{
  "abstract": "Hydrogen lines are widely used as tracers of stellar and planetary accretion. In classical T Tauri stars, hydrogen lines are usually interpreted as arising from magnetospheric accretion columns, whereas in lower-mass counterparts (subsolar-mass objects including brown dwarfs and gas giant planets), the post-accretion-shock region can directly emit a substantial fraction of the hydrogen-line luminosity. However, the boundary between nonshock-dominated and shock-dominated cases has remained unclear. In this study, we compare hydrogen-line profiles predicted by the shock emission model with 254 observations of 164 low-mass accreting objects (≲0.5 M ⊙ ) in the Very Large Telescope/X-shooter archive. We simultaneously fit seven hydrogen lines (H β , H γ , H6, H8, H9, Pa β , and Br γ ), testing both line profiles and flux ratios within a single framework, and introduce a phenomenological broad-component-subtracted fit for cases with mixed nonshock and shock contributions. We find that shock emission dominates the hydrogen-line emission at object masses M ≲ 0.05 M ⊙ or freefall velocities v ff < 175 km s ⁻¹ , but becomes minor at M ≳ 0.2 M ⊙ . The inferred flow velocities at the shock front are often significantly smaller than the freefall velocity from infinity, implying smaller truncation radii and surface dipole magnetic field strengths of sub-kG. The accretion luminosities inferred from the shock-emission fitting are systematically larger than literature values, often by orders of magnitude, likely because conventional estimates neglect line emission that is nonnegligible in low-mass objects. We also confirm that H α is more susceptible than the other hydrogen lines to additional nonshock components.",
  "abstract_shown": true,
  "affiliations": [
    "Sun Yat-sen University",
    "The University of Tokyo",
    "Academia Sinica Institute of Astronomy & Astrophysics (ASIAA)",
    "National Institutes of Natural Sciences",
    "Universität Duisburg-Essen",
    "University of Bern",
    "Max-Planck-Institut für Astronomie",
    "European Southern Observatory",
    "INAF—Osservatorio Astronomico di Capodimonte",
    "Peking University",
    "Université Paris-Saclay",
    "National Astronomical Observatory of Japan"
  ],
  "article_type": null,
  "arxiv": null,
  "author_total": 8,
  "authors": [
    {
      "affs": [
        0,
        1
      ],
      "name": "Yuhiko Aoyama",
      "orcid": "0000-0003-0568-9225"
    },
    {
      "affs": [
        2,
        3
      ],
      "name": "Jun Hashimoto",
      "orcid": "0000-0002-3053-3575"
    },
    {
      "affs": [
        4,
        5,
        6
      ],
      "name": "Gabriel-Dominique Marleau",
      "orcid": "0000-0002-2919-7500"
    },
    {
      "affs": [
        7
      ],
      "name": "Carlo F. Manara",
      "orcid": "0000-0003-3562-262X"
    },
    {
      "affs": [
        8
      ],
      "name": "Juan Manuel Alcalá",
      "orcid": "0000-0001-8657-095X"
    },
    {
      "affs": [
        9
      ],
      "name": "Gregory Herczeg",
      "orcid": "0000-0002-7154-6065"
    },
    {
      "affs": [
        10
      ],
      "name": "Camille Bergez-Casalou",
      "orcid": "0000-0002-1787-7883"
    },
    {
      "affs": [
        1,
        3,
        11
      ],
      "name": "Masahiro Ikoma",
      "orcid": "0000-0002-5658-5971"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/bg2y8b4yk58mn.bib",
  "cited_by": 0,
  "doi": "10.3847/1538-3881/aeada0",
  "fulltext_url": null,
  "id": "bg2y8b4yk58mn",
  "issns": [
    "0004-6256",
    "1538-3881"
  ],
  "landing_url": "https://iopscience.iop.org/article/10.3847/1538-3881/aeada0",
  "language": null,
  "licence": "CC-BY-4.0",
  "licence_label": "CC BY 4.0",
  "licence_source": "article",
  "licence_url": "https://creativecommons.org/licenses/by/4.0/",
  "metadata_licence": null,
  "native_subjects": [
    [
      "openalex",
      "3103|Astrophysics and Star Formation Studies"
    ]
  ],
  "openalex": "W7214003769",
  "pdf_url": "https://iopscience.iop.org/article/10.3847/1538-3881/aeada0/pdf",
  "pmcid": null,
  "pmid": null,
  "published": "2026-10-27",
  "publisher": "American Astronomical Society",
  "ris": "https://research.draconitic.com/paper/bg2y8b4yk58mn.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:18:14Z",
      "native_id": "10.3847/1538-3881/aeada0",
      "source": "crossref",
      "url": "https://iopscience.iop.org/article/10.3847/1538-3881/aeada0"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "openalex:3103|Astrophysics and Star Formation Studies",
      "subfield": "stellar"
    }
  ],
  "title": "Hydrogen-line Emission in Accreting Low-mass Objects. I. Spectral Analysis of Shock-origin Narrow Component",
  "title_text": "Hydrogen-line Emission in Accreting Low-mass Objects. I. Spectral Analysis of Shock-origin Narrow Component",
  "type": "peer-reviewed",
  "updated_at": "2026-10-30T12:18:14Z",
  "url": "https://research.draconitic.com/paper/bg2y8b4yk58mn",
  "venue": "The Astronomical Journal",
  "versions": [
    {
      "doi": "10.48550/arxiv.2609.23778",
      "pid": "4g1j53mhavgtz",
      "relation": "preprint",
      "title": "Hydrogen Line Emission in Accreting Low-Mass Objects I: Spectral Analysis of Shock-Origin Narrow Component",
      "venue": "arXiv"
    }
  ]
}