Shock Chemistry in the Young Bipolar Outflow L1157

Abstract

We present the first results from a recent survey of molecular lines in L1157, a highly collimated bipolar molecular outflow driven by a class 0 protostar. These observations are used to study the chemical alterations produced by a violent, highly collimated outflow. Different molecular lines are observed to trace different components of the gas. Some molecules (C3H2, N2H+, H13CO+, DCO+) are abundant in the quiescent medium but are not observed at high velocities. Lines from these molecules are the best tracers of the concentration of dense gas around the protostar. In addition, we find that some otherwise rare molecules (e.g., SiO, CH3OH, H2CO, HCN, CN, SO, and SO2) are enhanced by at least an order of magnitude at the shocked region. Our observations provide estimates of the enhancement factors for such species. Strong gradients in the chemical composition are observed along the outflow blue lobe, which could be due to the evolution in time of the chemical processes. We briefly discuss the chemistry of the most important molecules, devoting special attention to the species that are thought to be abundant in interstellar ice mantles.