Ortiz Morales, Jorge L. and Ceccarelli, Cecilia and Lis, Dariusz C. and Olmi, Luca and Plume, René and Schilke, Peter (2014) Ionization toward the high-mass star-forming region NGC 6334 I. Astronomy and Astrophysics, 563 . Art. No. A127. ISSN 0004-6361. http://resolver.caltech.edu/CaltechAUTHORS:20140509-151247082
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Context. Ionization plays a central role in the gas-phase chemistry of molecular clouds. Since ions are coupled with magnetic fields, which can in turn counteract gravitational collapse, it is of paramount importance to measure their abundance in star-forming regions. Aims. We use spectral line observations of the high-mass star-forming region NGC 6334 I to derive the abundance of two of the most abundant molecular ions, HCO^+ and N_2H^+, and consequently, the cosmic ray ionization rate. In addition, the line profiles provide information about the kinematics of this region. Methods. We present high-resolution spectral line observations conducted with the HIFI instrument on board the Herschel Space Observatory of the rotational transitions with J_(up) ≥ 5 of the molecular species C^(17)O, C^(18)O, HCO^+, H^(13)CO^+, and N_2H^+. Results. The HCO^+ and N_2H^+ line profiles display a redshifted asymmetry consistent with a region of expanding gas. We identify two emission components in the spectra, each with a different excitation, associated with the envelope of NGC 6334 I. The physical parameters obtained for the envelope are in agreement with previous models of the radial structure of NGC 6334 I based on submillimeter continuum observations. Based on our new Herschel/HIFI observations, combined with the predictions from a chemical model, we derive a cosmic ray ionization rate that is an order of magnitude higher than the canonical value of 10^(-17) s^(-1). Conclusions. We find evidence of an expansion of the envelope surrounding the hot core of NGC 6334 I, which is mainly driven by thermal pressure from the hot ionized gas in the region. The ionization rate seems to be dominated by cosmic rays originating from outside the source, although X-ray emission from the NGC 6334 I core could contribute to the ionization in the inner part of the envelope.
|Additional Information:||© ESO 2014. Article published by EDP Sciences. Received 12 June 2013. Accepted 24 January 2014. Published online 20 March 2014. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA. HIFI has been designed and built by a consortium of institutes and university departments from across Europe, Canada and the United States under the leadership of SRON Netherlands Institute for Space Research, Groningen, The Netherlands, and with major contributions from Germany, France and the US. Consortium members are: Canada: CSA, U.Waterloo; France: CESR, LAB, LERMA, IRAM; Germany: KOSMA, MPIfR, MPS; Ireland: NUI Maynooth; Italy: ASI, IFSI-INAF, Osservatorio Astrofisico di Arcetri-INAF; Netherlands: SRON, TUD; Poland: CAMK, CBK; Spain: Observatorio Astronómico Nacional (IGN), Centro de Astrobiología (CSIC-INTA); Sweden: Chalmers University of Technology − MC2, RSS & GARD; Onsala Space Observatory; Swedish National Space Board, Stockholm University − Stockholm Observatory; Switzerland: ETH Zurich, FHNW; USA: Caltech, JPL, NHSC. Support for this work was provided by NASA through an award issued by JPL/Caltech. J.M.O. acknowledges the support of NASA, through the PR NASA Space Grant Doctoral Fellowship, and from the Institut de Planétologie et d’Astrophysique de Grenoble (IPAG). C.C. acknowledges the financial support from the French Agence Nationale pour la Recherche (ANR) (project FORCOMS, contract ANR-08-BLAN-0225) and the French spatial agency CNES. The authors wish to thank Marco Padovani for providing the models for computing the CR ionization rate, as well as the anonymous referee and Ana López-Sepulcre, whose comments much contributed to improve this work.|
|Subject Keywords:||stars: formation, ISM: clouds, ISM: molecules, cosmic rays, ISM: abundances|
|Official Citation:||Ionization toward the high-mass star-forming region NGC 6334 I Jorge L. Morales Ortiz, Cecilia Ceccarelli, Dariusz C. Lis, Luca Olmi, René Plume and Peter Schilke A&A 563 A127 (2014) DOI: http://dx.doi.org/10.1051/0004-6361/201322071|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Ruth Sustaita|
|Deposited On:||09 May 2014 22:53|
|Last Modified:||09 May 2014 22:53|
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