SPITZER SPECTRAL LINE MAPPING OF PROTOSTELLAR OUTFLOWS. II. H2 EMISSION IN L1157

We present an analysis of Spitzer-IRS spectroscopic maps of the L1157 protostellar outflow in the H-2 pure-rotational lines from S(0) to S(7). The aim of this work is to derive the physical conditions pertaining to the warm molecular gas and study their variations within the flow. The mid-IR H-2 emission follows the morphology of the precessing flow, with peaks correlated with individual CO clumps and H-2 2.12 mu m ro-vibrational emission. More diffuse emission delineating the CO cavities is detected only in the low-laying transitions, with J(lower) <= 2. The H-2 line images have been used to construct two-dimensional maps of N(H-2), H-2 ortho-to-para ratio (OPR), and temperature spectral index beta, in the assumption of a gas temperature stratification where the H-2 column density varies as T-beta. Variations of these parameters are observed along the flow. In particular, the OPR ranges from similar to 0.6 to 2.8, highlighting the presence of regions subject to recent shocks where the OPR has not had time yet to reach the equilibrium value. Near-IR spectroscopic data on ro-vibrational H-2 emission have been combined with the mid-IR data and used to derive additional shock parameters in the brightest blueshifted and redshifted emission knots. A high abundance of atomic hydrogen (H/H-2 similar to 0.1-0.3) is implied by the observed H-2 column densities, assuming n(H-2) values as derived by independent SiO observations. The presence of a high fraction of atomic hydrogen indicates that a partially dissociative shock component should be considered for the H-2 excitation in these localized regions. However, planar shock models, either of C-or J-type, are not able to consistently reproduce all the physical parameters derived from our analysis of the H-2 emission. Globally, H-2 emission contributes to about 50% of the total shock radiated energy in the L1157 outflow. We find that the momentum flux through the shocks derived from the radiated luminosity is comparable to the thrust of the associated molecular outflow, supporting the scenario where the latter is driven by the shock working surface.