Organic Chemistry in the Dark Clouds L1448 and L183: A Unique Grain Mantle Composition

We present high sensitivity observations of the complex organic molecules(COMs) CH3OH, C2H5OH, HCOOCH3, HCOOH and H2CO and of SiO toward the quiescentdark cloud L183 and the molecular outflow L1448-mm. We have not detectedC2H5OH, HCOOCH3 and SiO in L183 and in the quiescent gas of L1448-mm. Theabundances of CH3OH, H2CO and SiO are enhanced by factors 4-20 in the shockprecursor component, and those of CH3OH and SiO by 3 and 4 orders of magnitudein the shocked gas, without substantial changes (< factor of 2) in theabundances of C2H5OH and HCOOCH3 relative to that of CH3OH. The largeenhancements of SiO and CH3OH can be explained by the shock ejection of animportant fraction of the grain mantle material into gas phase. Our upperlimits to the C2H5OH/CH3OH and HCOOCH3/CH3OH ratios are consistent with therather constant ratios measured in hot cores and Galactic center clouds.However, the upper limits to the HCOOCH3/CH3OH and the HCOOH/CH3OH ratios areat least one order of magnitude smaller than those found in"hot corinos"surrounding low mass protostars. We speculate that the observed abundances ofCOMs in different objects are consistent with a sort of "universal" grainmantle composition which is locally changed by the processes of low mass starformation.Comment: 10 pages, 1 figure. Accepted ApJ Letter