吸积低质量天体中的氢线发射 I：激波起源窄成分的光谱分析

中文摘要 AI 辅助

氢线被广泛用作恒星和行星吸积的示踪剂。在经典T~Tauri星中，氢线通常被解释为起源于磁层吸积柱，而在质量更低的天体（包括褐矮星和气态巨行星在内的亚太阳质量天体）中，吸积后激波区域可以直接发射相当一部分氢线光度。然而，非激波主导与激波主导情形之间的界限仍不清楚。在本研究中，我们将激波发射模型预测的氢线轮廓与VLT/X-Shooter档案中164个低质量吸积天体（$\lesssim 0.5\\,M_\odot$）的254次观测进行比较。我们在单一框架内同时拟合七条氢线（H$\beta$、H$\gamma$、H6、H8、H9、Pa$\beta$和Br$\gamma$），同时检验线轮廓和通量比，并针对非激波与激波贡献混合的情形引入一种唯象的宽成分扣除拟合方法。我们发现，在天体质量$M\lesssim0.05\\,M_\odot$或自由落体速度$v_\mathrm{ff}<175\\,\mathrm{km\\,s^{-1}}$时，激波发射主导氢线发射，但在$M \gtrsim 0.2\\,M_\odot$时变得次要。推断出的激波前沿流动速度通常显著小于从无穷远自由落体的速度，这意味着较小的截断半径和亚千高斯（sub-kG）的表面偶极磁场强度。从激波发射拟合推断出的吸积光度系统性地大于文献值，通常大几个数量级，这可能是因为传统估计忽略了在低质量天体中的不可忽略的线发射。我们还确认H$\alpha$比其他氢线更容易受到额外非激波成分的影响。

英文摘要

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.

发表机构

• School of Physics and Astronomy, Sun Yat-sen University（中山大学物理学院）
• Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo（东京大学理学研究科地球行星科学专攻）
• Academia Sinica Institute of Astronomy & Astrophysics (ASIAA)（中央研究院天文及天文物理研究所）
• Astrobiology Center, National Institutes of Natural Sciences（国立自然科学研究所天体生物学中心）
• Fakultät für Physik, Universität Duisburg-Essen（杜伊斯堡-埃森大学物理系）
• Division of Space Research and Planetary Sciences, Physics Institute, University of Bern（伯尔尼大学物理系空间研究与行星科学部）
• Max-Planck-Institut für Astronomie（马克斯·普朗克天文学研究所）
• European Southern Observatory（欧洲南方天文台）
• INAF - Osservatorio Astronomico di Capodimonte（意大利国家天体物理研究所卡波迪蒙特天文台）
• Kavli Institute for Astronomy and Astrophysics, Peking University（北京大学 Kavli 天体物理与天文研究所）

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42 pages, 18 figures, accepted for publication in The Astronomical Journal