标题： HM Cancri中的5.4分钟轨道周期的光谱证据 显示英文标题

标题： Spectroscopic Evidence for a 5.4-Minute Orbital Period in HM Cancri

摘要： HM Cancri 是一个超紧凑的双白矮星候选体，其明显的轨道周期仅为 5.4 分钟，这由 X 射线和光学光变曲线在该周期上的调制以及更长周期变化的缺失所表明。在本信中，我们呈现 Keck-I 光谱学数据，显示氦发射线在 5.4 分钟周期上具有径向速度和振幅的清晰调制，且没有其他调制。这些数据强烈表明，该双星系统从较冷、质量较小的恒星被照射的一面发出 He I 4471，从质量较大的恒星周围的环发出 He II 4686。根据它们的相对径向速度，我们测得质量比 q=0.50+/-0.13。我们得出结论，观测到的 5.4 分钟周期几乎可以肯定代表了一个相互作用的双白矮星的轨道周期。因此我们确认 HM Cnc 是已知轨道周期最短的双星：这是恒星演化理论的独特检验，并且是激光干涉空间天线（LISA）已知的最强引力波源之一。 显示英文摘要

摘要： HM Cancri is a candidate ultracompact binary white dwarf with an apparent orbital period of only 5.4 minutes, as suggested by X-ray and optical light-curve modulations on that period, and by the absence of longer-period variability. In this Letter we present Keck-I spectroscopy which shows clear modulation of the helium emission lines in both radial velocity and amplitude on the 5.4-minute period and no other. The data strongly suggest that the binary is emitting He I 4471 from the irradiated face of the cooler, less massive star, and He II 4686 from a ring around the more massive star. From their relative radial velocities, we measure a mass ratio q=0.50+/-0.13. We conclude that the observed 5.4-minute period almost certainly represents the orbital period of an interacting binary white dwarf. We thus confirm that HM Cnc is the shortest-period binary star known: a unique test for stellar evolution theory, and one of the strongest known sources of gravitational waves for the Laser Interferometer Space Antenna (LISA).