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

Journal article, 2026

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 mu m) 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 (M-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 mu m. 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 alpha at 12.37 mu m, followed by Humphreys beta and Pfund alpha. 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.