SOLID METHANE TOWARD DEEPLY EMBEDDED PROTOSTARS

We report on the detection of an absorption feature near 7.67 mu m tow ard the deeply embedded protostellar objects W 33A and NGC 7538 : IRS9 . Comparison with laboratory spectra shows that this feature can be id entified as the v(4) ('deformation') mode of solid state CH4 embedded in polar molecules (i.e., H2O and/or CH3OH) in icy grain mantles. The solid CH4 column density relative to solid H2O is 0.4-1.9%. Unlike sol id CO, solid CH4 does not seem to be subjected to out-gassing toward t he warm source W 33A. The low gas-to-solid ratio for CH4 argues that C H4 is formed on grains either by hydrogenation of accreted C or by ult raviolet processing of CH3OH-rich ices.