2022A&A...667L...4F

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 J_u = 10 up to 15 and K_a ≤ 2 for HCCCHCCC and J_u = 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 T_rot = 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.