Images of shock-excited molecular hydrogen in young stellar outflows

We present high-angular resolution molecular hydrogen (H-2) v = 1-0 S(1) line images of several well-known molecular outflow sources (DR 21, NGC 2071, HH 7-11, and HH 12). These images were obtained through a 1% narrow-band filter using an InSb infrared detector array with a plate scale of 1.2 arcsec per pixel. It is found that all of the outflow sources possess highly collimated jets of shock-excited gas that are extremely clumpy and often consist of bright knots that are placed periodically along the jet axis. The H-2, emission-line jets are significantly more collimated than their corresponding CO outflows and remain collimated to large distances from their driving source. In DR 21 and NGC 2071, the H, jets appear to become more collimated with increasing distance from the central source, suggesting that they are pressure-confined by the ambient cloud medium. In HH 7-11 and HH 12 the shock-excited ionized and molecular components are spatially coincident and appear to be of comparable shape and size; this suggests that the shocked gas is highly mixed on scales significantly smaller than the current spatial resolution (similar to 0.005 pc at 500 pc). We discuss the shock patterns observed internal to these jets and propose that they are formed by either (i) a varying external pressure, (ii) the growth of shear instabilities along the walls of a pressure-confined jet, or (iii) the collision of a highly collimated wind with numerous preexisting, subsolar mass clumps in the ambient cloud medium.