<i>Herschel</i>observations of EXtra-Ordinary Sources (HEXOS): The present and future of spectral surveys with<i>Herschel</i>/HIFI

We present initial results from the <i>Herschel<i/> GT key program: <i>Herschel<i/> observations of EXtra-Ordinary Sources (HEXOS) and outline the promise and potential of spectral surveys with <i>Herschel<i/>/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 HD<sup>18<sup/>O 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/H<sub>2<sub/>O ~ 0.02. We discuss the implications of this detection for the water D/H ratio in hot cores.