标题： IC 4665中的长期监测：非常低质量天体的快速旋转和弱变异性 显示英文标题

标题： Long-term monitoring in IC4665: Fast rotation and weak variability in very low mass objects

摘要： 我们展示了针对年轻疏散星团 IC4665（年龄约40 Myr）中的超低质量（VLM）恒星和棕矮星的三次测光监测活动的综合结果。在所有三次观测活动中，我们观测了约100个星团成员，首次允许我们对完全对流天体的斑点演化和磁场活动进行时间尺度为数年的限制性研究。 对于覆盖从0.05到0.5太阳质量的20个目标，我们检测到了周期性的流量调制，表明存在与天体共转的磁斑。测光周期的探测率（约20%）显著低于相同年龄的太阳质量恒星，这表明斑点特性具有质量依赖性。除了两个例外情况外，没有一个天体在一个以上季节表现出变化和斑点活动。这与太阳质量恒星的情况相反，表明能够产生测光调制的斑点配置相对罕见且短暂存在于VLM天体中。 本文推导出的旋转周期范围从3到30小时，表明VLM天体中缺乏慢速旋转者。这些周期符合一个旋转演化场景，其中包括主序前收缩和由于风制动导致的适度（40-50%）角动量损失。 通过结合我们的发现和文献结果，我们确定了两种旋转和磁场特性的状态，称为C序列和I序列。C序列的主要特征是快速旋转、弱风制动、Hα发射线和饱和的活动水平，而I序列则以缓慢旋转、强风制动、无Hα发射线和线性活动-旋转关系为特点。旋转速率和恒星质量是决定天体处于哪个序列的主要参数。 （摘要） 显示英文摘要

摘要： We present the combined results of three photometric monitoring campaigns targeting very low mass (VLM) stars and brown dwarfs in the young open cluster IC4665 (age ~40 Myr). In all three runs, we observe ~100 cluster members, allowing us for the first time to put limits on the evolution of spots and magnetic activity in fully convective objects on timescales of a few years. For 20 objects covering masses from 0.05 to 0.5 Msol we detect a periodic flux modulation, indicating the presence of magnetic spots co-rotating with the objects. The detection rate of photometric periods (~20%) is significantly lower than in solar-mass stars at the same age, which points to a mass dependence in the spot properties. With two exceptions, none of the objects exhibit variability and thus spot activity in more than one season. This is contrary to what is seen in solar-mass stars and indicates that spot configurations capable of producing photometric modulations occur relatively rarely and are transient in VLM objects. The rotation periods derived in this paper range from 3 to 30h, arguing for a lack of slow rotators among VLM objects. The periods fit into a rotational evolution scenario with pre-main sequence contraction and moderate (40-50%) angular momentum losses due to wind braking. By combining our findings with literature results, we identify two regimes of rotational and magnetic properties, called C- and I-sequence. Main properties on the C-sequence are fast rotation, weak wind braking, Halpha emission, and saturated activity levels, while the I-sequence is characterised by slow rotation, strong wind braking, no Halpha emission, and linear activity-rotation relationship. Rotation rate and stellar mass are the primary parameters that determine in which regime an object is found. (abridged)