13C-methyl formate: Observations of a sample of high-mass star-forming regions including Orion-KL and spectroscopic characterization
We have surveyed a sample of massive star-forming regions located over a range of distances from the Galactic center for methyl formate, HCOOCH3, and its isotopologues H13COOCH3 and HCOO13CH3. 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 13C-methyl formate isotopologue HCOO13CH3 toward 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 13C-methyl formate species in Orion–KL and image its spatial distribution. Our analysis shows that the 12C/13C isotope ratio in methyl formate toward the Orion–KL Compact Ridge and Hot Core-SW components (68.4 ± 10.1 and 71.4 ± 7.8, respectively) are, for both the 13C-methyl formate isotopologues, commensurate with the average 12C/13C ratio of CO derived toward Orion–KL. Likewise, regarding the other sources, our results are consistent with the 12C/13C in CO. We also report the spectroscopic characterization, which includes a complete partition function, of the complex H13COOCH3 and HCOO13CH3 species. New spectroscopic data for both isotopomers H13COOCH3 and HCOO13CH3, presented in this study, have made it possible to measure this fundamentally important isotope ratio in a large organic molecule for the first time.