Abstract

Investigating Protostellar Accretion (IPA) is a JWST Cycle 1 GO program that uses NIRSpec integral field units and MIRI Medium Resolution Spectrograph to obtain 2.9-28 mu m spectral cubes of young, deeply embedded protostars with luminosities of 0.2-10,000 L circle dot and central masses of 0.15-12 M circle dot. In this Letter, we report the discovery of a highly collimated atomic jet from the Class 0 protostar IRAS 16253-2429, the lowest-luminosity source (L bol = 0.2 L circle dot) in the IPA program. The collimated jet is detected in multiple [Fe ii] lines and [Ne ii], [Ni ii], and H i lines but not in molecular emission. The atomic jet has a velocity of about 169 +/- 15 km s-1, after correcting for inclination. The width of the jet increases with distance from the central protostar from 23 to 60 au, corresponding to an opening angle of 2.degrees 6 +/- 0.degrees 5. By comparing the measured flux ratios of various fine-structure lines to those predicted by simple shock models, we derive a shock speed of 54 km s-1 and a preshock density of 2.0 x 103 cm-3 at the base of the jet. From these quantities and using a suite of jet models and extinction laws, we compute a mass-loss rate between 0.4 and 1.1 x10-10 M circle dot yr -1. The low mass-loss rate is consistent with simultaneous measurements of low mass accretion rate (2.4 +/- 0.8 x 10-9 M circle dot yr-1) for IRAS 16253-2429 from JWST observations, indicating that the protostar is in a quiescent accretion phase. Our results demonstrate that very low-mass protostars can drive highly collimated, atomic jets, even during the quiescent phase.