  Context. The high energies of protostellar jets, implied by recent
observations of X-rays from such flows, came very much as a surprise. Inferred
shock velocities are considerably higher than what was previously known, hence
putting even larger energy demands on the driving sources of the jets. The
statistics of X-ray emitting jets are still poor, yet a few cases exist which
seem to imply a correlation between the presence of HeI 1.0830 mu emission and
X-ray radiation in a given source. Aims. This tentative correlation needs
confirmation and explanation. If the jet regions of HeI 1.0830 mu emission are
closely associated with those producing X-rays, high resolution infared
spectroscopy can be used to observationally study the velocity fields in the
hot plasma regions of the jets. This would provide the necessary evidence to
test and further develop theoretical models of intermediately fast (> 500 -
1500 km/s) interstellar shock waves. Methods. The HH 154 jet flow from the
embedded protostellar binary L 1551 IRS 5 provides a case study, since adequate
IR and X-ray spectroscopic data are in existence. The thermal X-ray spectrum is
fed into a photoionization code to compute, in particular, the line emission of
HeI and HI and to account for the observed unusual line intensity ratios.
Results. The advanced model is capable of accounting for most observables, but
shows also major weaknesses. It seems not unlikely that these could, in
principle, be overcome by a time dependent hydrodynamical calculation with
self-consistent cooling. However, such sophisticated model development is
decisively beyond the scope of the present work. Conclusions. Continued X-ray
observations, coordinated with simultaneous high resolution infrared
spectroscopy, are highly desirable.
