The gas/solid methane abundance ratio toward deeply embedded protostars

Abstract

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, interstellar 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. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA.