SiO shocks in the L 1157 molecular outflow ?

We report high angular resolution IRAM Plateau de Bure interferometric observations of the SiO v = 0 J = 2→1 and J = 5 → 4 transitions in the southern lobe of the young L 1157 molecular outflow. The resolution of these observations (∼ 2.5′′) makes them directly comparable to available highresolution CO maps of the flow. The known precession of the L 1157 flow is fully confirmed. We find a remarkable morphological agreement between the strong SiO shocks revealed by these observations and the two CO cavities of the southern lobe of the outflow: the positions, shapes and opening angles are similar in both tracers, with the SiO emission ahead of or at the edges of the CO emission. Each CO cavity is associated with a shock which is placed exactly at its apex and exhibits a linear feature pointing exactly towards the protostellar position. The CO appears in the wake of these leading shocks. These coincidences, as well as the presence of two independent shock/cavity systems, strongly support shock-entrainment models for the formation of molecular outflows. These observations also provide detailed information on the internal structure of the shocked regions. They confirm that a strong enhancement of the SiO abundance occurs within shocks. The comparison with high-angular resolution images of other shock-tracers (NH3, H2) shows that chemical and evolution effects play a crucial role in the observed brightness distributions. The SiO velocity distribution is mainly forward, but the kinematics seems to result from both the complex formation processes of SiO and the velocity field produced by bow-shocks. Finally, we briefly discuss the apparent density structure of the shocks, and especially the possible origin of the linear precursor seen downstream from the main bow-shock.