2004ApJ...615..344B

Absorption and emission lines originating from the ν₃CH stretching manifold of gas-phase CH₄were 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 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]<0.4x10^–6) and at least a factor of 10 larger in the central core. The discovery of solid CH₄in independent ground- and space-based data sets shows that methane is nearly entirely frozen onto grains in the envelope. It thus appears that CH₄is 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.