Cumulene carbenes in TMC-1: Astronomical discovery of $l$-H$_2$C$_5$

TL;DR

This paper reports the first astronomical detection of the cumulene carbene $l$-H$_2$C$_5$ in TMC-1, providing new insights into the abundance and chemistry of carbon chain molecules in space.

Contribution

The study presents the first detection of $l$-H$_2$C$_5$ in space and compares its abundance with other cumulene carbenes, advancing understanding of interstellar carbon chemistry.

Findings

$l$-H$_2$C$_5$ detected with a column density of (1.8±0.5)×10^{10} cm^{-2}

$l$-H$_2$C$_5$ is ~4 times less abundant than $l$-H$_2$C$_6$

$l$-H$_2$C$_5$ is 300-500 times less abundant than shorter chains $l$-H$_2$C$_3$ and $l$-H$_2$C$_4$

Abstract

We report the first detection in space of the cumulene carbon chain $l$-H$_2$C$_5$. A total of eleven rotational transitions, with $J_{up}$ = 7-10 and $K_a$ = 0 and 1, were detected in TMC-1 in the 31.0-50.4 GHz range using the Yebes 40m radio telescope. We derive a column density of (1.8$\pm$0.5)$\times$10$^{10}$ cm$^{-2}$. In addition, we report observations of other cumulene carbenes detected previously in TMC-1, to compare their abundances with the newly detected cumulene carbene chain. We find that $l$-H$_2$C$_5$ is $\sim$4.0 times less abundant than the larger cumulene carbene $l$-H$_2$C$_6$, while it is $\sim$300 and $\sim$500 times less abundant than the shorter chains $l$-H$_2$C$_3$ and $l$-H$_2$C$_4$. We discuss the most likely gas-phase chemical routes to these cumulenes in TMC-1 and stress that chemical kinetics studies able to distinguish between different isomers are needed to shed light on the chemistry of C$_n$H$_2$ isomers with $n$\,$>$\,3.