Methane Abundance Variations toward the Massive Protostar NGC 7538 IRS 9

Abstract

Absorption and emission lines originating from the ν₃ CH stretching manifold of gas-phase CH₄ 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 CH₄, 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 CH₄ abundance varies by an order of magnitude in this line of sight: it is low in the envelope and the outflow (X[CH₄]<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 CH₄ 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.

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.