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

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 CH 2 CHCN and CS 2 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 QUIJOTE 1 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)×10 11 cm −2 for a rotational temperature of 8.5 ± 1.0 K. The abundance ratio of thioketene and its cyano derivative, H 2 CCS/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 3 σ upper level to the column density of 3.0 × 10 10 cm −2 , which results in an abundance ratio H 2 CCO/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 + H 2 CS and CN + H 2 CCS could be a source of NCCHCS in TMC-1.

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