JOYS+: A JWST/MIRI survey of the evolution of H ₂ winds and jets from low-mass protostars

Context . The base of protostellar outflows can display both wide-angle, low-velocity winds and high-velocity, collimated jets, the magnetocentrifugal launching of which enables accretion onto the protostar. In outflows from the youngest protostars, the majority of the ejected or entrained mass is likely molecular H 2 . How the H 2 outflow evolves as the central protostar grows and the envelope dissipates is important for understanding the nature of the launching mechanism and assembly of the nascent protostar. Aims . Using JWST MIRI/MRS observations with an unprecedented spatial resolution down to 0.3" towards 13 single and 20 multiple Class 0 and I protostars, we aim to investigate the H 2 wind and jet morphology, mass outflow rate, velocity and temperature structure, and the evolution of these properties with protostellar class. Methods . We constructed continuum-subtracted maps of the H 2 S(1) and S(7) line flux and velocity towards the outflows in our sample and ALMA sub-millimetre CO maps. Towards the base of each blueshifted outflow lobe (typically within 300 au), we extracted representative spectra and measured the outflow opening angles from the H 2 S(1) line emission. A rotation-diagram fitting of the H 2 lines was used to determine the column density and temperature, which was combined with measurements of the outflow width and H 2 line velocity to measure the mass-loss rates. Results . Low- J ( J ≤ 4) transitions of H 2 largely trace extended wide-angle, low-velocity (0-20 km s −1 ) winds within the contours of the low-velocity (<30 km s −1 ) sub-millimetre CO emission, while high- J ( J > 5) transitions are associated with shocks and knots. In Class 0 sources with a known high-velocity (>30 km s −1 ) molecular CO or SiO jet, higher H 2 velocities are observed along the jet axis. The opening angle of the wind traced by the H 2 S(1) line broadens from ∼20° to ∼90° through the Class 0 to the Class I stage. The rotation diagrams in the blueshifted outflow lobes show a clear separation between a warm, ∼600 K and a hot, 1500-3000 K component, with no clear sign of evolution in the excitation temperature. The warm component contains two orders of magnitude more mass than the hot component, and the H 2 outflow mass-loss rate declines by two orders of magnitude from the Class 0 to the Class II stage. A correlation with the bolometric luminosity of the driving source is observed. A factor of 10–1000 mismatch between the warm H 2 and cold sub-millimetre CO outflow rate and momentum flux is also seen, which is consistent with the presence of cold and likely entrained (<150 K) molecular H 2 that cannot be detected with JWST/MIRI. Conclusions . The declining warm H 2 mass-loss rates and increasing opening angles from the Class 0 to I stages – and the absence of H 2 jets in the Class I sources – are consistent with the predictions of magnetohydrodynamical (MHD) disc wind models; however, the relatively constant temperatures of the warm and hot components with evolutionary stage may reflect the typical conditions in the outflow shocks rather than temperature stratification of the wind.