Kinematics of W Ursae Majoris type binaries and evidence of the two types of formation
Kinematics of W Ursae Majoris type binaries and evidence of the two types of formation
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DOI：
10.1111/j.1365-2966.2005.08609.x
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发表时间：
2004-11
影响因子：
4.8
通讯作者：
S. Bilir;Y. Karataş;O. Demircan;Z. Eker
中科院分区：
文献类型：
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作者：
S. Bilir;Y. Karataş;O. Demircan;Z. Eker
我们研究了 129 个 W UMa 双星的运动学，并讨论了它对接触双星演化的影响。发现样本在速度空间中是不均匀的。也就是说，样本中存在运动学上较年轻和较老的接触双星。通过选择满足运动群运动学标准的系统来形成运动学年轻（0.5 Gyr）子样本（运动群）。除去可能的移动群成员和已知属于疏散星团成员的系统后，样本的其余部分称为场接触二元（FCB）群。 FCB组根据轨道周期范围进一步分为四组。然后，在较短周期较小质量系统比较长周期较大质量系统具有更大速度弥散的意义上发现了相关性。速度空间中的分散表明 FCB 组的运动年龄为 5.47 Gyr。与场色球活跃双星（CAB）（可能是接触系统的分离双星祖先）相比，FCB 群似乎要老 1.61 Gyr。假设银河系中 CAB 和 W UMa 系统的形成和破坏达到平衡，则该年龄差异被视为根据经验推导出的接触阶段的寿命。由于 FCB 组的四个子组的运动年龄（3.21、3.51、7.14 和 8.89 Gyr）远长于接触阶段的 1.61 Gyr 寿命，因此 FCB 组的预接触阶段必定主要产生大色散。运动学年轻（0.5 Gyr）移动组覆盖与 FCB 组相同的总质量、周期和光谱范围。然而，这个群体的年龄非常小，没有为预接触阶段留下足够的空间，因此这些系统很可能是在主层序开始时或在前主层序收缩阶段形成的，要么是通过裂变过程，要么很可能是通过共同包络中两个组件的快速螺旋形成。
We study the kinematics of 129 W UMa binaries and we discuss its implications on the contact binary evolution. The sample is found to be heterogeneous in the velocity space. That is, kinematically younger and older contact binaries exist in the sample. A kinematically young (0.5 Gyr) subsample (moving group) is formed by selecting the systems that satisfy the kinematical criteria of moving groups. After removing the possible moving group members and the systems that are known to be members of open clusters, the rest of the sample is called the field contact binary (FCB) group. The FCB group is further divided into four groups according to the orbital period ranges. Then, a correlation is found in the sense that shorter-period less-massive systems have larger velocity dispersions than the longer-period more-massive systems. Dispersions in the velocity space indicate a 5.47-Gyr kinematical age for the FCB group. Compared with the field chromospherically active binaries (CABs), presumably detached binary progenitors of the contact systems, the FCB group appears to be 1.61 Gyr older. Assuming an equilibrium in the formation and destruction of CAB and W UMa systems in the Galaxy, this age difference is treated as an empirically deduced lifetime of the contact stage. Because the kinematical ages (3.21, 3.51, 7.14 and 8.89 Gyr) of the four subgroups of the FCB group are much longer than the 1.61-Gyr lifetime of the contact stage, the pre-contact stages of the FCB group must dominantly be producing the large dispersions. The kinematically young (0.5 Gyr) moving group covers the same total mass, period and spectral ranges as the FCB group. However, the very young age of this group does not leave enough room for pre-contact stages, and thus it is most likely that these systems were formed in the beginning of the main sequence or during the pre-main-sequence contraction phase, either by a fission process or most probably by fast spiralling in of two components in a common envelope.