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 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.

Detection of vibrationally excited methyl formate in W51 e2

Methyl formate in its first torsionally excited state (vt=1 at 131 cm-1) is detected for the first time toward W51 e2. All transitions from excited methyl formate within the observed spectral range are actually detected (82 transitions) and no strong lines are missing. The column density of the excited state is comparable to that of the ground state. For a source size of 7 we find that Trot = 104 +/- 14 K and N = 9.4 +4.0/-2.8 x 10^16 cm-2 for the excited state and Trot = 176 +/- 24 K and N = 1.7 +.2/-.2 x 10^17 cm-2 for the ground state. Lines from ethyl cyanide in its two first excited states (vt=1, torsion mode at 212 cm-1) and (vb=1, CCN in-plane bending mode at 206 cm-1) are also present in the observed spectrum. However blending problems prevent a precise estimate of its abundance. With regard to the number of lines of excited methyl formate and ethyl cyanide detected in W51 e2, it appears that excited states of large molecules certainly account for a large number of unidentified lines in spectral survey of molecular clouds.