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Spitzer-IRS maps of H_2 pure rotational lines from S(0) to S(7) in 3 outflowsfrom Class 0 sources - L1448, BHR71, and NGC2071 - are presented. These linesare used, in conjunction with available rovibrational, near-IR H_2 lines, toprobe the physical conditions of the warm gas between hundreds and thousands ofKelvin. We have constructed maps of the molecular hydrogen column density,ortho-to-para ratio and volume density, together with the index beta of thepower law describing the distribution of gas temperature. In all threeoutflows, the present ortho-to-para ratio significantly deviates from the hightemperature equilibrium of 3, being on average between 2.0 and 2.3. These lowvalues, that reflect the young age of these flows, are found also in regions ofrelatively high temperature (~ 1000 K), likely indicating that shocks areoccurring in a time shorter than that needed for a complete para to orthoconversion. Density maps indicate upper limits close to LTE conditions, i.e.between 10^6-10^7 cm^-3; moreover we demonstrate, based on the detections of HDemission spots (R(3) and R(4) lines), that a density stratification does exist,with the low density components (10^4-10^5 cm^-3) associated with the coldestgas. The beta index is found in all flows to be above 3.8: this value isconsistent with predictions of multiple C-type bow shocks with a range ofvelocities, some of which insufficient to achieve the temperature at which H_2partially dissociates. The contribution of H_2 to the total cooling isquantitatively similar to that of other abundant molecules emitting in thefar-infrared, such as water and CO; moreover, the luminosity radiated away iscomparable with the estimated kinetic energy of the swept-out outflow: thissupports the scenario in which the shock from which H_2 is emitted is alsocapable of accelerating the molecular outflow driven at the shock workingsurface.

Spitzer spectral line mapping of protostellar outflows: III - H_2 emission in L1448, BHR71, and NGC2071