Accretion diagnostics with JWST/MIRI of YSOs

Although Pfund {alpha} (HI 6-5) and Humphreys {alpha} (HI 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 HI (8-6) at 7.502{mu}m and HI (10-7) at 8.760{mu}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 HI line luminosities into accretion luminosity for 6 different HI lines in the MIRI wavelength range. Moreover, comparison of observed line ratios with theoretical models shows that MIR HI 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 (<10^10^cm^-3^), approaching the optically thin regime.