Submillimeter wave spectroscopy of ethyl isocyanide and its searches in Orion

Abstract

Context. About 40 cyanide compounds have been detected in the interstellar medium, but only 3 examples of organic isocyanide compounds were observed in this medium. Ethyl isocyanide is one of the best candidates for possible detection. Aim. To date, measurements of rotational spectra are limited to 40 GHz. The extrapolation of the prediction in the millimeter wave domain is inaccurate and does not permit an unambiguous detection. Methods. The rotational spectra were reinvestigated from 0.15 to 1 THz. Using the new prediction, we searched for the compound ethyl isocyanide in Orion KL and Sgr B2. Results. We newly assigned 2906 transitions and fitted these new data with those from previous studies, reaching quantum numbers up to J = 103 and Ka = 30. The asymmetric top Hamiltonian proposed by Watson in the Ir representation was used for the analysis, and both reductions A and S were tested. The search for CH3CH2NC in Sgr B2 (IRAM 30m) and Orion KL (IRAM 30m, ALMA Science Verification) result in a non-detection; upper limits to the column density were derived.