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

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) × 1011 cm−2 with a rotational temperature of 6±1 K, while for HCCCCS we derived N = (9.5±0.8) × 1010 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.