Radio Jets from Young Stellar Objects
  Jets and outflows are ubiquitous in the process of formation of stars since
outflow is intimately associated with accretion. Free-free (thermal) radio
continuum emission is associated with these jets. This emission is relatively
weak and compact, and sensitive radio interferometers are required to study it.
Observations in the cm range are most useful to trace the base of the ionized
jets, close to the central protostar, where optical or near-IR imaging is made
difficult by the high extinction present. Radio recombination lines in jets (in
combination with proper motions) should provide their 3D kinematics at very
small scale. Future instruments such as the Square Kilometre Array (SKA) and
the Next Generation Very Large Array (ngVLA) will be crucial to perform this
kind of sensitive observations. Thermal jets are associated with both high and
low mass protostars and possibly even with substellar objects. The ionizing
mechanism of these radio jets appears to be related to shocks in the associated
outflows, as suggested by the observed correlation between the cm luminosity
and the outflow momentum rate. Some protostellar jets show indications of
non-thermal emission in their lobes. Linearly polarized synchrotron emission
has been found in the jet of HH 80-81, allowing one to map the jet magnetic
field, a key ingredient to determine the collimation and ejection mechanisms.
As only a fraction of the emission is polarized, very sensitive observations
such as those that will be feasible with the interferometers previously
mentioned are required to perform studies in a large sample of sources. Jets
are present in many kinds of astrophysical scenarios. Characterizing radio jets
in young stars, where thermal emission allows one to determine their physical
conditions, would also be useful in understanding acceleration and collimation
mechanisms in all kinds of astrophysical jets.
