METHANOL ENHANCEMENT IN YOUNG BIPOLAR OUTFLOWS

We report observations of the 2k-1k, 3k-2k, and 5k-4k thermal lines of CH3OH toward young bipolar outflows. Strong emission (greater-than-or -similar-to 6 K, 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 mole cular core is rather weak (less-than-or-similar-to 1 K). We have deriv ed the methanol abundance by combining the CH3OH data with new C18O ob servations. It results that methanol is enhanced by large factors towa rd the shocked regions (approximately 400 toward the blue-shifted lobe of L 1157). Although the shocked gas is known to be heated to about 1 00 K, the CH3OH rotation temperatures have moderate values: in L 1157 T(rot) increases from 8 K in the ambient gas to 12 K in the shocked re gion. Radiative transfer calculations, carried out to simulate the CH3 OH excitation in different physical circumstances, confirm that in the range of physical conditions discussed here the methanol molecules ar e very subthermally excited, and T(rot) is not a good measure of the k inetic temperature (T(rot) << T(K)). The observed methanol abundance e nhancements are likely caused by processes of desorption of grain mant les in shocks.