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

We present a comprehensive study of mid-infrared neutral hydrogen (H ) emission lines in 79 nearby (d $<$ 200 pc) young stars using the ( ) Mid-Infrared Instrument (MIRI). This work extends accretion diagnostics to mid-infrared H transitions, which are less affected by extinction and outflow emission compared to optical and near-infrared H lines. i James Webb Space Telescope JWST i i We aim to identify mid-infrared H transitions that can serve as reliable accretion diagnostics in young stars, and evaluate their utility in deriving physical conditions of the accreting gas. i We identified and measured 22 H transitions in the MIRI wavelength regime (5–28 μ m) and performed LTE slab modelling to remove the H O contribution from selected H transitions. We examined the spatial extent of MIR H emission and assessed contamination from molecular and jet-related emission. i 2 i i We find that mid-IR H line emission is spatially compact, even for sources with spatially extended i Ne ii and Fe ii jets, suggesting minimal contamination from extended jet. Although Pfund α (H 6--5) and Humphreys α (H 7--6) are the strongest lines in the mid-infrared, they are blended with H_2O transitions. This blending necessitates additional processing to remove molecular contamination, thereby limiting their use as accretion diagnostics. Instead, we identify the H (8--6) at 7.502 μ m and H (10--7) at 8.760 μ m 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 line luminosities into accretion luminosity for six different H lines in the MIRI wavelength range. Moreover, a comparison of the observed line ratios with theoretical models shows that MIR H lines offer robust constraints on the hydrogen gas density in accretion columns, n_ = 10 i i i i i i i H 10.6 $ to 10^11.2 cm^-3 in most stars, with some stars exhibiting lower densities (<10^10 cm$^ -3 ), approaching the optically thin regime.