The precession of the giant HH 34 outflow: A possible jet deceleration mechanism

The giant jets represent a fundamental trace of the historical evolution of the outflow activity over timescales of similar to10(4) yr, i.e., a timescale comparable to the accretion time of the outflow sources in their main protostellar phase. The study of such huge jets provides the possibility of retrieving important elements related to the life of the outflow sources. In this paper, we study the role of precession (combined with jet velocity variability and the resulting enhanced interaction with the surrounding environment) as a deceleration mechanism for giant jets, using a numerical approach. This thesis was proposed for the first time by Devine et al., but it could not be numerically explored until now, because it is intrinsically difficult to reproduce, at the same time, the large range of scales from similar to100 AU up to a few parsecs. In the present paper, we obtain predictions of Halpha intensity maps and position-velocity diagrams from three-dimensional simulations of the giant HH 34 jet (including an appropriate ejection velocity time variability and a precession of the outflow axis), and we compare them with previously published observations of this object. Our simulations represent a step forward from previous numerical studies of HH objects, in that the use of a seven-level, binary adaptive grid has allowed us to compute models that appropriately cover all relevant scales of a giant jet, from the similar to100 AU jet radius close to the source to the similar to1 pc length of the outflow. A good qualitative and quantitative agreement is found between the model predictions and the observations, indicating that a precession of the jet axis can indeed be the probable cause of the deceleration of the giant jets. Moreover, we show that a critical parameter for obtaining a better or worse agreement with the observations is the ratio rho(j)/rho(a) between the jet and the environmental densities. The implications of this result in the context of the current star formation models are discussed.