13C-methyl formate: observations of a sample of high mass star-forming regions including Orion-KL and spectroscopic characterization

Abstract

We have surveyed a sample of massive star-forming regions located over a range of distances from the Galactic centre for methyl formate, HCOOCH${}_3$, and its isotopologues H${}^{13}$COOCH${}_3$ and HCOO${}^{13}$CH${}_3$. The observations were carried out with the APEX telescope in the frequency range 283.4--287.4~GHz. Based on the APEX observations, we report tentative detections of the ${}^{13}$C-methyl formate isotopologue HCOO${}^{13}$CH${}_3$ towards the following four massive star-forming regions: Sgr~B2(N-LMH), NGC~6334~IRS~1, W51 e2 and G19.61-0.23. In addition, we have used the 1~mm ALMA science verification observations of Orion-KL and confirm the detection of the ${}^{13}$C-methyl formate species in Orion-KL and image its spatial distribution. Our analysis shows that the ${}^{12}$C/${}^{13}$C isotope ratio in methyl formate toward Orion-KL Compact Ridge and Hot Core-SW components (68.4$\pm$10.1 and 71.4$\pm$7.8, respectively) are, for both the ${}^{13}$C-methyl formate isotopologues, commensurate with the average ${}^{12}$C/${}^{13}$C ratio of CO derived toward Orion-KL. Likewise, regarding the other sources, our results are consistent with the ${}^{12}$C/${}^{13}$C in CO. We also report the spectroscopic characterization, which includes a complete partition function, of the complex H${}^{13}$COOCH${}_3$ and HCOO${}^{13}$CH${}_3$ species. New spectroscopic data for both isotopomers H${}^{13}$COOCH${}_3$ and HCOO${}^{13}$CH${}_3$, presented in this study, has made it possible to measure this fundamentally important isotope ratio in a large organic molecule for the first time.

arXiv commentAccepted for publication in The Astrophysical Journal Supplement Series

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