Letter to the Editor

Laboratory and astronomical discovery of cyanothioketene, NCCHCS, in the cold starless core TMC-1^⋆,⋆⋆

¹ Dept. de Astrofísica Molecular, Instituto de Física Fundamental (IFF-CSIC), C/ Serrano 121, 28006 Madrid, Spain
e-mail: carlos.cabezas@csic.es; jose.cernicharo@csic.es
² Department of Applied Chemistry, Science Building II, National Yang Ming Chiao Tung University, 1001 Ta-Hsueh Rd., Hsinchu 300098, Taiwan
³ Observatorio Astronómico Nacional (OAN, IGN), C/ Alfonso XII, 3, 28014 Madrid, Spain
⁴ Centro de Desarrollos Tecnológicos, Observatorio de Yebes (IGN), 19141 Yebes, Guadalajara, Spain
⁵ Center for Emergent Functional Matter Science, National Yang Ming Chiao Tung University, Hsinchu 300093, Taiwan

Received: 11 April 2024
Accepted: 2 May 2024

Abstract

We present the detection of cyanothioketene, NCCHCS, in the laboratory and toward TMC-1. This transient species was produced through a discharge of a gas mixture of CH₂CHCN and CS₂ using argon as carrier gas, and its rotational spectrum between 9 and 40 GHz was characterized using a Balle-Flygare narrowband-type Fourier-transform microwave spectrometer. A total of 21 rotational transitions were detected in the laboratory, all of them exhibiting hyperfine structure induced by the spin of the N nucleus. The spectrum for NCCHCS was predicted in the domain of our line surveys using the derived rotational and distortion constants. The detection in the cold starless core TMC-1 was based on the QUIJOTE¹ line survey performed with the Yebes 40 m radio telescope. Twenty-three lines were detected with K_a = 0, 1, and 2 and J_u = 9 up to 14. The derived column density is (1.2 ± 0.1)×10¹¹ cm⁻² for a rotational temperature of 8.5 ± 1.0 K. The abundance ratio of thioketene and its cyano derivative, H₂CCS/NCCHCS, is 6.5 ± 1.3. Although ketene is more abundant than thioketene by ∼15 times, its cyano derivative NCCHCO surprisingly is not detected with a 3σ upper level to the column density of 3.0 × 10¹⁰ cm⁻², which results in an abundance ratio H₂CCO/NCCHCO > 430. Hence, the chemistry of CN derivatives seems to be more favored for S-bearing than for O-bearing molecules. We carried out chemical modeling calculations and found that the gas-phase neutral-neutral reactions CCN + H₂CS and CN + H₂CCS could be a source of NCCHCS in TMC-1.