Hydrogen-line Emission in Accreting Low-mass Objects. I. Spectral Analysis of Shock-origin Narrow Component

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.