NGC 3766中令人困惑的新一类变星：老朋友变星？
The puzzling new class of variable stars in NGC 3766 : old friend pulsators?
摘要：
最近对NGC 3766星团的变星调查显示，在H-R图上没有脉动的区域有相当数量的周期性变星。这一区域位于盾牌座delta星和SPB星的不稳定带之间。此外，新现象的周期，P~0.1-0.7 d，不允许将其先验地与这两种类型的脉动中的任何一种联系起来。NGC 3766星团中的恒星被称为快速旋转体，其旋转速度通常大于临界速度的一半。转动既可以影响脉动的几何性质，也可以影响脉动的周期域。它还通过引力暗化改变了恒星的视光度，这种效应在旋转脉动相互作用的理论研究中很少被考虑。我们探讨这两种效应是否能够为NGC 3766中的“新变量”的观测特性提供一致的解释：解释它们在已知不稳定带及其变化周期之外的存在。我们在传统旋转近似的框架内对SPB模型进行了不稳定性分析，并根据视角和旋转研究了模式的可见性。我们还检查重力暗化如何影响有效的温度和光度的恒星模型，不同的视角和旋转速度。在不稳定带的红边（冷边），赤道上的SPB扇形模被优先激发，其振动强度最大，而且由于重力变暗，赤道上的低质量SPB模型可以出现在非旋转SPB和盾牌座δ星之间的间隙中。在这种情况下，由于科里奥利力的影响，这些最明显的模式的周期被移动到0.2-0.5 d的范围。因此，我们认为在NGC 3766中观测到的新变星实际上是快速旋转的SPB变星。
英文摘要：
The recent variability survey of the NGC 3766 cluster revealed a considerable number of periodic variable stars in a region of the H-R diagram where no pulsation is expected. This region lies between the instability strips of the delta Scuti and SPB stars. Moreover the periods of the new phenomenon, P~0.1-0.7 d, do not allow to associate it a priori to either of these two types of pulsations. Stars in the NGC 3766 cluster are known as fast rotators with rotational velocities typically larger than half of their critical velocity. Rotation can affect both the geometrical properties and period domain of pulsations. It also alters the apparent stellar luminosity through gravity darkening, effect seldom taken considered in theoretical studies of the rotation-pulsation interaction. We explore if both of these effects are able to deliver a consistent interpretation for the observed properties of the "new variables" in NGC 3766: explaining their presence outside the known instability strips and their variability periods. We carry out an instability analysis of SPB models within the framework of the Traditional Approximation of Rotation and study the visibility of modes according to the angle of view and rotation. We also check how gravity darkening affects the effective temperature and luminosity of stellar models for different angles of view and rotation velocities. At the red (cold) border of the instability strip, prograde sectoral modes are preferentially excited and their visibilities are maximum when seen equator-on. Furthermore low-mass SPB models seen equator-on can appear in the gap between non-rotating SPB and delta Scuti stars due to gravity darkening. In that case, periods of these most visible modes are shifted to the 0.2-0.5 d range due to the effects of the Coriolis force. We hence suggest that the new variable stars observed in NGC 3766 are actually fast rotating SPB pulsators.
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