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

Abstract

Spitzer-IRS maps of H_2 pure rotational lines from S(0) to S(7) in 3 outflows from Class 0 sources - L1448, BHR71, and NGC2071 - are presented. These lines are used, in conjunction with available rovibrational, near-IR H_2 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, being on average between 2.0 and 2.3. These low values, that reflect the young age of these flows, are found also in regions of relatively high temperature (~ 1000 K), likely indicating that shocks are occurring in a time shorter than that needed for a complete para to ortho conversion. Density maps indicate upper limits close to LTE conditions, i.e. between 10^6-10^7 cm^-3; moreover we demonstrate, based on 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 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 emitting in the far-infrared, such as water and CO; moreover, the luminosity radiated away is comparable with the estimated kinetic energy of the swept-out outflow: this supports the scenario in which the shock from which H_2 is emitted is also capable of accelerating the molecular outflow driven at the shock working surface.