2007MNRAS.381.1127B

We present high spatial resolution maps, obtained with the Plateau de Bure Interferometer, of the blue lobe of the L1157 outflow. We observed four lines at 3 mm, namely CH₃ OH (2_K-1_K), HC₃N (11-10), HCN (1-0) and OCS (7-6). Moreover, the bright B1 clump has also been observed at better spatial resolution in CS (2-1), CH₃ OH (2₁-1₁)A^– and ³⁴SO (3₂-2₁). These high spatial resolution observations show a very rich structure in all the tracers, revealing a clumpy structure of the gas superimposed to an extended emission. In fact, the three clumps detected by previous IRAM 30-m single-dish observations have been resolved into several subclumps and new clumps have been detected in the outflow. The clumps are associated with the two cavities created by two shock episodes driven by the precessing jet. In particular, the clumps nearest the protostar are located at the wall of the younger cavity with a clear arch shape form while the farthest clumps have slightly different observational characteristics indicating that they are associated with the older shock episode. The emission of the observed species peaks in different part of the lobe: the eastern clumps are brighter in HC₃ N (11-10), HCN (1-0) and CS (2-1) while the western clumps are brighter in CH₃ OH (2_K-1_K), OCS (7-6) and ³⁴SO (3₂-2₁). This peak displacement in the line emission suggests a variation of the physical conditions and/or the chemical composition along the lobe of the outflow at small scale, likely related to the shock activity and the precession of the outflow. In particular, we observe the decoupling of the silicon monoxide and methanol emission, common shock tracers, in the B1 clump located at the apex of the bow shock produced by the second shock episode.