The HH 154 jet coming from the YSO binary L1551 IRS5 is one of the closest (about 150 pc) astrophysical jet known. It is therefore a unique laboratory for studies of outflow mechanisms and of the shocks forming at the interaction front between the expanding material and the ambient medium. The substructures (knots) observed within the HH 154 jet were imaged in several spectral bands using the Hubble Space Telescope. This allows us to derive a simple characterization of the physical conditions in different structures as well as to measure the proper motion of the knots in the jet, their flux variability and shock emission over a time base of about ten years. These knots in the jet undergo signicant morphological variations: some of them disappear in a few years and collision between different knots may occur. Our results suggest the presence of a shock front at the base of the jet identified with an internal working surface; more important, the knot at the base of the jet is coincident with the X‐ray source discovered in HH 154. In fact the optical data are compared to X‐rays data collected with Chandra with the goal of understanding the energetics of this jet, in particular, and of low‐mass star jets, in general.

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