RACE-OC计划：疏散星团NGC2099（M37）的旋转和变化
RACE-OC Project: Rotation and variability in the open cluster NGC2099 (M37)
摘要：
在G或更高光谱类型的恒星中，自转和太阳型磁活动密切相关。磁场活动的存在和水平取决于恒星的自转，而自转本身又受到磁场的强度和拓扑结构的强烈影响。疏散星团是研究自转和活动之间联系的特别有用的目标。疏散星团NGC 2099是RACE-OC项目的一部分，该项目旨在探索疏散星团中不同年龄的晚型成员的自转和磁活动的演化。2004年1月收集了该星团的时间序列CCD测光观测数据。研究了光变曲线振幅等活动表现与恒星整体参数之间的关系。我们已经发现了135个周期变量，其中122个是候选簇成员。G型和K型恒星自转周期的测定使我们能够更好地探索大约5亿年的角动量演化。与较老的毕宿星团（~ 6.25亿年）的比较表明，新确定的旋转周期分布与主序星斑点星随着年龄的增长而旋转制动的情况是一致的。然而，与年轻的M34星团（~ 2亿年）的比较表明，这些星团的G8-K5成员具有相同的旋转周期分布，即NGC 2099中的G8-K5成员似乎在~ 2亿年至~ 5亿年的年龄范围内没有经历明显的制动。最后，NGC 2099成员有一定水平的光球磁活动，以光变曲线振幅测量，比相同质量和旋转的年轻恒星小，这表明活动水平也取决于其他一些年龄相关的参数。
英文摘要：
Rotation and solar-type magnetic activity are closely related to each other in stars of G or later spectral types. Presence and level of magnetic activity depend on star's rotation and rotation itself is strongly influenced by strength and topology of the magnetic fields. Open clusters represent especially useful targets to investigate the connection between rotation and activity. The open cluster NGC2099 has been studied as a part of the RACE-OC project, which is aimed at exploring the evolution of rotation and magnetic activity in the late-type members of open clusters of different ages. Time series CCD photometric observations of this cluster were collected during January 2004. The relations between activity manifestations, such as the light curve amplitude, and global stellar parameters are investigated. We have discovered 135 periodic variables, 122 of which are candidate cluster members. Determination of rotation periods of G- and K-type stars has allowed us to better explore evolution of angular momentum at an age of about 500 Myr. A comparison with the older Hyades cluster (~625 Myr) shows that the newly determined distribution of rotation periods is consistent with the scenario of rotational braking of main-sequence spotted stars as they age. However, a comparison with the younger M34 cluster (~200 Myr) shows that the G8-K5 members of these clusters have the same rotation period distribution, that is G8-K5 members in NGC2099 seem to have experienced no significant braking in the age range from ~200 to ~500 Myr. Finally, NGC2099 members have a level of photospheric magnetic activity, as measured by light curve amplitude, smaller than in younger stars of same mass and rotation, suggesting that the activity level also depends on some other age-dependent parameters.
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