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 ($\lesssim 0.5\,M_\odot$) 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.