Discovery of allenyl acetylene, H2CCCHCCH, in TMC-1. A study of the isomers of C5H4

TL;DR

This study reports the first detection of allenyl acetylene in TMC-1, compares its abundance with related isomers, and discusses implications for interstellar chemistry based on observational and modeling data.

Contribution

First identification of allenyl acetylene in space, with detailed analysis of its abundance and comparison to related isomers in TMC-1.

Findings

Allenyl acetylene has a similar abundance to methyl diacetylene and vinyl acetylene.

The rotational temperature of allenyl acetylene is about 7 K.

Other isomers like 1.4-pentadiyne are not detected, with upper limits established.

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)e13 cm-2. The other well known isomer of this molecule, methyl diacetylene (CH3C4H), has also been observed and we derived a similar rotational temperature, Trot = 7.0 +/- 0.3 K, and a column density for its two states (A and E) of (6.5 +/- 0.3)e12 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 3sigma upper limit of 2.5e12 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.