A multi-epoch HST study of the herbig-haro flow from XZ Tauri

We present nine epochs of Hubble Space Telescope optical imaging of the bipolar outflow from the pre-main-sequence binary XZ Tauri. Our data monitor the system from 1995 to 2005 and include emission line images of the flow. The northern lobe appears to be a succession of bubbles, the outermost of which expanded ballistically from 1995 to 1999 but in 2000 began to deform and decelerate along its forward edge. It reached an extent of 6″ from the binary in 2005. A larger and fainter southern counterbubble was detected for the first time in deep Advanced Camera for Surveys images from 2004. Traces of shocked emission are seen as far as 20″ south of the binary. The bubble emission nebulosity has a low excitation overall, as traced by the [S II]/Hα line ratio, requiring a nearly comoving surrounding medium that has been accelerated by previous ejections or stellar winds. Within the broad bubbles there are compact emission knots whose alignments and proper motions indicate that collimated jets are ejected from each binary component. The jet from the southern component, XZ Tau A, is aligned with the outflow axis of the bubbles and has tangential knot velocities of 70-200 km s^-1. Knots in the northern flow are seen to slow and brighten as they approach the forward edge of the outermost bubble. The knots in the jet from the other star, XZ Tau B, have lower velocities of 100 km s^-1. To explain the observations of the outer bubble, we propose that the XZ Tau A stellar jet underwent a large velocity pulse circa 1980. This ejection quickly overtook older, slower-moving ejecta very near the star, producing a 70 km s^-1 shock in a hot (T 80, 000 K), compact "fireball." The initial thermal pressure of this gas parcel drove the expansion of a spherical bubble. Subsequent cooling caused the bubble to transition to ballistic expansion, followed by slowing of its forward edge by mass loading from the pre-shock medium. Repeated pulses may explain the multiple bubbles seen in the data. Collimated jets continue to flow through the bubble's interior, and with the fading of the original fireball they are becoming the primary energizing mechanism for the emission line structures. Future evolution of the flow should see the outer bubble structures fade from view, and the emergence of a more typical Herbig-Haro jet/bow shock morphology. We present a preliminary numerical model of a pulsed jet to illustrate this scenario.