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 resol ution of these observations (similar to 2.5 '') makes them directly co mparable to available high-resolution CO maps of the flow. The known p recession of the L1157 flow is fully confirmed. We find a remarkable m orphological agreement between the strong SiO shocks revealed by these observations and the two CO cavities of the southern lobe of the outf low: the positions, shapes and opening angles are similar in both trac ers, 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 p rotostellar position. The CO appears in the wake of these leading shoc ks. These coincidences, as well as the presence of two independent sho ck/cavity systems, strongly support shock-entrainment models for the f ormation of molecular outflows. These observations also provide detail ed information on the internal structure of the shocked regions. They confirm that a strong enhancement of the SiO abundance occurs within s hocks. The comparison with high-angular resolution images of other sho ck-tracers (NH3, H-2) shows that chemical and evolution effects play a crucial role in the observed brightness distributions. The SiO veloci ty distribution is mainly forward, but the kinematics seems to result from both the complex formation processes of SiO and the velocity fiel d produced by bow-shocks. Finally, we briefly discuss the apparent den sity structure of the shocks, and especially the possible origin of th e linear precursor seen downstream from the main bow-shock.