2009A&A...493..565M

Ethyl cyanide is an abundant molecule in hot molecular clouds. Its rotational spectrum is very dense and several hundreds of rotational transitions within the ground state have been identified in molecular clouds in the 40-900GHz frequency range. Lines from ¹³C isotopically substituted ethyl cyanide were identified in Orion. To enable the search and the possible detection of other isotopologues of ethyl cyanide in interstellar objects, we have studied the rotational spectrum of deuterated ethyl cyanide: CH₂DCH₂CN (in-plane and out-of-plane) and CH₃CHDCN and the spectrum of ¹⁵N substituted ethyl cyanide CH₃CH₂C¹⁵N. Using these experimental data, we have searched for these species in Orion. The rotational spectrum of each species in the ground state was measured in the microwave and millimeter-submillimeter wavelength range using a waveguide Fourier Transform spectrometer (8-17GHz) and a source-modulated spectrometer employing backward-wave oscillators (BWOs) (150-260 and 580-660 GHz). More than 300 lines were identified for each species, for J values in the range 71-80 and K_a values in the range 28-31 depending on the isotopologues. The experimental spectra were analyzed using a Watson's Hamiltonian in the A-reduction. From the fitting procedure, accurate spectroscopic constants were derived for each of the species. These new sets of spectroscopic constants enable us to predict reliably the rotational spectrum (lines frequencies and intensities) in the 4-1000 GHz frequency range and for J and K_a up to 80 and 31, respectively. Combined with IRAM 30m antenna observations of Orion, this experimental study allowed us to detect ¹⁵N substituted ethyl cyanide CH₃CH₂C¹⁵N for the first time in Orion. The derived column density and rotational temperature are 10¹³cm^–2 and 150K for the plateau and 3x10¹⁴cm^–2 and 300K for the hot core. The deuterated species were searched for but were not detected. The upper limit to the column density of each deuterated isotopologues was 10¹⁴cm^–2.