直接成像褐矮星伴星的宿主恒星金属丰度与运动学
Host-star metallicities and kinematics of directly imaged brown-dwarf companions
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- Università degli Studi di Milano（米兰大学）
- Indian Institute of Astrophysics（印度天体物理研究所）
- Visvesvaraya Technological University（维斯瓦纳塔亚理工大学）
- Vikram Sarabhai Space Centre, ISRO（印度空间研究组织维克拉姆·萨拉巴伊太空中心）
- St. Albert’s College（圣阿尔伯特学院）
- European Southern Observatory（欧洲南方天文台）
- Physical Research Laboratory（物理研究实验室）
- Indian Institute of Technology, Dhanbad（印度理工学院丹巴德分校）
- Pontificia Universidad Católica de Chile（智利天主教 Pontificia 大学）
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中文总结 AI 辅助
本研究分析直接成像褐矮星伴星宿主恒星金属丰度，发现其呈太阳丰度分布、无富金属偏差，表明宽轨道褐矮星可能主要由盘不稳定性或云碎裂形成，而非经典核吸积。
中文摘要 AI 辅助
褐矮星作为自由漂浮天体很常见，但作为类太阳恒星的近距离伴星却很罕见，这种差异被称为“褐矮星沙漠”。宿主恒星的金属丰度可以约束这些伴星主要是通过金属敏感的核吸积形成，还是通过较少依赖金属的盘或云碎裂形成。我们将对直接成像行星宿主恒星的均匀光谱分析扩展到褐矮星领域，并将其金属丰度与行星宿主恒星及近距离褐矮星宿主恒星进行比较。我们汇编了54个独特的直接成像褐矮星系统，这些系统选自包容性的13-80倍木星质量区间，投影间距从约5天文单位到数千天文单位。靠近模型依赖的70-75倍木星质量氢燃烧边界的天体可能实际上是极低质量恒星。对于31个具有档案高分辨率光谱的宿主恒星，我们使用贝叶斯光谱合成推导了大气参数和金属丰度。文献中的伴星质量和投影间距具有异质性，仅用于人口统计背景。我们计算了46个宿主恒星的运动速度，仅用于表征偏向年轻天体的成像样本。宿主恒星具有大致太阳金属丰度分布，中位数[Fe/H]为+0.06 dex，中位数绝对偏差为0.11 dex，未显示出强烈的富金属偏差。在较低质量和较高质量的直接成像子样本之间，未检测到统计学上显著的金属丰度差异。宿主恒星在运动学上是冷的，这符合直接成像巡天偏向年轻天体的选择。缺乏强烈的富金属偏差表明，经典核吸积并不主导宽轨道褐矮星种群。盘不稳定性和云碎裂仍然是合理的形成机制，但当前样本和异质的伴星性质不允许对这些通道进行逐天体区分。
英文摘要
Brown dwarfs are common as free-floating objects but rare as close companions to Sun-like stars, a disparity known as the "brown-dwarf desert". Host-star metallicity can constrain whether these companions form mainly through metal-sensitive core accretion or through less metal-dependent disc or cloud fragmentation. We extend our homogeneous spectroscopic analysis of directly imaged planet hosts into the brown-dwarf regime and compare their metallicities with those of planet hosts and close-in brown-dwarf hosts. We compiled 54 unique directly imaged brown-dwarf systems selected over an inclusive 13-80 M_Jup interval and projected separations from about 5 au to several thousand au. Objects near the model-dependent 70-75 M_Jup hydrogen-burning boundary may instead be very-low-mass stars. For 31 hosts with archival high-resolution spectra, we derived atmospheric parameters and metallicities using Bayesian spectral synthesis. Literature companion masses and projected separations are heterogeneous and are used only for demographic context. Galactic velocities were calculated for 46 hosts solely to characterise the youth-biased imaging sample. The host stars have a broadly solar metallicity distribution, with a median [Fe/H] of +0.06 dex and a median absolute deviation of 0.11 dex, and show no strong metal-rich bias. No statistically significant metallicity difference is detected between the lower- and higher-mass directly imaged subsamples. The hosts are kinematically cold, as expected from the youth-biased selection of direct-imaging surveys. The absence of a strong metal-rich bias suggests that classical core accretion does not dominate the wide-orbit brown-dwarf population. Disc instability and cloud fragmentation remain plausible, but the current sample and heterogeneous companion properties do not permit object-by-object discrimination between these channels.
