Rotational spectra of isotopic species of methyl cyanide, CH$_3$CN, in
  their ground vibrational states up to terahertz frequencies
  Methyl cyanide is an important trace molecule in star-forming regions. It is
one of the more common molecules used to derive kinetic temperatures in such
sources. As preparatory work for Herschel, SOFIA, and in particular ALMA we
want to improve the rest frequencies of the main as well as minor isotopologs
of methyl cyanide. The laboratory rotational spectrum of methyl cyanide in
natural isotopic composition has been recorded up to 1.63 THz. Transitions with
good signal-to-noise ratio could be identified for CH$_3$CN, $^{13}$CH$_3$CN,
CH$_3^{13}$CN, CH$_3$C$^{15}$N, CH$_2$DCN, and $^{13}$CH$_3^{13}$CN in their
ground vibrational states up to about 1.2 THz. The main isotopic species could
be identified even in the highest frequency spectral recordings around 1.6 THz.
The highest $J'$ quantum numbers included in the fit are 64 for
$^{13}$CH$_3^{13}$CN and 89 for the main isotopic species. Greatly improved
spectroscopic parameters have been obtained by fitting the present data
together with previously reported transition frequencies. The present data will
be helpful to identify isotopologs of methyl cyanide in the higher frequency
bands of instruments such as the recently launched Herschel satellite, the
upcoming airplane mission SOFIA or the radio telescope array ALMA.
