Absorption and emission lines originating from the nu3 C-H 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 : IRS9. 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 um,
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]<0.4e-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.Comment: Scheduled for publication in ApJ 615, 01 Nov. 2004. 11 page
