Solid methane toward deeply embedded protostars

A.C.A. Boogert - W.A. Schutte - A.G.G.M. Tielens - D.C.B. Whittet - F.P. Helmich - P. Ehrenfreund - P.R. Wesselius - Th. de Graauw - T. Prusti

Abstract:

We report on the detection of an absorption feature near 7.67 microns 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 v₄ (`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 solid CO, solid CH₄ does not seem to be subjected to out-gassing toward the warm source W 33A. The low gas-to-solid ratio for CH₄ argues that CH₄ is formed on grains either by hydrogenation of accreted C or by ultraviolet processing of CH₃OH-rich ices.

Keywords:
ISM: dust, extinction - ISM: molecules - ISM: abundances - infrared: ISM:lines and bands - stars: individual: NGC 7538 : IRS9 - stars: individual: W 33A

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