Boogert, A. C. A. and Blake, G. A. and Öberg, K. (2004) Methane Abundance Variations toward the Massive Protostar NGC 7538 IRS 9. Astrophysical Journal, 615 (1). pp. 344-353. ISSN 0004-637X http://resolver.caltech.edu/CaltechAUTHORS:20120917-154344773
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Absorption and emission lines originating from the ν_3 C–H stretching manifold of gas-phase CH_4 were discovered in the high-resolution (R = 25,000) infrared L-band spectrum along the line of sight toward NGC 7538 IRS 9. These observations provide a diagnostic of the complex dynamics and chemistry in a massive star-forming region. The line shapes resemble P Cygni profiles with the absorption and emission components shifted by ~7 km s^(-1) with respect to the systemic velocity. Similar velocity components were observed in CO at 4.7 μm, but in contrast to CH_4, the CO shows deep absorption due to a high-velocity outflow as well as absorption at the systemic velocity due to the cold outer envelope. It is concluded that the gas-phase CH_4 abundance varies by an order of magnitude in this line of sight: it is low in the envelope and the outflow (X[CH_4] < 0.4 × 10^(-6)) and at least a factor of 10 larger in the central core. The discovery of solid CH4 in independent ground- and space-based data sets shows that methane is nearly entirely frozen onto grains in the envelope. It thus appears that CH_4 is formed by grain surface reactions, evaporates into the gas phase in the warm inner regions of protostellar cores, and is efficiently destroyed in shocks related to outflows.
|Additional Information:||© 2004 The American Astronomical Society. Received 2004 March 4; accepted 2004 July 8. Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands, and the United Kingdom), and with the participation of ISAS and NASA. The research of A. C. A. B. and G. A. B. is supported by the Spitzer Legacy Science program and by the Owens Valley Radio Observatory through NSF grant AST 02-28955. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. We thank Jan Cami for providing a copy of ISO SWS IA, Fred Lahuis for an independent reduction, Tim Brooke for suggesting the possible ethane identification, and Yuk Yung for help with the wavelength calibration. Suggestions by an anonymous referee have led to an improved presentation of the data.|
|Subject Keywords:||astrochemistry; infrared: ISM; ISM: abundances; ISM: molecules; stars: formation; stars: individual (NGC 7538 IRS 9)|
|Official Citation:||Methane Abundance Variations toward the Massive Protostar NGC 7538 IRS 9 A. C. A. Boogert et al. 2004 ApJ 615 344|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Tony Diaz|
|Deposited On:||18 Sep 2012 17:17|
|Last Modified:||26 Dec 2012 16:11|
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