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Methanol enhancement in young bipolar outflows

R. Bachiller, S. Liechti, C.M. Walmsley, and F. Colomer
Centro Astronómico de Yebes, Apartado 148, E-19080 Guadalajara, Spain
Physikalisches Institut, Universität zu Köln, Zulpicherstr. 77, D-50937 Köln, Germany
Abstract: We report observations of the 2--1, 3--2, and 5--4 thermal lines of CHOH toward young bipolar outflows. Strong emission ( > 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 molecular core is rather weak ( < 1 K). We have derived the methanol abundance by combining the CHOH data with new CO observations. It results that methanol is enhanced by large factors toward the shocked regions ( 400 toward the blue-shifted lobe of L 1157). Although the shocked gas is known to be heated to about 100 K, the CHOH rotation temperatures have moderate values: in L 1157 increases from 8 K in the ambient gas to 12 K in the shocked region. Radiative transfer calculations, carried out to simulate the CHOH excitation in different physical circumstances, confirm that in the range of physical conditions discussed here the methanol molecules are very subthermally excited, and is not a good measure of the kinetic temperature ( << ). The observed methanol abundance enhancements are likely caused by processes of desorption of grain mantles in shocks.

Robert Lucas
Fri May 12 14:34:35 METDST 1995