===== On C<sub>5</sub>H<sub>4</sub> isomers in the ISM =====

=== Ethynylallene ===

J. Cernicharo, C. Cabezas, M. Agúndez, B. Tercero, N. Marcelino, J. R. Pardo, F. Tercero, J. D. Gallego, J. A. López-Pérez, and P. de Vicente\\
reported on\\
**[[https://doi.org/10.1051/0004-6361/202140482|Discovery of allenyl acetylene, H<sub>2</sub>CCCHCCH in TMC-1 − study of the isomers of C<sub>5</sub>H<sub>4</sub>]]**\\
//Astron. Astrophys.// **647**, Art. No. L3 (2021).\\
Ethynylallene, aka allenylacetylene or 1,2-pentadien-4-yne, was identified through 16 unblended or slightly blended //a//-type transitions with 6 ≤ //J// ≤ 10 and //K<sub>a</sub>// ≤ 3 in the course of a molecular line survey of the prototypical cold dark molecular cloud TMC-1 carried out with the Yebes 40 m radio telescope between 32.8 and 49.2 GHz. The signal-to-noise ratio of the //K<sub>a</sub>// = 3 lines of these is on the edge, but the model supports the assignments. A value of //T//<sub>rot</sub> = 9 ± 1 K was derived through a rotation diagram, but the model is more compatible with 7 K. The column density of butenyne is about a factor of 4.5 higher than that of the isoelectronic allenyl cyanide. It is, in addition, similar to that of butenyne, a factor of 5 lower than that of the methyldiacetylene isomer (aka butadienyne), and a factor of 10 lower than that of methylacetylene (aka propyne).\\
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=== 1,4-Pentadiyne ===

R. Fuentetaja, M. Agúndez, C. Cabezas, B. Tercero, N. Marcelino, J. R. Pardo, P. de Vicente and J. Cernicharo \\
announced the\\
**[[https://doi.org/10.1051/0004-6361/202348777|Discovery of allenyl acetylene, Discovery of HCCCH<sub>2</sub>CCH in TMC-1 with the QUIJOTE line survey]]**\\
//Astron. Astrophys.// **688**, Art. No. L15 (2024).\\
1,4-Pentadiyne was identified through 18 mostly unblended or slightly blended //b//-type transitions with 2 ≤ //J// ≤ 12 and //K<sub>a</sub>// ≤ 2 in the course of a molecular line survey of the prototypical cold dark molecular cloud TMC-1 carried out with the Yebes 40 m radio telescope between 31.5 and 49.6 GHz. A value of //T//<sub>rot</sub> = 9.5 ± 0.5 K was derived. The column density of 1,4-pentadiyne is about a factor of 1.4 higher than that of the isoelectronic mono cyanide and about a factor of 1.4 less abundant than the ethynylallene isomer.\\
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Contributor(s): H. S. P. Müller; 05, 2021; 08, 2024

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