The gas / solid methane abundance ratio toward deeply embedded protostars ?

We present the detection of ro-vibrational absorption lines of the deformation mode of gaseous CH 4 toward the massive protostars W 33A, and NGC 7538 : IRS9, using the SWS spectrometer on board of the Infrared Space Observatory. The observed lines indicate that the CH 4 gas is warm (T ∼ 90 K), and has a low abundance (∼ 4 10 −7). The CH 4 ice in these lines of sight is embedded in a polar matrix (Boogert et al. 1996), and the gas/solid state abundance ratio is low (∼ 0.5). We discuss models for the formation of interstellar CH 4. The observations impose strong limitations on time dependent gas phase models, e.g. a low initial CO/C ratio would be required, the CH 4 must have been formed and subsequently condensed on the grains within a narrow time window, and an additional mechanism would be necessary to form polar ice mantles. More likely, in-terstellar CH 4 is formed through grain surface reactions at a high CO/C ratio, which explains the low observed CH 4 abundance (∼ 10 −6), the presence of CH 4 in a polar ice, the low gas/solid ratio, and the absence of a strong cold CH 4 gas component. The observed warm CH 4 gas probably has sublimated from the grains in the 'hot core' region surrounding the protostar.