higher frequencies (34-50 GHz) compared to the GOTHAM survey, direct abundance comparisons should be taken with a degree of caution as they can be highly degenerate with the assumed or fitted source size. Cernicharo et al (2021b) assumed a source size radius of 40 for allenyl acetylene, while the derived full width at half maximum source sizes of the individual velocity components for cyanoacetyleneallene here ranged from 70-259 for Source Model B. Nevertheless, as shown in Table 1, the hydrocarbon to CN derivative ratio ranges from 30-120, with a large variation due to a combination of different telescopes, analysis techniques, and production routes; it is clear, however, that the column density derived from Source Model B is most consistent with previous determinations, whereas Source Model A represents a scenario not yet seen toward this source.…”

“…This species, recently analyzed in the laboratory by McCarthy et al (2020), could plausibly represent such a chemical "missing link". The first detection of the unsubstituted form of this molecule, allenyl acetylene (H 2 CCCHCCH), also in TMC-1, has recently been reported by Cernicharo et al (2021b); they also detected a number of new interstellar species in that source using data taken with the Yebes 40 m radio telescope (Cernicharo et al 2021a,c;Marcelino et al 2021;Cabezas et al 2021;Agundez et al 2021), thereby allowing for further examinations of their abundances and potential chemical connections.…”