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 quiescent dark cloud L183 and the molecular outflow L1448-mm. We have not detected C2H5OH, HCOOCH3, or SiO in L183 or in the quiescent gas of L1448-mm. The abundances of CH3OH, H2CO, and SiO are enhanced by factors of 4-20 in the shock precursor component, and those of CH3OH and SiO by 3 and 4 orders of magnitude in the shocked gas, without substantial changes (less than a factor of 2) in the abundances of C2H5OH and HCOOCH3 relative to that of CH3OH. The large enhancements of SiO and CH3OH can be explained by the shock ejection of an important fraction of the grain mantle material into the gas phase. Our upper limits to the C2H5OH/CH3OH and HCOOCH3/CH3OH ratios are consistent with the rather constant ratios measured in hot cores and Galactic center clouds. However, the upper limits to the HCOOCH3/CH3OH and HCOOH/CH3OH ratios are at least 1 order of magnitude smaller than those found in hot corinos surrounding low-mass protostars. We speculate that the observed abundances of COMs in different objects are consistent with a sort of universal grain mantle composition that is locally changed by the processes of low-mass star formation.