Spectroscopic characterization and detection of Ethyl Mercaptan in Orion
  New laboratory data of ethyl mercaptan, CH$_{3}$CH$_{2}$SH, in the millimeter
and submillimeter-wave domains (up to 880 GHz) provided very precise values of
the spectroscopic constants that allowed the detection of
$gauche$-CH$_3$CH$_2$SH towards Orion KL. 77 unblended or slightly blended
lines plus no missing transitions in the range 80-280 GHz support this
identification. A detection of methyl mercaptan, CH$_{3}$SH, in the spectral
survey of Orion KL is reported as well. Our column density results indicate
that methyl mercaptan is $\simeq$ 5 times more abundant than ethyl mercaptan in
the hot core of Orion KL.
