DISCOVERY OF SOLID FORMALDEHYDE TOWARD THE PROTOSTAR GL-2136 - OBSERVATIONS AND LABORATORY SIMULATION

A search for the nu(5) band of solid formaldehyde toward the protostel lar source GL 2136 has resulted in the detection of absorption feature s near 3.47 and 3.54 mu m. A 8ood match of the 3.47 mu m feature exist s with solid H2CO features in laboratory analogs of interstellar ice c ondensates. The abundance of H2CO and the composition of the ice in wh ich it is found are constrained by the precise wavelength of the 3.47 mu m absorption, the profile of the 3.54 mu m absorption feature, and the previously observed 9.75 (CH3OH), 6.0, and 6.8 mu m features. From comparison with laboratory spectra, we conclude that the H2CO and CH3 OH are intimately mixed and that both contribute to the 3.54 mu m abso rption, while the profiles of the 6.0 and 6.8 mu m bands indicate a co ntribution of the solid H2CO nu(2) and nu(3) features at 5.8 and 6.7 m u m. We estimate that the abundance of H2CO is similar to 7 % of that of H2O seen along the same line of sight, but that only a small fracti on 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 c onsisting of almost pure H2O (type I ice) and the other rich in, if no t dominated by H2CO and CH3OH (type II ice). The possible origin of so lid H2CO and the implication of our results for the physical and chemi cal conditions in the cloud are discussed