2026ApJ...998..282F

We present observations of jets within 2000 au of three deeply embedded protostars using 2.9-27 μm observations with JWST. These observations show the morphologies and kinematics of the collimated jets from three protostars, the low-mass Class 0 protostars B335 and HOPS 153, and the intermediate-mass protostar HOPS 370. These jets are traced by shock-ionized fine-structure line emission observed with the JWST NIRSpec and MIRI integral field units. We find that [Fe II] emission traces the full extent of the inner 1000-2000 au of the jets, depending on the distance to the protostar, while other ions mostly trace isolated shocked knots. The jets show evidence of wiggling motion in the plane of the sky as well as asymmetries between blue- and redshifted lobes. The widths of the jets increase nonmonotonically with distance from the central protostar, with opening angles ranging from $2\mathop{.}\limits^{^\circ }1$ to $\lt 10\mathop{.}\limits^{^\circ }1$ for the three protostars in the sample. The jets have total velocities ranging from 147 to 184 km s^–1 after correcting for disk inclination. For B335, an 8 month gap between NIRSpec and MIRI Medium Resolution Spectroscopy observations enabled measurement of the tangential velocity of a shocked knot; in combination with the radial velocity, this shows that the jet has a different inclination than the outflow cavity. We find multiple knots before and during a recent outburst in B335, although the knots were more frequent during the burst. The asymmetries between the blue- and redshifted lobes strongly suggest complex interactions between the circumstellar disks and magnetic fields.