1998A&A...336..352B

We present the detection of ro-vibrational absorption lines of the deformation mode of gaseous CH₄ 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 CH₄ gas is warm (T∼90K), and has a low abundance (∼4x10^–7). The CH₄ 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 CH₄. The observations impose strong limitations on time dependent gas phase models, e.g. a low initial CO/C ratio would be required, the CH₄must 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, interstellar CH₄is formed through grain surface reactions at a high CO/C ratio, which explains the low observed CH₄ abundance (∼10^–6), the presence of CH₄ in a polar ice, the low gas/solid ratio, and the absence of a strong cold CH₄ gas component. The observed warm CH₄ gas probably has sublimated from the grains in the `hot core' region surrounding the protostar.