ROSAT all-sky survey of W Ursae Majoris stars and the problem of supersaturation
ROSAT all-sky survey of W Ursae Majoris stars and the problem of supersaturation
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DOI：
10.1051/0004-6361:20010197
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发表时间：
2001-04
影响因子：
6.5
通讯作者：
K. Stȩpień;J. Schmitt;W. Voges
中科院分区：
文献类型：
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作者：
K. Stȩpień;J. Schmitt;W. Voges
从ROSAT全天空巡天（RASS）数据，我们得到了57 W UMa型接触系统的X射线通量。在我们的样本中，我们检测到三颗恒星，这是有史以来发现的最短周期的主序星双星作为X射线源。对于恒星（B V）0 - 0：6的高原达到，类似的饱和水平观察到的单个，快速旋转的恒星。WUMa星的X射线通量比旋转最快的单星的X射线通量弱约4{5}倍。由于早期型低活动变星的周期较长，在我们的恒星中存在明显的周期-活动关系，而（B V）0 > 0：6且自转周期在0.23 ~ 0.45天之间的冷星则不存在这种关系。单个超快旋转星（UFR）和WUMa星较低的X射线发射被解释为日冕因子降低的结果。UFRs和WUMa系统表面覆盖度降低的物理机制是一致的。对于UFRs，我们提出了强烈的极性上升气流对流区，从下面的不均匀加热驱动。上升气流在对流层底部附近应伴有大尺度向极流动，在近地层中应伴有向赤道流动。这些气流将发电机产生的elds拖向两极，并创造了一个没有elds的赤道区域，其宽度取决于恒星的旋转速率。对于W UMa星，我们提出，一个大规模的水平流包围两颗星将防止磁场产生长寿命的结构与热X射线发射等离子体发光。最快旋转的UFR的活动减少增加了过饱和状态下恒星的角动量损失时间尺度。因此，可以很自然地解释WUMa星的周期cuto和极限质量的存在。
From ROSAT all-sky survey (RASS) data we obtained X-ray fluxes for 57 W UMa type contact systems. In our sample we detected three stars which are the shortest period main sequence binaries ever found as X-ray sources. For stars with (B V )0 0:6 a plateau is reached, similar to the saturation level observed for single, rapidly rotating stars. The X-ray flux of W UMa stars is about 4{5 times weaker than that of the fastest rotating single stars. Because early type, low activity variables have longer periods, an apparent period-activity relation is seen among our stars, while cool stars with (B V )0 > 0:6 and rotation periods between 0.23 and 0.45 days do not show any such relation. The lower X-ray emission of the single, ultra fast rotators (UFRs) and W UMa stars is interpreted as the result of a decreased coronal lling factor. The physical mechanisms responsible for the decreased surface coverage diers for UFRs and W UMa systems. For UFRs we propose strong polar updrafts within a convection zone, driven by nonuniform heating from below. The updrafts should be accompanied by large scale poleward flows near the bottom of the convective layer and equatorward flows in the surface layers. The flows drag dynamo generated elds toward the poles and create a eld-free equatorial region with a width depending on the stellar rotation rate. For W UMa stars we propose that a large scale horizontal flow embracing both stars will prevent the magnetic eld from producing long-lived structures lled with hot X-ray emitting plasma. The decreased activity of the fastest rotating UFRs increases the angular momentum loss time scale of stars in a supersaturated state. Thus the existence of a period cuto and a limiting mass of W UMa stars can be naturally explained.