Improving accretion diagnostics for young stellar objects with mid-infrared hydrogen lines from JWST/MIRI
/ Authors
B. Shridharan, P. Manoj, V. C. Pathak, A. C. O. Garatti, Bihan Banerjee, T. Henning, I. Kamp, E. V. Dishoeck, H. Tyagi, R. Arun
and 21 more authors
B. Mathew, M. Gudel, P.-O. Lagage Tifr, Mumbai, -INAF, Naples, Mpia, Heidelberg, Kapetyn Astronomical Institute, Groningen, L. Observatory, Leiden, Mpie, Garching, Iia, Bangalore, C. University, E. Zurich, Zurich, Universit'e Paris-Saclay, France
/ Abstract
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.
Journal: Astronomy & Astrophysics