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

Springer Link (Chiba Institute of Technology)

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 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 (<1010 cm−3), approaching the optically thin regime.

Authors 13

1. Baskaran Shridharan

   Tata Institute of Fundamental Research

   Affiliation as printed

   Department of Astronomy and Astrophysics , Tata Institute of Fundamental Research , Homi Bhabha Road , Mumbai 400005 , India

2. P. Manoj

   Tata Institute of Fundamental Research

   Affiliation as printed

   Department of Astronomy and Astrophysics , Tata Institute of Fundamental Research , Homi Bhabha Road , Mumbai 400005 , India

3. Vinod Chandra Pathak

   Tata Institute of Fundamental Research

   Affiliation as printed

   Department of Astronomy and Astrophysics , Tata Institute of Fundamental Research , Homi Bhabha Road , Mumbai 400005 , India

4. A. Caratti o Garatti

   Astronomical Observatory of Capodimonte

   Affiliation as printed

   INAF -Osservatorio Astronomico di Capodimonte , Salita Moiariello 16 , 80131 Napoli , Italy

5. Bihan Banerjee

   Tata Institute of Fundamental Research

   Affiliation as printed

   Department of Astronomy and Astrophysics , Tata Institute of Fundamental Research , Homi Bhabha Road , Mumbai 400005 , India

6. Thomas K. Henning
7. I. Kamp

   University of Groningen

   Affiliation as printed

   Kapteyn Astronomical Institute , Rijksuniversiteit Groningen , Postbus 800 , 9700AV Groningen , The Netherlands

8. E. van Dishoeck Aachen Leiden Observatory

   Leiden University · Max Planck Institute for Extraterrestrial Physics · Leiden Observatory

   Affiliation as printed

   Leiden Observatory , Universiteit Leiden , Leiden , Zuid-Holland , The Netherlands

   Max-Planck-Institut für Extraterrestrische Physik , D-85748 Garching bei München , Germany

9. H. Tyagi

   Tata Institute of Fundamental Research

   Affiliation as printed

   Department of Astronomy and Astrophysics , Tata Institute of Fundamental Research , Homi Bhabha Road , Mumbai 400005 , India

10. Roy Arun

    Indian Institute of Astrophysics · Christ University

    Affiliation as printed

    Department of Physics and Electronics , CHRIST (Deemed to be University) , Bangalore 560029 , India

    Indian Institute of Astrophysics , 2nd Block Koramangala , Bangalore 560034 , India

11. B. Mathew

    Christ University

    Affiliation as printed

    Department of Physics and Electronics , CHRIST (Deemed to be University) , Bangalore 560029 , India

12. M. Güdel

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

    Affiliation as printed

    Department of Astrophysics , University of Vienna , Türkenschanzstr. 17 , 1180 Vienna , Austria

    ETH Zürich , Institute for Particle Physics and Astrophysics , Wolfgang-Pauli-Str. 27 , 8093 Zürich , Switzerland

13. 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 , 91191 Gif-sur-Yvette , France