ENHANCED CHEMICAL ABUNDANCES IN THE L1157 OUTFLOW: SiO AND CH_3OH OBSERVATIONS

摘要

Millimeter and submillimeter wavelength spectral lines of CH_3OH and SiO have been observed in the remarkable outflow source L1157. Statistical equilibrium analyses indicate that the abundances of both compounds are enhanced greatly in the blueshifted, southern lobe which is known to show evidence of active shock heating. Both the SiO and CH_3OH emission in the blueshifted lobe peak at the same position as the CO J = 1-0 line. The SiO and CH_3OH lines originate from warm gas. The temperature deduced from the methanol observations matches the laboratory temperature at which pure CH_3OH ice separates from an H_2O:CH_3OH mixture, and sublimates rapidly into the gas phase. The SiO shows evidence of kinematically distinct regions, with the higher velocity gas being more highly clumped, warmer, and enriched in SiO abundance by a factor of more than 10~5 relative to quiescent, dark cloud values. The slower gas is somewhat cooler and less dense, with an SiO abundance enhancement factor of 10~6, well above that seen in the Orion KL outflow. The CH_3OH abundance, relative to H_2, is 1.5 x 10~(-7), and its emission originates in gas with T_(kin) ≥ 100 K. A number of outflows are now known that show strong abundance enhancements in SiO and/or CH_3OH due to evaporation of grain mantles or dust destruction due to shocks. This probably occurs in the bow shocks associated with supersonic jets, and the resulting abundance enhancements may be relatively short-lived after cessation of the jet, due to depletion back onto the grains and gas-phase chemical processing of CH_3OH.