Radio jets in young stellar objects with the SKA

Abstract

Jets and outflows are ubiquitous in the process of formation of stars since accretion is intimately associated with outflow. Free-free radio continuum emission in the centimeter domain is associated with these jets. The emission is weak, and sensitive telescopes are required to detect it. One of the key problems in the study of outflows is to determine how they are accelerated and collimated. Observations in the cm range are most useful to trace the base of the ionized jets, close to the young central object and its accretion disk, where optical or near-IR images are obscured by the high extinction present. Radio recombination lines in jets (in combination with proper motions) should provide their 3D kinematics at very small scale (near their origin). SKA will be crucial to perform this kind of observations. Thermal jets are associated with both low and high mass protostars. 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 centimeter luminosity and the outflow momentum rate. From this correlation and that with the bolometric luminosity of the driving star it will be possible to discriminate with SKA between unresolved HII regions and jets, and to infer physical properties of the embedded objects. Some jets associated with young stellar objects (YSOs) show indications of non-thermal emission (negative spectral indices) in part of their lobes. Linearly polarized synchrotron emission has been found in the jet of HH 80-81, allowing us to measure the direction and intensity of the jet magnetic field, a clue ingredient in determining 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 SKA are required to perform these studies. Jets are common in many kinds of astrophysical scenarios. Characterizing radio jets in YSOs, where thermal emission allows us to determine their physical conditions in a reliable way, would be also useful in understanding acceleration and collimation mechanisms in all kinds of astrophysical jets.