Detection of Methanol in a Class 0 Protostellar Disk

We report the detection of emission from methanol in a compact source coincident with the position of the L1157 infrared source, which we attribute to molecules in the disk surrounding this young, class 0 protostellar object. Using the Caltech Owens Valley Millimeter Array with a synthesized beam size of 2'', we detect spatially unresolved methanol emission in the 2_k-1_k transitions at 3 mm wavelength, which is coincident in position with the peak of the continuum emission. The gas-phase methanol could be located in the central region (<100 AU radius) of a flat disk or in an extended heated surface layer (~200 AU radius) of a flared disk. The fractional abundance of methanol X(CH₃OH) is ~2 × 10^-8 in the flat disk model and ~3 × 10^-7 for the surface layer of a flared disk. The large variation in the fractional abundance between the warm portion of the flared disk and the disk as a whole makes it plausible that substantial chemical processing via depletion and desorption has occurred.