Improving accretion diagnostics for young stellar objects with mid-infrared hydrogen lines from JWST/MIRI

¹ Department of Astronomy and Astrophysics, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India
² INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
³ Max-Planck-Institut für Astronomie (MPIA), Königstuhl 17, 69117 Heidelberg, Germany
⁴ Kapteyn Astronomical Institute, Rijksuniversiteit Groningen, Postbus 800, 9700 AV Groningen, The Netherlands
⁵ Leiden Observatory, Universiteit Leiden, Leiden, Zuid-Holland, The Netherlands
⁶ Max-Planck-Institut für Extraterrestrische Physik, D-85748 Garching bei München, Germany
⁷ Indian Institute of Astrophysics, 2nd Block Koramangala, Bangalore 560034, India
⁸ Department of Physics and Electronics, CHRIST (Deemed to be University), Bangalore 560029, India
⁹ Department of Astrophysics, University of Vienna, Türkenschanzstr. 17, 1180 Vienna, Austria
¹⁰ ETH Zürich, Institute for Particle Physics and Astrophysics, Wolfgang-Pauli-Str. 27, 8093 Zürich, Switzerland
¹¹ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France

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Received: 12 July 2025
Accepted: 30 November 2025

Abstract

Context. We present a comprehensive study of mid-infrared neutral hydrogen (H i) emission lines in 79 nearby (d < 200 pc) young stars using the James Webb Space Telescope (JWST) Mid-Infrared Instrument (MIRI). This work extends accretion diagnostics to mid-infrared H i transitions, which are less affected by extinction and outflow emission compared to optical and near-infrared H i lines.

Aims. We aim to identify mid-infrared H i transitions that can serve as reliable accretion diagnostics in young stars, and evaluate their utility in deriving physical conditions of the accreting gas.

Methods. We identified and measured 22 H i transitions in the MIRI wavelength regime (5–28 µm) and performed LTE slab modelling to remove the H₂O contribution from selected H i transitions. We examined the spatial extent of MIR H i emission and assessed contamination from molecular and jet-related emission.

Results. We find that mid-IR H i line emission is spatially compact, even for sources with spatially extended [Ne II] and [Fe II] jets, suggesting minimal contamination from extended jet. Although Pfund α (H i 6–5) and Humphreys α (H i 7–6) are the strongest lines in the mid-infrared, they are blended with H₂O transitions. This blending necessitates additional processing to remove molecular contamination, thereby limiting their use as accretion diagnostics. Instead, we identify the H i (8–6) at 7.502 µm and H i (10–7) at 8.760 µm transitions as better alternatives, as they are largely unaffected by molecular contamination and offer a more reliable means of measuring accretion rates from MIRI spectra. We provide updated empirical relations for converting mid-IR H i line luminosities into accretion luminosity for six different H i lines in the MIRI wavelength range. Moreover, a comparison of the observed line ratios with theoretical models shows that MIR H i lines offer robust constraints on the hydrogen gas density in accretion columns, n_H = 10^10.6 to 10^11.2 cm⁻³ in most stars, with some stars exhibiting lower densities (<10¹⁰ cm⁻³), approaching the optically thin regime.