论文标题
一个非常热的白色矮人，迅速旋转的K型子级伴侣：UCAC2 46706450
An extremely hot white dwarf with a rapidly rotating K-type subgiant companion: UCAC2 46706450
论文作者
论文摘要
UCAC2 46706450是紫外线（UV）过量的晚期恒星。它被认为是建立具有白色矮人（WD）伴侣的FGK恒星样本的候选人，该样本可用于测试二进制进化模型。为了验证伴侣的WD性质，Parsons等人进行了紫外线光谱。 （2016）。通过详细的模型 - 大气分析，我们表明，UV源是非常热的WD，具有有效温度$ t_ \ t_ \ mathrm {eff} $ = $ 105 \，000 \ pm5000 $ k，质量$ $ $ $ m_ \ odot = 0.54 \ odot = 0.54 \ pm0.02 $，radius $ r_ \ r_ \ r_ \ odot = 0.40 $} 0.040 = 0.040 = 0.040 = 0.040 = 0.040 = 0.040 = 0.040 = 0.040 = 0.040 = 0.040^= 0.040 = 0.040^{+05555}和亮度$ l/l_ \ odot = 176^{+55} _ { - 49} $，即，紧凑对象即将输入WD冷却序列。调查酷星的调查光谱（$ t_ \ mathrm {eff} $ = $ 4945 \ pm250 $ k），我们发现它是一个K-Type子巨头，具有$ m/m_ \ odot = 0.8-2.4 $，$ r/r_ \ odot = 5.9 19^{+5} _ { - 5} $，它以$ v \ sin（i）= 81 $ km s $^{ - 1} $迅速旋转。光光曲线显示了两天的时间，而O波段峰峰幅度为0.06 mag。我们建议，它是由与恒星斑有关的出色旋转引起的。使用半径，我们推断出$ v _ {\ mathrm {rot}} = 151^{+18} _ { - 13} $ km s $ s $^{ - 1} $的极高的旋转速度，从而将恒星标记为最快的旋转子位。这解释了h $α$发射和发射线核在CAII H和K中观察到的色球活性以及NUV通量过量。从WD和K子巨头的相等和恒定径向速度以及从拟合到光谱能量分布，我们推断它们形成了物理，宽但未解决的二元系统。这两个组件都表现出相似的金属丰度，并显示出具有略微超极（最高0.6个dex）丰度的铁组元素，这意味着由于残留的，较弱的辐射驱动的风，WD大气中的原子扩散尚未活跃。 （简略）
UCAC2 46706450 is a late-type star with an ultraviolet (UV) excess. It was considered a candidate to establish a sample of FGK stars with white dwarf (WD) companions that can be used to test binary evolution models. To verify the WD nature of the companion, UV spectroscopy was performed by Parsons et al. (2016). By a detailed model-atmosphere analysis we show that the UV source is an extremely hot WD with effective temperature $T_\mathrm{eff}$ = $105\,000\pm5000$ K, mass $M/M_\odot = 0.54\pm0.02$, radius $R/R_\odot = 0.040^{+0.005}_{-0.004}$, and luminosity $L/L_\odot= 176^{+55}_{-49}$, i.e., the compact object is just about to enter the WD cooling sequence. Investigating spectra of the cool star ($T_\mathrm{eff}$ = $4945\pm250$ K) we found that it is a K-type subgiant with $M/M_\odot = 0.8-2.4$, $R/R_\odot = 5.9^{+0.7}_{-0.5}$, and $L/L_\odot= 19^{+5}_{-5}$, that is rapidly rotating with $v \sin(i)=81$ km s$^{-1}$. Optical light curves reveal a period of two days and an o-band peak-to-peak amplitude of 0.06 mag. We suggest, that it is caused by stellar rotation in connection with star spots. With the radius we infer an extremely high rotational velocity of $v_{\mathrm{rot}}=151^{+18}_{-13}$ km s$^{-1}$, thus marking the star as one of the most rapidly rotating subgiants known. This explains chromospheric activity observed by H$α$ emission and emission-line cores in CaII H and K as well as NUV flux excess. From equal and constant radial velocities of the WD and the K subgiant as well as from a fit to the spectral energy distribution we infer that they form a physical, wide though unresolved binary system. Both components exhibit similar metal abundances and show iron-group elements with slightly oversolar (up to 0.6 dex) abundance, meaning that atomic diffusion in the WD atmosphere is not yet active due to a residual, weak radiation-driven wind. (abridged)
