Hubble Space Telescope WFPC2 imaging of XZ Tauri: Time evolution of a Herbig-Haro bow shock

We report observations of dramatic morphological changes in the extremely y oung Herbig-Haro object ejected by the pre-main-sequence binary XZ Tauri. N ubble Space Telescope images taken in 1995 showed a filled bubble of emissi on nebulosity extending 4" north-northeast of the system in the direction o f a previously known jet. New images obtained in 1998 show that the bubble has undergone a remarkable transition into a limb-brightened structure as i t continues its motion away from the binary. The new images suggest that we may be witnessing the initial formation of the postshock cooling zone in a newly emerging Herbig-Haro bow shock system, probably accompanied by signi ficant temporal evolution in the nebula's emission line spectrum.