Rotational spectrum of ~(13)C_2-methyl formate (HCOO~(13)CH_3)and detection of the two ~(13)C-methyl formate in Orion~*

摘要

Context. Laboratory measurements and analysis of the microwave and millimeter-wave spectra of potential interstellar moleculesare a prerequisite for their subsequent identification by radioastronomical techniques. The spectral analysis provides spectroscopicparameters that are used in the assignment procedure of the laboratory spectra, and that also predict the frequencies of transitions notmeasured in the laboratory with a high degree of precision.Aims. An experimental laboratory study and its theoretical analysis is presented for 13C2-methyl formate (HCOO13CH3) allowing asearch for this isotopologue in the Orion molecular cloud. The ~(13)C_1-methyl formate (H~(COOCH-3) molecule was also searched forin this interstellar cloud, using previously published spectroscopic data.Methods. The experimental spectra of 13C2-methyl formate were recorded in the microwave and sub-mm energy ranges (4-20 GHz,8-80 GHz, 150-700 GHz). The spectra were analyzed using the Rho-Axis Method (RAM), which takes the CH3 internal rotation andthe coupling between internal rotation and global rotation into account.Results. Twenty-seven spectroscopic constants of 13C2-methyl formate have been obtained from a fit of 936 transitions of the groundtorsional state with a standard (unitless) deviation of 1.08. A prediction of line positions and intensities is also produced. This predic-tion allowed us to identify 230 ~(13)C_2-methyl formate lines in the Orion interstellar molecular cloud. We refitted all previously publishedground state transitions of the ~(13)C_1-methyl formate molecule in order to provide a prediction of its ground state spectrum. 234 linesof 13C1-methyl formate were detected in the Orion interstellar cloud using that prediction.