SPITZER SPECTRAL LINE MAPPING OF PROTOSTELLAR OUTFLOWS. III. H2 EMISSION IN L1448, BHR71, AND NGC2071

Spitzer Infrared Spectrograph maps of H-2 pure rotational lines from S(0) to S(7) in three outflows from Class 0 sources, namely, L1448, BHR71, and NGC2071, are presented. These lines are used in conjunction with available rovibrational, near-infrared H2 lines to probe the physical conditions of the warm gas between hundreds and thousands of kelvin. We have constructed maps of the molecular hydrogen column density, ortho-to-para ratio, and volume density, together with the index beta of the power law describing the distribution of gas temperature. In all three outflows, the present ortho-to-para ratio significantly deviates from the high temperature equilibrium of 3, ranging between 2.0 and 2.3. These low values, which in general reflect the young age of these flows, are found also in regions of relatively high temperature (around 1000 K), likely indicating that shocks are occurring in a time period shorter than that needed for a complete para-to-ortho conversion. Density maps indicate upper limits close to LTE conditions, i.e., between 10(6) and 10(7) cm(-3); moreover, we demonstrate on the basis of the detections of HD emission 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 coldest gas. The beta index is found in all flows to be above 3.8: this value is consistent with predictions of multiple C-type bow shocks with a range of velocities, some of which are insufficient to achieve the temperature at which H-2 partially dissociates. The contribution of H-2 to the total cooling is quantitatively similar to that of other abundant molecules, such as water and CO, emitting in the far-infrared; moreover, the luminosity radiated away is comparable to the estimated kinetic energy of the swept-out outflow: this supports the scenario in which the shock from which H2 is emitted is also capable of accelerating the molecular outflow driven at the shock working surface.