Solid methane toward deeply embedded protostars.
Prusti, T.; de Graauw, T.; Whittet, D. C. B.; Schutte, W. A.; Tielens, A. G. G. M.; Boogert, A. C. A.; Ehrenfreund, P.; Helmich, F. P.; Wesselius, P. R.
Abstract
We report on the detection of an absorption feature near 7.67μ m toward the deeply embedded protostellar objects W 33A and NGC 7538 : IRS9. Comparison with laboratory spectra shows that this feature can be identified as the ν_4_ (`deformation') mode of solid state CH_4_ embedded in polar molecules (i.e., H_2_O and/or CH_3_OH) in icy grain mantles. The solid CH_4_ column density relative to solid H_2_O is 0.4-1.9%. Unlike solid CO, solid CH_4_ does not seem to be subjected to out-gassing toward the warm source W 33A. The low gas-to-solid ratio for CH_4_ argues that CH_4_ is formed on grains either by hydrogenation of accreted C or by ultraviolet processing of CH_3_OH-rich ices.