  Recently X-ray emission from protostellar jets has been detected with both
XMM-Newton and Chandra satellites, but the physical mechanism which can give
rise to this emission is still unclear. We performed an extensive exploration
of a wide space of the main parameters influencing the jet/ambient interaction.
Aims include: 1) to constrain the jet/ambient interaction regimes leading to
the X-ray emission observed in Herbig-Haro objects in terms of the emission by
a shock forming at the interaction front between a continuous supersonic jet
and the surrounding medium; 2) to derive detailed predictions to be compared
with optical and X-ray observations of protostellar jets; 3) to get insight
into the protostellar jet's physical conditions. We performed a set of
bidimensional hydrodynamic numerical simulations, in cylindrical coordinates,
modeling supersonic jets ramming a uniform ambient medium. The model takes into
account the most relevant physical effects, namely the thermal conduction and
the radiative losses. Our model explains the observed X-ray emission from
protostellar jets in a natural way. In particular we find that the case of a
protostellar jet less dense than the ambient medium reproduces well the
observations of the nearest Herbig-Haro object, HH154, and allows us to make
detailed predictions of a possible X-ray source proper motion (vsh = 500 km/s),
detectable with Chandra. Furthermore our results suggest that the simulated
protostellar jets which best reproduce the X-rays observations cannot drive
molecular outflows.

Astrophysics