2010ApJ...724...69N

We present an analysis of Spitzer-IRS spectroscopic maps of the L1157 protostellar outflow in the H₂pure-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 H₂ emission follows the morphology of the precessing flow, with peaks correlated with individual CO clumps and H₂2.12 µm ro-vibrational emission. More diffuse emission delineating the CO cavities is detected only in the low-laying transitions, with J_lower ≤ 2. The H₂ line images have been used to construct two-dimensional maps of N(H₂), H₂ortho-to-para ratio (OPR), and temperature spectral index β, in the assumption of a gas temperature stratification where the H₂ column density varies as T ^–β. Variations of these parameters are observed along the flow. In particular, the OPR ranges from ∼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 H₂ 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/H₂∼ 0.1-0.3) is implied by the observed H₂column densities, assuming n(H₂) 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 H₂excitation 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 H₂emission. Globally, H₂ 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.