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The puzzling new class of variable stars in NGC 3766 : old friend pulsators?

机译:NGC 3766中令人费解的新型变星:老朋友脉动器?

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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.
机译:最近对NGC 3766星团进行的变异性调查显示,在H-R图的某个区域中有大量的周期性变星,预计不会出现脉动。该区域位于Scuti三角星和SPB星星的不稳定带之间。此外,新现象的周期P〜0.1-0.7 d,不允许先验地将其与这两种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器,其旋转速度通常大于其临界速度的一半。旋转会影响脉动的几何特性和周期域。它还通过引力变暗改变了表观的恒星发光度,这在旋转-脉冲相互作用的理论研究中很少考虑。我们探讨了这两种影响是否都能对NGC 3766中“新变量”的观察特性做出一致的解释:解释它们在已知不稳定性带之外的存在及其变异期。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,并根据视角和旋转角度研究了模式的可见性。我们还检查了重力变暗如何影响不同视角和旋转速度的恒星模型的有效温度和光度。在不稳定带的红色(冷)边界处,优先激发前进的扇形模式,并且当在赤道上看到时,其可见性最大。此外,由于重力变暗,在赤道上空出现的低质量SPB模型可能出现在非旋转SPB和Scuti三角星之间的间隙中。在这种情况下,由于科里奥利力的作用,这些最可见模式的周期移至0.2-0.5 d范围。因此,我们建议在NGC 3766中观测到的新的变星实际上是快速旋转的SPB脉冲星。

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