Polaris: Amplitude, Period Change, and Companions
Polaris: Amplitude, Period Change, and Companions
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北极星：振幅、周期变化和同伴
DOI：
10.1086/338583
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发表时间：
2000
期刊：
影响因子：
--
通讯作者：
C. Short
中科院分区：
文献类型：
--
作者：
N. Evans;D. Sasselov;C. Short
北极星向我们展示了一个罕见的现象，即造父变星的脉动幅度在过去50年中有所下降。在这项研究中，我们已经使用了这个属性，看看是否在过去的15年中的振幅下降有任何影响高层大气加热。我们获得了IUE高分辨率和低分辨率光谱，但没有发现任何变化，无论是镁II色球发射或流量在1800度之间的1978年和1993年的脉动幅度下降了50%（从2.8到1.6公里秒-1）。从1700埃到V、B、R（KC）和I（KC）的能量分布与同色不变的超巨星相似，而不是全振幅造父变星，因为它在1800埃的通量比全振幅造父变星δ Cep的通量大。北极星也有一个快速变化的周期（3.2 s yr-1），与其他泛音周期相同。我们认为，这是一个自然的结果，不同的信封的位置，占主导地位的基本和泛音脉动的脉动增长率。在基模振荡器中，较深的包络线在确定增长率方面比泛音振荡器更重要。对于基模振荡器，半径的演化变化产生近似线性的周期变化。在泛音衰减器中，脉动以更不稳定的方式对小的演化变化做出反应，因为模式对高包络特征（例如不透明度凸起）更敏感，并且许多紧密间隔的泛音模式的增长率容易改变。最后，爱因斯坦观测的X射线通量上限意味着天体测量轨道上的伴星早于F4 V。IUE和爱因斯坦观测的伴星的上限和下限的组合分别捕获了1.7和1.4 M之间的伴星质量。X射线的极限与更远的伴星α UMi B是一个三级系统中的物理伴星是一致的。然而，X射线的限制要求更遥远的伴星α UMi C和D太老了，不能与北极星联系在一起。
Polaris has presented us with the rare phenomenon of a Cepheid with a pulsation amplitude that has decreased over the last 50 yr. In this study we have used this property to see whether the amplitude decrease during the last 15 yr has had any effect on upper atmosphere heating. We obtained IUE high- and low-resolution spectra but found no change in either the Mg II chromospheric emission or the flux at 1800 Å between 1978 and 1993 when the pulsation amplitude dropped by 50% (from 2.8 to 1.6 km s-1). The energy distribution from 1700 Å through V, B, R(KC), and I(KC) is like that of a nonvariable supergiant of the same color rather than a full amplitude Cepheid in that it has more flux at 1800 Å than the full amplitude Cepheid δ Cep. Polaris also has a rapidly changing period (3.2 s yr-1), in common with other overtone pulsators. We argue that this is a natural consequence of the different envelope locations that dominate pulsation growth rates in fundamental and overtone pulsation. In fundamental mode pulsators, the deeper envelope is more important in determining growth rates than for overtone pulsators. For fundamental mode pulsators, evolutionary changes in the radius produce approximately linear changes in period. In overtone pulsators, pulsation reacts to small evolutionary changes in a more unstable way because the modes are more sensitive to high envelope features such as opacity bumps, and the growth rates for the many closely spaced overtone modes change easily. Finally, the upper limit to the X-ray flux from an Einstein observation implies that the companion in the astrometric orbit is earlier than F4 V. The combination of upper and lower limits on the companion from IUE and Einstein respectively catch the companion mass between 1.7 and 1.4 M☉. The X-ray limit is consistent with the more distant companion α UMi B being a physical companion in a hierarchal triple system. However the X-ray limits require that the even more distant companions α UMi C and D are too old to be physically associated with Polaris.