{
  "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 non-shock-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 ($\\lesssim 0.5\\,M_\\odot$) in the VLT/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 non-shock and shock contributions. We find that shock emission dominates the hydrogen-line emission at object masses $M\\lesssim0.05\\,M_\\odot$ or free-fall velocities $v_\\mathrm{ff}<175\\,\\mathrm{km\\,s^{-1}}$, but becomes minor at $M \\gtrsim 0.2\\,M_\\odot$. The inferred flow velocities at the shock front are often significantly smaller than the free-fall 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 non-negligible in low-mass objects. We also confirm that H$α$ is more susceptible than the other hydrogen lines to additional non-shock components.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": null,
  "arxiv": "2609.23778",
  "author_total": 8,
  "authors": [
    {
      "name": "Yuhiko Aoyama"
    },
    {
      "name": "Jun Hashimoto"
    },
    {
      "name": "Gabriel-Dominique Marleau"
    },
    {
      "name": "Carlo F. Manara"
    },
    {
      "name": "Juan Manuel Alcalá"
    },
    {
      "name": "Gregory Herczeg"
    },
    {
      "name": "Camille Bergez-Casalou"
    },
    {
      "name": "Masahiro Ikoma"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/4g1j53mhavgtz.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2609.23778",
  "fulltext_url": null,
  "id": "4g1j53mhavgtz",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2609.23778",
  "language": null,
  "licence": "LicenseRef-arXiv-nonexclusive-1.0",
  "licence_label": "arXiv licence",
  "licence_source": "article",
  "licence_url": "http://arxiv.org/licenses/nonexclusive-distrib/1.0/",
  "metadata_licence": "CC0-1.0",
  "native_subjects": [
    [
      "arxiv",
      "astro-ph.EP"
    ],
    [
      "arxiv",
      "astro-ph.SR"
    ]
  ],
  "openalex": null,
  "pdf_url": "https://arxiv.org/pdf/2609.23778",
  "pmcid": null,
  "pmid": null,
  "published": "2026-09-20",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/4g1j53mhavgtz.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:35:50Z",
      "native_id": "2609.23778",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2609.23778"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.EP",
      "subfield": "planetary"
    },
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.SR",
      "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": "preprint",
  "updated_at": "2026-10-30T12:35:50Z",
  "url": "https://research.draconitic.com/paper/4g1j53mhavgtz",
  "venue": "arXiv",
  "versions": [
    {
      "doi": "10.3847/1538-3881/aeada0",
      "pid": "bg2y8b4yk58mn",
      "relation": "published-as",
      "title": "Hydrogen-line Emission in Accreting Low-mass Objects. I. Spectral Analysis of Shock-origin Narrow Component",
      "venue": "The Astronomical Journal"
    }
  ]
}