1996A&A...309..633S

A search for the ν₅ band of solid formaldehyde toward the protostellar source GL 2136 has resulted in the detection of absorption features near 3.47 and 3.54µm. A good match of the 3.47µm feature exists with solid H₂CO features in laboratory analogs of interstellar ice condensates. The abundance of H₂CO and the composition of the ice in which it is found are constrained by the precise wavelength of the 3.47µm absorption, the profile of the 3.54µm absorption feature, and the previously observed 9.75 (CH₃OH), 6.0, and 6.8µm features. From comparison with laboratory spectra, we conclude that the H₂CO and CH₃OH are intimately mixed and that both contribute to the 3.54µm absorption, while the profiles of the 6.0 and 6.8µm bands indicate a contribution of the solid H₂CO ν₂ and ν₃ features at 5.8 and 6.7µm. We estimate that the abundance of H₂CO is ∼7% of that of H₂O seen along the same line of sight, but that only a small fraction of the H₂O is intimately mixed with the H₂CO. There appear to be at least two types of ice mantles present along the line of sight, one consisting of almost pure H₂O (type I ice) and the other rich in, if not dominated by H₂CO and CH₃OH (type II ice). The possible origin of solid H₂CO and the implication of our results for the physical and chemical conditions in the cloud are discussed