Warm SiO gas in molecular bullets associated with protostellar outflows

Context. We present the first SiO multiline analysis (from J = 2-1 to J = 11-10) of the molecular bullets along the outflows of the Class 0 sources L1448-mm and L1157-mm, obtained through observations with IRAM and JCMT. Aims. We investigate the physical properties of different types of bullets, in order to explore their excitation conditions and constrain their formation and evolution. Methods. We have computed the main physical parameters (n(H2), T-kin, N-SiO) in each bullet and compared them with other tracers of warm and dense gas and with models for the SiO excitation in shocks. Results. The bullets close to L1448-mm, associated with high velocity gas, have higher excitation conditions (n(H2) similar to 10(6) cm(-3), T greater than or similar to 500 K) than the L1157 bullets (n(H2) similar to 1-5 x 10(5) cm(-3), T similar to 100-300 K). In both the sources, there is clear evidence of the presence of velocity components having different excitation conditions, with the denser and/or warmer gas associated with the gas at the higher speed. In L1448 the bulk of the emission is due to the high-excitation and high velocity gas, while in L1157 most of the emission comes from the low excitation gas at ambient velocity. The observed velocity-averaged line ratios are well reproduced by shocks with speeds vs larger than similar to 30 km s(-1) and densities similar to 10(5) - 10(6) cm(-3). Plane-parallel shock models, however, fail to predict all the observed line profiles and in particular the very similar profiles shown by both low and high excitation lines. Conclusions. The observations support the idea that the L1157 clumps are shock interaction events older than the L1448 bullets close to the driving source. In the latter objects, the velocity structure and the variations of physical parameters with the velocity very closely resemble those found in optical/IR jets near the protostar, suggesting that similar launching and excitation mechanisms are also at the origin of collimated jets seen at millimetre wavelengths.