CN and CCH derivatives of ethylene and ethane: Confirmation of the detection of CH${}_3$CH${}_2$CCH in TMC-1

Abstract

We present a study of CH${}_3$CH${}_2$CCH, CH${}_3$CH${}_2$CN, CH${}_2$CHCCH, and CH${}_2$CHCN in TMC-1 using the QUIJOTE${}^1$ line survey. We confirm the presence of CH${}_3$CH${}_2$CCH in TMC-1, which was previously reported as tentative by our group. From a detailed study of the ethynyl and cyanide derivatives of CH${}_2$CH${}_2$ and CH${}_3$CH${}_3$ in TMC-1, we found that the CH${}_2$CHCCH/CH${}_2$CHCN and CH${}_3$CH${}_2$CCH/CH${}_3$CH${}_2$CN abundance ratios are 1.5$\pm$0.1 and 4.8$\pm$0.5, respectively. The derived CH${}_2$CHCCH/CH${}_3$CH${}_2$CCH abundance ratio is 15.3$\pm$0.8, and that of CH${}_2$CHCN over CH${}_3$CH${}_2$CN is 48$\pm$5. All the single substituted isotopologs of vinyl cyanide have been detected, and we found that the first and second carbon substitutions in CH${}_2$CHCN provide a ${}^{12}$C/${}^{13}$C ratio in line with that found for other three-carbon bearing species such as HCCNC and HNCCC. However, the third ${}^{13}$C isotopolog, CH${}_2$CH${}^{13}$CN, presents an increase in its abundance similar to that found for HCCCN. Finally, we observed eight $b$-type transitions of CH${}_2$CHCN, and we find that their intensity cannot be fitted adopting the dipole moment ${\mu }_b$ derived previously. These transitions involve the same rotational levels as those of the $a$-type transitions. From their intensity, we obtain ${\mu }_b$=0.80$\pm$0.03\,D, which is found to be in between earlier values derived in the laboratory using intensity measurements or the Stark effect. Our chemical model indicates that the abundances of CH${}_3$CH${}_2$CCH, CH${}_3$CH${}_2$CN, CH${}_2$CHCCH, and CH${}_2$CHCN observed in TMC-1 can be explained in terms of gas-phase reactions.

Accepted as a Letter in Astronomy & Astrophysics on March 8th 2024

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