Spectroscopic study and astronomical detection of doubly 13C-substituted
  ethyl cyanide
  We have performed a spectral line survey called EMoCA toward Sagittarius
B2(N) between 84 and 114 GHz with ALMA. Line intensities of the main isotopic
species of ethyl cyanide and its singly 13C-substituted isotopomers observed
toward the hot molecular core Sgr B2(N2) suggest that the doubly
13C-substituted isotopomers should be detectable also. We want to determine the
spectroscopic parameters of all three doubly 13C-substituted isotopologues of
ethyl cyanide to search for them in our ALMA data. We investigated the
laboratory rotational spectra of the three species between 150 and 990 GHz. We
searched for emission lines produced by these species in the ALMA spectrum of
Sgr B2(N2). We modeled their emission as well as the emission of the 12C and
singly 13C-substituted isotopologues assuming local thermodynamic equilibrium.
We identified more than 5000 rotational transitions, pertaining to more than
3500 different transition frequencies, in the laboratory for each of the three
isotopomers. The quantum numbers reach J ~ 115 and K_a ~ 35, resulting in
accurate spectroscopic parameters and accurate rest frequency calculations
beyond 1000 GHz for strong to moderately weak transitions of either isotopomer.
All three species are unambiguously detected in our ALMA data. The 12C/13C
column density ratio of the isotopomers with one 13C atom to the ones with two
13C atoms is about 25. Ethyl cyanide is the second molecule after methyl
cyanide for which isotopologues containing two 13C atoms have been securely
detected in the interstellar medium. The model of our ethyl cyanide data
suggests that we should be able to detect vibrational satellites of the main
species up to at least v_19 = 1 at 1130 K and up to v_13 + v_21 = 2 at 600 K
for the isotopologues with one 13C atom in our present ALMA data. Such
satellites may be too weak to be identified unambiguously for isotopologues
with two 13C atoms.
