KMTNet超新星计划发现短周期和异常缺氦矮新星KSP-OT-201701 a
Discovery of a Short-Period and Unusually Helium-Deficient Dwarf Nova KSP-OT-201701a by the KMTNet Supernova Program
摘要：
我们介绍了韩国微透镜望远镜网络超新星计划首次发现的短周期和异常缺氦的矮新星KSP-OT-201701 a。源显示三个超级爆发，每一个导致的前兆爆发，和几个正常的爆发在英属维尔京群岛的跨度约2.6年的超级周期和正常周期长度分别约360和76天。超级爆发结束时的光谱观察揭示了强双峰HI发射线和弱HeI发射线的存在。HeIλ5876和Hα谱线测得的氦氢强度比在静止期为0.10 { \pm } 0.01，在爆发期为0.26 { \pm } 0.04，与长周期矮新星的强度比相似，但明显低于氦激变变星（He CV）的强度比。它的轨道周期为51.91 { \pm } 2.50分钟，这是基于时间序列光谱学估计的，比超级驼峰周期56.52 { \pm } 0.19分钟要短一点，正如偏心盘和次要恒星之间的引力相互作用所预期的那样。利用超峰周期超过0.089 { \pm } 0.053，测得其质量比为0.37^{+0.32}-0.21}。KSP-OT-201701 a的短轨道周期（低于周期极小值）、不寻常的氦缺乏和大的质量比表明它是一个从长周期矮新星演化为氦CV的过渡天体，其中有一颗演化的次级恒星。
英文摘要：
We present the first ever discovery of a short-period and unusually helium-deficient dwarf nova KSP-OT-201701a by the Korea Microlensing Telescope Network Supernova Program. The source shows three superoutbursts, each led by a precursor outburst, and several normal outbursts in BVI during the span of ~2.6 years with supercycle and normal cycle lengths of about 360 and 76 days, respectively. Spectroscopic observations near the end of a superoutburst reveal the presence of strong double-peaked HI emission lines together with weak HeI emission lines. The helium-to-hydrogen intensity ratios measured by HeIλ5876 and Hα lines are 0.10 {\pm} 0.01 at a quiescent phase and 0.26 {\pm} 0.04 at an outburst phase, similar to the ratios found in long-period dwarf novae while significantly lower than those in helium cataclysmic variables (He CVs). Its orbital period of 51.91 {\pm} 2.50 minutes, which is estimated based on time series spectroscopy, is a bit shorter than the superhump period of 56.52 {\pm} 0.19 minutes, as expected from the gravitational interaction between the eccentric disk and the secondary star. We measure its mass ratio to be 0.37^{+0.32}_{-0.21} using the superhump period excess of 0.089 {\pm} 0.053. The short orbital period, which is under the period minimum, the unusual helium deficiency, and the large mass ratio suggest that KSP-OT-201701a is a transition object evolving to a He CV from a long-period dwarf nova with an evolved secondary star.
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