EVR-CB-004: An Inflated Hot Subdwarf O Star + Unseen WD Companion in a Compact Binary Discovered with the Evryscope
EVR-CB-004: An Inflated Hot Subdwarf O Star + Unseen WD Companion in a Compact Binary Discovered with the Evryscope
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DOI：
10.3847/1538-4357/abb5b2
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发表时间：
2020-09
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通讯作者：
J. Ratzloff;T. Kupfer;B. Barlow;D. Schneider;T. Marsh;U. Heber;Kyle A. Corcoran;E. Bauer;S. Hämmerich;H. Corbett;A. Glazier;W. Howard;N. Law
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作者：
J. Ratzloff;T. Kupfer;B. Barlow;D. Schneider;T. Marsh;U. Heber;Kyle A. Corcoran;E. Bauer;S. Hämmerich;H. Corbett;A. Glazier;W. Howard;N. Law
我们介绍了EVR-CB-004的发现，它是一颗近距离双星，有一个残存的恒星核和一个看不见的白矮星(WD)伴星。这项工作中的分析表明，初级恒星可能是一颗膨胀的热次矮星(SDO)，更有可能是一颗更罕见的后蓝色水平分支(Post-BHB)恒星。后BHBs是蓝色水平恒星或热矮星在过渡到WD之前短暂的贝壳燃烧的最后阶段。这个天体是在南部一次全天热次矮星可变性调查中使用EvryScope的光度数据发现的。EVR-CB-004的光度光曲线显示了主星的椭球形变和多普勒增强以及重力边缘变暗的多分量变率。双星EVR-CB-004是为数不多的已知系统之一，对于这些极其接近的双星系统，它具有长周期(6.08426小时)和大幅度椭球调制(从最大到最小亮度变化16.0%)，而初级系统的属性使其成为真正独特的系统。它还显示出一种特殊的低幅度(小于1%)的正弦光曲线变化，其周期是二进制周期的1/3共振。我们初步确定这个额外的变化源是潮汐诱发的共振脉动，我们建议后续的观测可以验证这一解释。从体系的演化状态、组分和质量分数来看，EVR-CB-004是形成单一高质量(≈1.2M⊙)WD的强有力的合并候选者。它让我们得以一窥残存核心演化和二次变化的短暂阶段，这是以前在紧凑双星中从未见过的。
We present the discovery of EVR-CB-004, a close binary with a remnant stellar core and an unseen white dwarf (WD) companion. The analysis in this work reveals that the primary is potentially an inflated hot subdwarf (sdO) and more likely is a rarer post–blue horizontal branch (post-BHB) star. Post-BHBs are the short-lived shell-burning final stage of a blue horizontal star or hot subdwarf before transitioning to a WD. This object was discovered using Evryscope photometric data in a southern all-sky hot subdwarf variability survey. The photometric light curve for EVR-CB-004 shows multicomponent variability from ellipsoidal deformation of the primary and Doppler boosting, as well as gravitational limb darkening. The binary EVR-CB-004 is one of just a handful of known systems and has a long period (6.08426 hr) and large-amplitude ellipsoidal modulation (16.0% change in brightness from maximum to minimum) for these extremely close binary systems, while the properties of the primary make it a truly unique system. It also shows a peculiar low-amplitude (less than 1%) sinusoidal light-curve variation with a period that is a 1/3 resonance of the binary period. We tentatively identify this additional variation source as a tidally induced resonant pulsation, and we suggest follow-up observations that could verify this interpretation. From the evolutionary state of the system, its components, and its mass fraction, EVR-CB-004 is a strong merger candidate to form a single high-mass (≈1.2 M⊙) WD. It offers a glimpse into a brief phase of remnant core evolution and secondary variation not seen before in a compact binary.