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

Astronomy and Astrophysics, vol. 708, pp. A22

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 H 2 O 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 H 2 O 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 ( < 10 10 cm −3 ), approaching the optically thin regime.

Authors 12

1. Baskaran Shridharan

   Tata Institute of Fundamental Research

   Affiliation as printed

   Department of Astronomy and Astrophysics, Tata Institute of Fundamental Research

2. P. Manoj

   Tata Institute of Fundamental Research

   Affiliation as printed

   Department of Astronomy and Astrophysics, Tata Institute of Fundamental Research

3. Vinod Chandra Pathak

   Tata Institute of Fundamental Research

   Affiliation as printed

   Department of Astronomy and Astrophysics, Tata Institute of Fundamental Research

4. A. Caratti o Garatti

   Astronomical Observatory of Capodimonte · National Institute for Astrophysics

   Affiliation as printed

   INAF – Osservatorio Astronomico di Capodimonte

5. Bihan Banerjee

   Tata Institute of Fundamental Research

   Affiliation as printed

   Department of Astronomy and Astrophysics, Tata Institute of Fundamental Research

6. Thomas K. Henning

   Max Planck Institute for Astronomy

   Affiliation as printed

   Max-Planck-Institut für Astronomie (MPIA)

7. I. Kamp

   University of Groningen

   Affiliation as printed

   Kapteyn Astronomical Institute, Rijksuniversiteit Groningen

8. Himanshu Tyagi

   Tata Institute of Fundamental Research

   Affiliation as printed

   Department of Astronomy and Astrophysics, Tata Institute of Fundamental Research

9. Roy Arun

   Indian Institute of Astrophysics · Christ University

   Affiliation as printed

   Department of Physics and Electronics, CHRIST (Deemed to be University)

   Indian Institute of Astrophysics

10. Blesson Mathew

    Christ University

    Affiliation as printed

    Department of Physics and Electronics, CHRIST (Deemed to be University)

11. M. Güdel

    University of Vienna · ETH Zurich · Institute for Particle Physics and Astrophysics

    Affiliation as printed

    Department of Astrophysics, University of Vienna

    ETH Zürich, Institute for Particle Physics and Astrophysics

12. P. -O. Lagage

    Centre National de la Recherche Scientifique · Université Paris Cité · Commissariat à l'Énergie Atomique et aux Énergies Alternatives · Université Paris-Saclay · Astrophysique, Instrumentation et Modélisation

    Affiliation as printed

    Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM