Herschel observations of EXtra-Ordinary Sources: the present and future
  of spectral surveys with Herschel/HIFI
  We present initial results from the Herschel GT key program: Herschel
observations of EXtra-Ordinary Sources (HEXOS) and outline the promise and
potential of spectral surveys with Herschel/HIFI. The HIFI instrument offers
unprecedented sensitivity, as well as continuous spectral coverage across the
gaps imposed by the atmosphere, opening up a largely unexplored wavelength
regime to high-resolution spectroscopy. We show the spectrum of Orion KL
between 480 and 560 GHz and from 1.06 to 1.115 THz. From these data, we confirm
that HIFI separately measures the dust continuum and spectrally resolves
emission lines in Orion KL. Based on this capability we demonstrate that the
line contribution to the broad-band continuum in this molecule-rich source is
~20-40% below 1 THz and declines to a few percent at higher frequencies. We
also tentatively identify multiple transitions of HD18O in the spectra. The
first detection of this rare isotopologue in the interstellar medium suggests
that HDO emission is optically thick in the Orion hot core with HDO/H2O ~ 0.02.
We discuss the implications of this detection for the water D/H ratio in hot
cores.
