Hydrogen Line Emission in Accreting Low-Mass Objects I: Spectral Analysis of Shock-Origin Narrow Component
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 (≲ 0.5 M⊙) in the VLT/X-Shooter archive. We simultaneously fit seven hydrogen lines (Hβ, Hγ, H6, H8, H9, Paβ, and Brγ), 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≲0.05 M⊙ or free-fall velocities vff<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 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 Hα is more susceptible than the other hydrogen lines to additional non-shock components.
Versions
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BibTeX
@misc{aoyama2026hydrogen,
title = {{Hydrogen Line Emission in Accreting Low-Mass Objects I: Spectral Analysis of Shock-Origin Narrow Component}},
author = {Yuhiko Aoyama and Jun Hashimoto and Gabriel-Dominique Marleau and Carlo F. Manara and Juan Manuel Alcalá and Gregory Herczeg and Camille Bergez-Casalou and Masahiro Ikoma},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.23778},
eprint = {2609.23778},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.23778},
}
RIS
TY - UNPB
TI - Hydrogen Line Emission in Accreting Low-Mass Objects I: Spectral Analysis of Shock-Origin Narrow Component
AU - Yuhiko Aoyama
AU - Jun Hashimoto
AU - Gabriel-Dominique Marleau
AU - Carlo F. Manara
AU - Juan Manuel Alcalá
AU - Gregory Herczeg
AU - Camille Bergez-Casalou
AU - Masahiro Ikoma
PY - 2026
DA - 2026/09/20
PB - arXiv
DO - 10.48550/arxiv.2609.23778
UR - https://arxiv.org/abs/2609.23778
AB - 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 (≲ 0.5 M_⊙) in the VLT/X-Shooter archive. We simultaneously fit seven hydrogen lines (Hβ, Hγ, H6, H8, H9, Paβ, and Brγ), 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≲0.05 M_⊙ or free-fall 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 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 Hα is more susceptible than the other hydrogen lines to additional non-shock components.
ER -