IRS SCAN-MAPPING OF THE WASP-WAIST NEBULA (IRAS 16253-2429). I. DERIVATION OF SHOCK CONDITIONS FROM H2 EMISSION AND DISCOVERY OF 11.3 μm PAH ABSORPTION

The outflow driven by the Class 0 protostar, IRAS 16253-2429, is associated with bipolar cavities visible in scattered mid-infrared light, which we refer to as the Wasp-Waist Nebula. InfraRed Spectometer (IRS) scan mapping with the Spitzer Space Telescope of a similar to 1' x 2' area centered on the protostar was carried out. The outflow is imaged in six pure rotational (0-0 S(2) through 0-0 S(7)) H-2 lines, revealing a distinct, S-shaped morphology in all maps. A source map in the 11.3 mu m polycyclic aromatic hydrocarbon (PAH) feature is presented in which the protostellar envelope appears in absorption. This is the first detection of absorption in the 11.3 mu m PAH feature. Spatially resolved excitation analysis of positions in the blue- and redshifted outflow lobes, with extinction-corrections determined from archival Spitzer 8 mu m imaging, shows remarkably constant temperatures of similar to 1000 K in the shocked gas. The radiated luminosity in the observed H-2 transitions is found to be 1.94 +/- 0.05 x 10(-5) L-circle dot in the redshifted lobe and 1.86 +/- 0.04 x 10(-5) L-circle dot in the blueshifted lobe. These values are comparable to the mechanical luminosity of the flow. By contrast, the mass of hot (T similar to 1000 K) H-2 gas is 7.95 +/- 0.19 x 10(-7) M-circle dot in the redshifted lobe and 5.78 +/- 0.17 x 10(-7) M-circle dot in the blueshifted lobe. This is just a tiny fraction, of order 10(-3), of the gas in the cold (30 K), swept-up gas mass derived from millimeter CO observations. The H-2 ortho/para ratio of 3:1 found at all mapped points in this flow suggests previous passages of shocks through the gas. Comparison of the H-2 data with detailed shock models of Wilgenbus et al. shows the emitting gas is passing through Jump (J-type) shocks. Pre-shock densities of 10(4) cm(-3) <= n(H) 10(5) cm(-3) are inferred for the redshifted lobe and n(H) <= 10(3) cm(-3) for the blueshifted lobe. Shock velocities are 5 km s(-1) <= upsilon(s) <= 10 km s(-1) for the redshifted gas and v(s) = 10 km s(-1) for the blueshifted gas. Initial transverse (to the shock) magnetic field strengths for the redshifted lobe are in the range 10-32 mu G, and just 3 mu G for the blueshifted lobe. A cookbook for using the CUBISM contributed software for IRS spectral mapping data is presented in the Appendix.