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

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, nH = 1010.6 to 1011.2 cm−3 in most stars, with some stars exhibiting lower densities (