隐藏在普通视线中：一个距离26 pc的双线白矮星和一个远亲
Hidden in Plain Sight: A Double-lined White Dwarf Binary 26 pc Away and a Distant Cousin
摘要：
我们提出了两个附近的白矮星不一致的光谱和视差距离的高分辨率光谱。第一个是PG 1632+177，是一颗13等的白矮星，距离我们只有25.6 pc。以前的光谱观测未能探测到这颗恒星的径向速度变化。在这里，我们发现PG 1632+177是一个2.05 d周期的双线光谱双星（SB 2），包含一个低质量的He核白矮星和一个更大质量的CO核白矮星伴星。在L 870-2之后，PG 1632+177成为目前已知的第二个最近的SB 2白矮星。我们的第二个目标WD 1534+503也是一个轨道周期为0.71 d的SB 2系统。对于每个系统，我们通过复合模型大气分析来约束两个组件的大气参数。我们还提出了一套新的NLTE合成光谱适合于模拟冷白矮星的高分辨率观测，并表明，NLTE的影响，在核心的H $α$线增加有效温度降低。讨论了在轨道约束条件下SB 2和食双星白矮星的轨道周期和质量分布，并证明了观测到的粒子数与双星粒子数合成模型预测的周期分布是一致的.后者预测更大质量的CO + CO白矮星双星在短（$<1$ d）周期，以及双星与几天的轨道周期;这样的系统仍在等待被发现的大量。
英文摘要：
We present high-resolution spectroscopy of two nearby white dwarfs with inconsistent spectroscopic and parallax distances. The first one, PG 1632+177, is a 13th magnitude white dwarf only 25.6 pc away. Previous spectroscopic observations failed to detect any radial velocity changes in this star. Here, we show that PG 1632+177 is a 2.05 d period double-lined spectroscopic binary (SB2) containing a low-mass He-core white dwarf with a more-massive, likely CO-core white dwarf companion. After L 870-2, PG 1632+177 becomes the second closest SB2 white dwarf currently known. Our second target, WD 1534+503, is also an SB2 system with an orbital period of 0.71 d. For each system, we constrain the atmospheric parameters of both components through a composite model-atmosphere analysis. We also present a new set of NLTE synthetic spectra appropriate for modeling high-resolution observations of cool white dwarfs, and show that NLTE effects in the core of the H$α$ line increase with decreasing effective temperature. We discuss the orbital period and mass distribution of SB2 and eclipsing double white dwarfs with orbital constraints, and demonstrate that the observed population is consistent with the predicted period distribution from the binary population synthesis models. The latter predict more massive CO + CO white dwarf binaries at short ($<1$ d) periods, as well as binaries with several day orbital periods; such systems are still waiting to be discovered in large numbers.
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