Detection of vibrationally excited methyl formate in W51 e2

Context. Hot cores in molecular clouds, such as Orion KL, Sgr B2, W51 e1/e2, are characterized by the presence of molecules at temperature high enough to significantly populate their low-frequency vibrationally excited states. For complex organic molecules, characterized by a dense spectrum both in the ground state and in the excited states, such as methyl formate, ethyl cyanide or dimethyl ether, lines from vibrationally excited states certainly participate to the spectral confusion. Aims. Thanks to the recent laboratory study of the first torsional excited mode of methyl formate, we have searched for methyl formate, HCOOCH3, in its first torsionally excited state (υt=1) in the molecular cloud W51 e2. Methods. We have performed observations of the molecular cloud W51 e2 in different spectral regions at 1.3, 2 and 3 mm with the IRAM 30 m single dish antenna. Results. Methyl formate in its first torsionally excited state (υt=1 at 131 cm−1) is detected for the first time toward W51 e2. 82 transitions have been detected among which 46 are unblended with other species. For a total of 16 A-E pairs in the observed spectrum, 9 are unblended; these 9 pairs are all detected. All transitions from excited methyl formate within the observed spectral range are actually detected 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 ×10 16 cm−2 for the excited state and Trot = 176 ± 24 K and N = 1.7+.2 −.2 × 10 17 cm−2 for the ground state. Lines from ethyl cyanide in its two first excited states (υt=1, torsion mode at 212 cm−1) and (υb=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. However as for methyl formate it should be comparable with the ground state for which we find Trot = 103 ± 9 K and N = 3.7+0.6 −0.4 ×10 15 cm−2 for a 7′′source size. Conclusions. 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.