Radio jets from young stellar objects

Abstract

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 in the centimeter domain is associated with these jets. The emission is relatively weak and compact, and sensitive radio interferometers of high angular resolution are required to detect and study 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 the inner parts of its accretion disk, 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 (near their origin). 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 objects in the substellar domain. 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 of the centimeter luminosity with the bolometric luminosity of the system it will be possible to discriminate between unresolved HII regions and jets, and to infer additional physical properties of the embedded objects. Some jets associated with young stellar objects (YSOs) show indications of nonthermal emission (negative spectral indices) in part of their lobes. Linearly polarized synchrotron emission has been found in the jet of HH 80-81, allowing one to measure the direction and intensity of 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 YSOs, where thermal emission allows one to determine their physical conditions in a reliable way, would also be useful in understanding acceleration and collimation mechanisms in all kinds of astrophysical jets, such as those associated with stellar and supermassive black holes and planetary nebulae.

Gas flow and accretion via spiral streamers and circumstellar disks in a young binary protostar

“…The result of the above equation is given in units of 10 -6 M ⊙ year -1 . We adopted θ = 10° since the usually high collimation of jets allow for a small opening angle, v = 200 km s -1 as typical jet velocity for low-mass objects, the inclination, i, determined from the source sizes (see main text) and T = 10 4 K is usually adopted (29). Despite the flux calibration uncertainties at centimetric-wavelength observations being normally lower than millimetric ones, we assume a conservative uncertainty for the VLA fluxes to be similar to the ALMA data, 15%.…”

The Role of Magnetic Fields in Protostellar Outflows and Star Formation

“…radiation with a positive spectral index α where the flux at frequency ν, S ν ∝ ν α ). Moreover, typical flux densities are very low and usually less than 1 mJy [Anglada et al, 2018]. Such emission is un-polarized and is often elongated in the direction of the known atomic/molecular outflow.…”

The VLA/ALMA Nascent Disk and Multiplicity (VANDAM) Survey of Orion Protostars. I. Identifying and Characterizing the Protostellar Content of the OMC-2 FIR4 and OMC-2 FIR3 Regions

“…For example, no clear outflow has been detected from HOPS-108 itself in 12 CO molecular line emission, and the free-free continuum source associated with HOPS-108 is very weak in comparison with HOPS-370. There is a known correlation between L bol and free-free continuum emission (Anglada 1995;Shirley et al 2007;Tychoniec et al 2018;Anglada et al 2018). If HOPS-108 is consistent with the correlation derived by Tychoniec et al (2018), then a 100 L source is expected to have a 4.1 cm flux density of ∼1.5 mJy at the distance to Orion.…”