  X-ray emission from about ten protostellar jets has been discovered and it
appears as a feature common to the most energetic jets. Although X-ray emission
seems to originate from shocks internal to jets, the mechanism forming these
shocks remains controversial. One of the best studied X-ray jet is HH 154 that
has been observed by Chandra over a time base of about 10 years. We analyze the
Chandra observations of HH 154 by investigating the evolution of its X-ray
source. We show that the X-ray emission consists of a bright stationary
component and a faint elongated component. We interpret the observations by
developing a hydrodynamic model describing a protostellar jet originating from
a nozzle and compare the X-ray emission synthesized from the model with the
X-ray observations. The model takes into account the thermal conduction and
radiative losses and shows that the jet/nozzle leads to the formation of a
diamond shock at the nozzle exit. The shock is stationary over the period
covered by our simulations and generates an X-ray source with luminosity and
spectral characteristics in excellent agreement with the observations. We
conclude that the X-ray emission from HH 154 is consistent with a diamond shock
originating from a nozzle through which the jet is launched into the ambient
medium. We suggest that the physical origin of the nozzle could be related to
the dense gas in which the HH 154 driving source is embedded and/or to the
magnetic field at the jet launching/collimation region.

Solar and Stellar Astrophysics