JOYS+: A JWST/MIRI survey of the evolution of H₂ winds and jets from low-mass protostars

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
² Max-Planck-Institut für Extraterrestrische Physik, Giessenbach-strasse 1, 85748 Garching, Germany
³ INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
⁴ Institut de Ciències de l’Espai (ICE-CSIC), Campus UAB, Can Magrans S/N, 08193 Cerdanyola del Vallès, Catalonia, Spain
⁵ Institut d’Estudis Espacials de Catalunya (IEEC), c/Gran Capita 2–4, 08034 Barcelona, Catalonia, Spain
⁶ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁷ Department of Physics, PO Box 64, 00014 University of Helsinki, Finland
⁸ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
⁹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
¹⁰ Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale, 91405 Orsay, France
¹¹ Department of Astronomy, University of Virginia, Charlottesville, VA 22903, USA
¹² Virginia Institute of Theoretical Astronomy, University of Virginia, Charlottesville, VA 22903, USA
¹³ School of Cosmic Physics, Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2, Ireland
¹⁴ National Radio Astronomy Observatory, 520 Edgemont Rd., Charlottesville, VA 22903, USA
¹⁵ INAF – Osservatorio Astronomico di Roma, Via di Frascati 33, 00078 Monte Porzio Catone, Italy
¹⁶ Joint ALMA Observatory, Alonso de Córdova 3107, Vitacura, Santiago, Chile
¹⁷ Institut de Ciències de l’Espai (ICE, CSIC), Can Magrans s/n, 08193 Cerdanyola del Vallès, Catalonia, Spain
¹⁸ Institut d’Estudis Espacials de de Catalunya (IEEC), 08034 Barcelona, Catalonia, Spain
¹⁹ Department of Astrophysics, University of Vienna, Türkenschanzs-trasse 17, 1180 Vienna, Austria
²⁰ ETH Zürich, Institute for Particle Physics and Astrophysics, Wolfgang-Pauli-Strasse 27, 8093 Zürich, Switzerland
²¹ Department of Astronomy, Oskar Klein Centre, Stockholm University, AlbaNova University Center, 10691 Stockholm, Sweden
²² UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
²³ School of Cosmic Physics, Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2, Ireland

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Received: 23 December 2025
Accepted: 13 April 2026

Abstract

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 H₂ . How the H₂ 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 H₂ 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 H₂ 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 H₂ S(1) line emission. A rotation-diagram fitting of the H₂ lines was used to determine the column density and temperature, which was combined with measurements of the outflow width and H₂ line velocity to measure the mass-loss rates.

Results. Low- J (J ≤ 4) transitions of H₂ 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 H₂ velocities are observed along the jet axis. The opening angle of the wind traced by the H₂ 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 H₂ 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 H₂ 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 H₂ that cannot be detected with JWST/MIRI.

Conclusions. The declining warm H₂ mass-loss rates and increasing opening angles from the Class 0 to I stages – and the absence of H₂ 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.