1995A&A...295L..51B

We report observations of the 2_k-1_k, 3_k-2_k, and 5_k-4_k thermal lines of CH₃OH toward young bipolar outflows. Strong emission (~>6K, in the case of L 1157) is observed toward the shocked molecular regions associated with the lobes of some outflows, whereas the emission from the cold quiescent material in the surrounding molecular core is rather weak (~<1K). We have derived the methanol abundance by combining the CH₃OH data with new C¹⁸O observations. It results that methanol is enhanced by large factors toward the shocked regions (∼400 toward the blue-shifted lobe of L1157). Although the shocked gas is known to be heated to about 100K, the CH₃OH rotation temperatures have moderate values: in L 1157 T_rot increases from 8K in the ambient gas to 12K in the shocked region. Radiative transfer calculations, carried out to simulate the CH₃OH excitation in different physical circumstances, confirm that in the range of physical conditions discussed here the methanol molecules are very subthermally excited, and T_rot is not a good meusure of the kinetic temperature (T_rot≪T_K). The observed methanol abundance enhancements are likely caused by processes of desorption of grain mantles in shocks.