Keck-HGCA试点巡天II：直接成像发现HD 63754 B——一颗位于氢燃烧极限附近、轨道间距约20 au的大质量伴星
The Keck-HGCA Pilot Survey II: Direct Imaging Discovery of HD 63754 B, a ~20 au Massive Companion Near the Hydrogen Burning Limit
AI总结：
本研究联合HGCA天体测量、径向速度与Keck/NIRC2高对比度成像，发现并刻画约20 au处近氢燃烧极限的伴星HD 63754 B，测得其质量、轨道与光度，判断其为L/T过渡褐矮星。
摘要：
我们展示了对HD 63754 B（HIP 38216 B）的天体测量与直接成像联合发现、质量测量和轨道分析；它是一颗位于恒星—亚恒星边界附近的伴星，围绕其类太阳宿主星运行，轨道间距约20 AU。HD 63754是在我们正在进行的高对比度成像巡天中被观测的，该巡天针对Hipparcos与Gaia之间具有显著自行加速度、且与大间距亚恒星伴星相符的恒星。我们利用存档的HIRES和HARPS径向速度（RV）数据，以及从Hipparcos-Gaia加速度目录（HGCA）提取的宿主星天体测量加速度，来预测候选伴星在HD 63754 A周围的位置。随后，我们在W. M. Keck天文台使用近红外相机2（NIRC2）在L′波段于预测位置对HD 63754 B进行了成像。接着，我们利用径向速度、Hipparcos-Gaia加速度以及我们新获得的相对天体测量数据，对HD 63754 B的轨道进行联合建模，测得动力学质量为81.9_{-5.8}^{+6.4} M_jup，偏心率为0.260_{-0.059}^{+0.065}，近正面倾角为174.81_{-0.50}^{+0.48}度。对于HD 63754 B，我们得到其L′波段绝对星等为L′=11.39±0.06 mag；通过与一批已测量光度的L型和T型矮星样本比较，我们由此推断其热光度为log(L_bol/L_⊙)=−4.55±0.08 dex。尽管年龄和动力学质量估计仍存在不确定性，但我们的分析表明，HD 63754 B更可能是一颗处于L/T过渡带的褐矮星，而非低质量恒星，这一判断由其推断热光度和模型估计的有效温度所指示。未来的径向速度、光谱和天体测量数据，例如来自JWST和Gaia DR4的数据，将明确HD 63754 B的质量，并实现光谱分型和大气表征。
英文摘要：
We present the joint astrometric and direct imaging discovery, mass measurement, and orbital analysis of HD 63754 B (HIP 38216 B), a companion near the stellar-substellar boundary orbiting ~20 AU from its Sun-like host. HD 63754 was observed in our ongoing high-contrast imaging survey targeting stars with significant proper-motion accelerations between Hipparcos and Gaia consistent with wide-separation substellar companions. We utilized archival HIRES and HARPS radial velocity (RV) data, together with the host star's astrometric acceleration extracted from the Hipparcos-Gaia Catalog of Accelerations (HGCA), to predict the location of the candidate companion around HD 63754 A. We subsequently imaged HD 63754 B at its predicted location using the Near Infrared Camera 2 (NIRC2) in the $L'$ band at the W. M. Keck Observatory. We then jointly modeled the orbit of HD 63754 B with RVs, Hipparcos-Gaia accelerations, and our new relative astrometry, measuring a dynamical mass of ${81.9}_{-5.8}^{+6.4} M_{jup}$, an eccentricity of ${0.260}_{-0.059}^{+0.065}$, and a nearly face-on inclination of $174.81_{-0.50}^{+0.48}$ degrees. For HD 63754 B, we obtain an L' band absolute magnitude of $L' = 11.39\pm0.06$ mag, from which we infer a bolometric luminosity of $log(L_{bol}/L_{\odot})= -4.55 \pm0.08$ dex using a comparison sample of L and T dwarfs with measured luminosities. Although uncertainties linger in age and dynamical mass estimates, our analysis points toward HD 63754 B's identity as a brown dwarf on the L/T transition rather than a low-mass star, indicated by its inferred bolometric luminosity and model-estimated effective temperature. Future RV, spectroscopic, and astrometric data such as those from JWST and Gaia DR4 will clarify HD 63754 B's mass, and enable spectral typing and atmospheric characterization.
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