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We present an analysis of Spitzer-IRS spectroscopic maps of the L1157protostellar outflow in the H2 pure-rotational lines from S(0) to S(7). The aimof this work is to derive the physical conditions pertaining to the warmmolecular gas and study their variations within the flow. The mid-IR H2emission follows the morphology of the precessing flow, with peaks correlatedwith individual CO clumps and H2 2.12{\mu}m ro-vibrational emission. Morediffuse emission delineating the CO cavities is detected only in the low-layingtransitions, with J(lower) less or equal to 2. The H2 line images have beenused to construct 2D maps of N(H2), H2 ortho-to-para ratio and temperaturespectral index beta, in the assumption of a gas temperature stratificationwhere the H2 column density varies as T^(beta). Variations of these parametersare observed along the flow. In particular, the ortho-to-para ratio ranges from0.6 to 2.8, highlighting the presence of regions subject to recent shocks wherethe ortho-to-para ratio has not had time yet to reach the equilibrium value.Near-IR spectroscopic data on ro-vibrational H2 emission have been combinedwith the mid-IR data and used to derive additional shock parameters in thebrightest blue- and red-shifted emission knots. A high abundance of atomichydrogen (H/H2 about 0.1-0.3) is implied by the observed H2 column densities,assuming n(H2) values as derived by independent SiO observations. The presenceof a high fraction of atomic hydrogen, indicates that a partially-dissociativeshock component should be considered for the H2 excitation in these localizedregions. However, planar shock models, either of C- or J-type, are not able toconsistently reproduce all the physical parameters derived from our analysis ofthe H2 emission. Globally, H2 emission contributes to about 50% of the totalshock radiated energy in the L1157 outflow.

Spitzer spectral line mapping of protostellar outflows: II H2 emission in L1157