JOYS: Unlocking accretion-rate diagnostics for high-mass protostars using JWST/MIRI HI lines

¹ Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
² Leiden Observatory, Leiden University, PO Box 9513, 2300 RA, Leiden, The Netherlands
³ Max Planck Institut für Extraterrestrische Physik (MPE), Giessen-bachstrasse 1, 85748 Garching, Germany
⁴ INAF–Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
⁵ Department of Physics, Maynooth University, Maynooth, Co. Kildare, Ireland
⁶ UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
⁷ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
⁸ Space Science and Astrobiology Division, NASA’s Ames Research Center, Moffett Field, CA 94035, USA
⁹ Institut de Ciencies de l’Espai (ICE-CSIC), Campus UAB, Carrer de Can Magrans S/N, 08193 Cerdanyola del Valles, Catalonia, Spain
¹⁰ Institut d’Estudis Espacials de Catalunya (IEEC), c/ Gran Capitá, 2–4, 08034 Barcelona, Spain
¹¹ School of Cosmic Physics, Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, D02 XF86 Dublin, Ireland
¹² INAF-Osservatorio Astronomico di Roma, Via di Frascati 33, 00078 Monte Porzio Catone, Italy
¹³ Laboratory for Astrophysics, Leiden Observatory, Leiden University, PO Box 9513, 2300 RA, Leiden, The Netherlands

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 15 December 2025
Accepted: 15 March 2026

Abstract

Context. While many aspects of high-mass star formation have been investigated, the accretion onto the central protostars is one of the most fundamental but less explored physical properties. The James Webb Space Telescope (JWST), through its Mid InfraRed Instrument (MIRI), offers a unique opportunity to explore tracers of accretion at less-extincted wavelengths (5–27 μm) than those studied so far, where it delivers unparalleled sensitivity and spectral resolution.

Aims. We probed the capability of MIRI in its MRS/IFU mode to detect and resolve atomic Hydrogen (H I) emission lines in such embedded objects to subsequently estimate accretion luminosities (L_acc) and accretion rates (Ṁ_acc) for the first time in a sample of (six) high-mass, star forming regions at different evolutionary stages.

Methods. We used the dereddened H I line luminosities as tracers of accretion by applying line-to-accretion-luminosity relations (L_acc-calibrations) from the literature. As such L_acc-calibrations were originally established for low-mass Class II objects, we assessed their applicability to our sample prior to estimating accretion rates. Extinction was derived from the silicate absorption feature at 9.7 μm.

Results. The infrared continuum reveals, at much higher spatial resolution than before, the location of new infrared sources (protostars), towards which we detected a handful of H I lines. While a few lines are secure detections, many are tentative. The most commonly detected line is Humphreys α at 12.37 μm, followed by Humphreys β and Pfund α. Assuming that their line fluxes are dominated by accretion, we find that two of the three existing L_acc-calibrations predict excessively high accretion luminosities that largely exceed their bolometric luminosities (L_bol), and that the third L_acc-calibration still overpredicts accretion luminosities for some sources. Considering the given uncertainties, estimated accretion rates are only tentative.

Conclusions. This work demonstrates the great potential of JWST/MIRI to probe H I line emission that originated in the innermost regions of high-mass protostellar systems, setting the ground floor for further investigations into accretion in these objects. While this project had the ambitious goal of robustly quantifying accretion rates, we shed light on what outstanding methodological challenges remain; the most critical being the development of new L_acc- calibrations for intermediate- to high-mass protostars.