Hydrogen-line Emission in Accreting Low-mass Objects. I. Spectral Analysis of Shock-origin Narrow Component
12 affiliations
- 1Sun Yat-sen University
- 2The University of Tokyo
- 3Academia Sinica Institute of Astronomy & Astrophysics (ASIAA)
- 4National Institutes of Natural Sciences
- 5Universität Duisburg-Essen
- 6University of Bern
- 7Max-Planck-Institut für Astronomie
- 8European Southern Observatory
- 9INAF—Osservatorio Astronomico di Capodimonte
- 10Peking University
- 11Université Paris-Saclay
- 12National Astronomical Observatory of Japan
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.
Versions
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BibTeX
@article{aoyama2026hydrogenline,
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},
journal = {The Astronomical Journal},
year = {2026},
month = oct,
doi = {10.3847/1538-3881/aeada0},
url = {https://iopscience.iop.org/article/10.3847/1538-3881/aeada0},
note = {CC BY 4.0},
}
RIS
TY - JOUR
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 - 2026/10/27
JO - The Astronomical Journal
DO - 10.3847/1538-3881/aeada0
UR - https://iopscience.iop.org/article/10.3847/1538-3881/aeada0
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 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.
SN - 0004-6256
SN - 1538-3881
ER -