超紧AM CVn二元系V407 Vul和HM Cnc的合成直接碰撞光变曲线
Synthetic Direct Impact Light Curves of the Ultracompact AM CVn Binary Systems V407 Vul and HM Cnc
摘要：
相互作用双星白矮星（AM CVn）系统HM Cnc和V407的轨道周期分别为5.4 min和9.5 min。这两个系统的特征在于在X射线光变曲线中的“开/关”行为，以及接近正弦的光学光变曲线，并且在这两个系统中，光学光变曲线在相位上领先X射线光变曲线约0.2。在已经提出的解释观测结果的模型中，似乎需要微调最少的是Marsh & Steeghs（2002）的直接影响模型。在这个模型中，主白矮星相对于半长轴足够大，吸积流直接撞击主白矮星的表面而不形成吸积盘。Marsh和Steeghs提出，在这种情况下，可能会在赤道周围建立一个流动，其表面温度随着距离撞击点的距离而降低。在这项研究中，我们估计的光变曲线，可能会导致这样的温度分布，并发现它们是合理的近似的观察。一个意想不到的结果是，必须存在两个不同的X射线斑点，以匹配X射线光变曲线的形状。
英文摘要：
The interacting binary white dwarf (AM CVn) systems HM Cnc and V407 have orbital periods of 5.4 min and 9.5 min, respectively. The two systems are characterized by an "on/off" behaviour in the X-ray light curve, and optical light curves that are nearly sinusoidal and which lead the X-ray light curves in phase by about 0.2 in both systems. Of the models that have been proposed to explain the observations, the one that seems to require the least fine tuning is the direct impact model of Marsh & Steeghs (2002). In this model, the white dwarf primary is large enough relative to the semi-major axis that the accretion stream impacts the surface of the primary white dwarf directly without forming an accretion disc. Marsh & Steeghs proposed that in this situation there could be a flow set up around the equator with a decreasing surface temperature the further one measured from the impact point. In this study, we estimate the light curves that might result from such a temperature distribution, and find them to be reasonable approximations to the observations. One unexpected result is that two distinct X-ray spots must exist to match the shape of the X-ray light curves.
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