Shock-induced PDR in the Herbig-Haro object HH 2

We report mid-infrared ($5-17\mu m$) and SO, CO, $^{13}$CO millimeter line observations of the protostellar jet HH~2 and the parental molecular cloud. We have detected for the first time mid-infrared emission along a protostellar jet. We find that the outflowing gas extends much further away than the Herbig-Haro object HH~2, showing direct evidence that downstream gas has been accelerated by previous outflow events. These gas layers appear to have been detached from the parental cloud, as they are distributed around a cavity, probably dug by protostellar outflow(s). SO emission is detected in shocked gas regions associated with outflows. The UV field produced in the strong shock region HH~2H-A has produced a low-excitation Photon-Dominated Region at the walls of the cavity, which is detected in the PAH emission bands and in the continuum between 5 and $17\mu m$. This continuum arises from very small grains transiently heated by a FUV field G $\simeq 20G_0$, which probably formed from evaporation of dust grain mantles in shocks.