Letter to the Editor

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

¹ Dept. de Astrofísica Molecular, Instituto de Física Fundamental (IFF-CSIC), C/ Serrano 121, 28006 Madrid, Spain
e-mail: r.fuentetaja@csic.es; jose.cernicharo@csic.es
² Centro de Desarrollos Tecnológicos, Observatorio de Yebes (IGN), 19141 Yebes, Guadalajara, Spain
³ Observatorio Astronómico Nacional (OAN, IGN), C/ Alfonso XII, 3, 28014 Madrid, Spain

Received: 19 September 2022
Accepted: 11 October 2022

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 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.