Discovery of two new interstellar molecules with QUIJOTE: HCCCHCCC and HCCCCS

TL;DR

This study reports the discovery of two new interstellar molecules, HCCCHCCC and HCCCCS, in the TMC-1 starless core, providing their spectral data, abundance measurements, and insights into their formation mechanisms.

Contribution

The paper presents the first identification and spectral characterization of HCCCHCCC and HCCCCS in space, along with chemical modeling of their formation pathways.

Findings

HCCCHCCC has a column density of (1.3±0.2)×10^11 cm^-2 and Trot of 6±1 K.

HCCCCS has a column density of (9.5±0.8)×10^10 cm^-2 and Trot of 10±1 K.

Abundance ratios suggest formation via gas-phase neutral-neutral and ion-neutral reactions.

Abstract

We report on the discovery of two new molecules, HCCCHCCC and HCCCCS, towards the starless core TMC-1 in the Taurus region from the QUIJOTE line survey in the 31.1-50.2 GHz frequency range. We identify a total of twenty-nine lines of HCCCHCCC and six rotational transitions of HCCCCS. The rotational quantum numbers range from Ju=10 up to 15 and Ka <=2 for HCCCHCCC and Ju=21/2 up to 31/2 for HCCCCS. We derived a column density for HCCCHCCC of N=(1.3+/-0.2)x10^11 cm-2 with a rotational temperature of 6+/-1 K, while for HCCCCS we derived N=(9.5+/-0.8)x10^10 cm-2 and Trot =10+/-1 K. The abundance of HCCCHCCC is higher than that of its recently discovered isomer, l-H2C6. If we compare HCCCCS with its related molecules, HCS and HCCS, we obtain abundance ratios HCS/HCCCCS=58 and HCCS/HCCCCS=7.2. We investigated the formation of these two molecules using chemical modelling calculations. The observed abundances can be accounted for by assuming standard gas-phase formation routes involving neutral-neutral reactions and ion-neutral reactions.