Herschel/HIFI Spectral Mapping of C^+, CH^+, and CH in Orion BN/KL: The Prevailing Role of Ultraviolet Irradiation in CH^+ Formation
Creators
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1.
Infrared Processing and Analysis Center
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National Science Foundation
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University of Cologne
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University of Toledo
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Jet Propulsion Lab
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California Institute of Technology
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Harvard-Smithsonian Center for Astrophysics
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Johns Hopkins University
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University of Michigan–Ann Arbor
Abstract
The CH^+ ion is a key species in the initial steps of interstellar carbon chemistry. Its formation in diverse environments where it is observed is not well understood, however, because the main production pathway is so endothermic (4280 K) that it is unlikely to proceed at the typical temperatures of molecular clouds. We investigate the formation of this highly reactive molecule with the first velocity-resolved spectral mapping of the CH^+ J = 1−0, 2−1 rotational transitions, three sets of CH Λ-doubled triplet lines, ^(12)C^+ and ^(13)C^+ ^(2)P_(3/2) - ^(2)P_(1/2), and CH_(3)OH 835 GHz E-symmetry Q-branch transitions, obtained with Herschel/HIFI over a region of ≈ 12 arcmin^2 centered on the Orion BN/KL source. We present the spatial morphologies and kinematics, cloud boundary conditions, excitation temperatures, column densities, and ^(12)C^+ optical depths. Emission from all of C^+, CH^+, and CH is indicated to arise in the diluted gas, outside the explosive, dense BN/KL outflow. Our models show that UV irradiation provides favorable conditions for steady-state production of CH^+ in this environment. Surprisingly, no spatial or kinematic correspondences of the observed species are found with H_2 S(1) emission tracing shocked gas in the outflow. We propose that C^+ is being consumed by rapid production of CO to explain the lack of both C^+ and CH^+ in the outflow. Hence, in star-forming environments containing sources of shocks and strong UV radiation, a description of the conditions leading to CH^+ formation and excitation is incomplete without including the important—possibly dominant—role of UV irradiation.
Additional Information
© 2016 The American Astronomical Society. Received 2015 December 31; revised 2016 March 22; accepted 2016 April 18; published 2016 September 14. This work is based on observations made with the HIFI instrument on the Herschel Space Observatory, which was designed and built by a consortium of institutes and university departments from across Europe, Canada, and the United States (the National Aeronautics and Space Administration, NASA) under the leadership of the Netherlands Institute for Space Research (SRON), Groningen, The Netherlands, and with major contributions from Germany, France, and the US. We express our gratitude to Nathan Crockett for helpful discussions and providing the Orion BN/KL continuum data, and to John Bally and Nathan Cunningham for providing the H2 observations of the BN/KL outflow. We also thank Octavio Roncero and Alexandre Zanchet for providing their CH+ state-to-state formation rates, and we especially appreciate support with the Meudon PDR code from Frank Le Petit. We are grateful to the HIFI Instrument Control Center team for its many years of dedicated work and support. We also thank an anonymous referee for thorough review of the manuscript and thoughtful comments to improve its quality. Support for this work was provided by NASA (Herschel GT funding) through an award issued by JPL/Caltech. V.O. acknowledges support by the Deutsche Forschungsgemeinschaft (DFG) via the collaborative research grant SFB 956, project C1. E.F. and M.G. thank the CNES and the INSU program PCMI for funding. A part of this research was performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. H.G. acknowledges support from the National Science Foundation (NSF). Any opinions, findings, and conclusions in this article are those of the authors, and do not necessarily reflect the views of the NSF.Attached Files
Published - apj_829_1_15.pdf
Submitted - 1604.05805v3.pdf
Files
1604.05805v3.pdf
Additional details
Identifiers
- Eprint ID
- 70636
- Resolver ID
- CaltechAUTHORS:20160928-111158868
Related works
- Describes
- https://arxiv.org/abs/1604.05805 (URL)
Funding
- NASA/JPL/Caltech
- Deutsche Forschungsgemeinschaft (DFG)
- SFB 956
- Centre National d'Études Spatiales (CNES)
- Institut National des Sciences de l'Univers (INSU)
- Programme National de Physique Chimie du Milieu Interstellaire (PCMI)
Dates
- Created
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2016-09-28Created from EPrint's datestamp field
- Updated
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2021-11-11Created from EPrint's last_modified field
Caltech Custom Metadata
- Caltech groups
- Infrared Processing and Analysis Center (IPAC)