2007A&A...476..791O

Spectral line surveys are useful since they allow identification of new molecules and new lines in uniformly calibrated data sets. The subsequent multi-transition analysis will provide improved knowledge of molecular abundances, cloud temperatures and densities, and may also reveal previously unsuspected blends of molecular lines, which otherwise may lead to erroneous conclusions. Nonetheless, large portions of the sub-millimetre spectral regime remain unexplored due to severe absorptions by H₂O and O₂ in the terrestrial atmosphere. The purpose of the measurements presented here is to cover wavelength regions at and around 0.55mm - regions largely unobservable from the ground. Using the Odin astronomy/aeronomy satellite, we performed the first spectral survey of the Orion KL molecular cloud core in the bands 486-492 and 541-576GHz with rather uniform sensitivity (22-25mK baseline noise). Odin's 1.1m size telescope, equipped with four cryo-cooled tuneable mixers connected to broad band spectrometers, was used in a satellite position-switching mode. Two mixers simultaneously observed different 1.1GHz bands using frequency steps of 0.5GHz (25h each). An on-source integration time of 20h was achieved for most bands. The entire campaign consumed ∼1100 orbits, each containing one hour of serviceable astro-observation. We identified 280 spectral lines from 38 known interstellar molecules (including isotopologues) having intensities in the range 80 to 0.05K. An additional 64 weak lines remain unidentified. Apart from the ground state rotational 1_1,0-1_0,1 transitions of ortho-H₂O, H₂¹⁸O and H₂¹⁷O, the high energy 6_2,4-7_1,7 line of para-H₂O (E_u=867K) and the HDO(2_0,2-1_1,1) line have been observed, as well as the 1₀-0₁ lines from NH₃ and its rare isotopologue ¹⁵NH₃. We suggest assignments for some unidentified features, notably the new interstellar molecules ND and SH^–. Severe blends have been detected in the line wings of the H₂¹⁸O, H₂¹⁷O and ¹³CO lines changing the true linewidths of the outflow emission.