Herschel Observations of Extraordinary Sources: Analysis of the HIFI 1.2 THz Wide Spectral Survey toward Orion KL. I. Methods

Nathan R. Crockett, Edwin A. Bergin, Justin L. Neill, Cécile Favre, Peter Schilke, Dariusz C. Lis, Tom A. Bell, Geoffrey Blake, José Cernicharo, Martin Emprechtinger, Gisela B. Esplugues, Harshal Gupta, Maria Kleshcheva, Steven Lord, Nuria Marcelino, Brett A. McGuire, John Pearson, Thomas G. Phillips, Rene Plume, Floris van der TakBelén Tercero, Shanshan Yu

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We present a comprehensive analysis of a broadband spectral line survey of the Orion Kleinmann-Low nebula (Orion KL), one of the most chemically rich regions in the Galaxy, using the HIFI instrument on board the Herschel Space Observatory. This survey spans a frequency range from 480 to 1907 GHz at a resolution of 1.1 MHz. These observations thus encompass the largest spectral coverage ever obtained toward this high-mass star-forming region in the submillimeter with high spectral resolution and include frequencies >1 THz, where the Earth's atmosphere prevents observations from the ground. In all, we detect emission from 39 molecules (79 isotopologues). Combining this data set with ground-based millimeter spectroscopy obtained with the IRAM 30 m telescope, we model the molecular emission from the millimeter to the far-IR using the XCLASS program, which assumes local thermodynamic equilibrium (LTE). Several molecules are also modeled with the MADEX non-LTE code. Because of the wide frequency coverage, our models are constrained by transitions over an unprecedented range in excitation energy. A reduced χ2 analysis indicates that models for most species reproduce the observed emission well. In particular, most complex organics are well fit by LTE implying gas densities are high (>106 cm-3) and excitation temperatures and column densities are well constrained. Molecular abundances are computed using H2 column densities also derived from the HIFI survey. The distribution of rotation temperatures, T rot, for molecules detected toward the hot core is significantly wider than the compact ridge, plateau, and extended ridge T rot distributions, indicating the hot core has the most complex thermal structure. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
Original languageEnglish
Pages (from-to)112-146
Number of pages35
JournalThe Astrophysical Journal
Issue number2
Publication statusPublished - Jun-2014


  • ISM: abundances
  • ISM: individual objects: Orion KL
  • ISM: molecules

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