The cluster Praesepe (age 650 Myr) is an ideal laboratory to study stellar evolution. Specifically, it allows us to trace the long-term decline of rotation and activity on the main-sequence. Here we present rotation periods measured for five stars in Praesepe with masses of 0.1-0.5Ms -- the first rotation periods for members of this cluster. Photometric periodicities were found from two extensive monitoring campaigns, and are confirmed by multiple independent test procedures. We attribute these variations to magnetic spots co-rotating with the objects, thus indicating the rotation period. The five periods, ranging from 5 to 84h, show a clear positive correlation with object mass, a trend which has been reported previously in younger clusters. When comparing with data for F-K stars in the coeval Hyades, we find a dramatic drop in the periods at spectral type K8-M2 (corresponding to 0.4-0.6Ms). A comparison with periods of VLM stars in younger clusters provides a constraint on the spin-down timescale: We find that the exponential rotational braking timescale is clearly longer than 200 Myr, most likely 400-800 Myr. These results are not affected by the small sample size in the rotation periods. Both findings, the steep drop in the period-mass relation and the long spin-down timescale, indicate a substantial change in the angular momentum loss mechanism for very low mass objects, possibly the breakdown of the solar-type (Skumanich) rotational braking. While the physical origin for this behaviour is unclear, we argue that parts of it might be explained by the disappearance of the radiative core and the resulting breakdown of an interface-type dynamo in the VLM regime. Rotational studies in this mass range hold great potential to probe magnetic properties and interior structure of main-sequence stars.
The effect of rotation on the abundances of the chemical elements of the A-type stars in the Praesepe cluster
我们研究如何化学丰度的后期B，A-和早期F-型恒星随时间的演变，我们寻找的化学元素的丰度和其他恒星参数，如有效温度和Vsini之间的相关性。我们已经观测到大量的B型、A型和F型恒星，它们属于不同年龄的疏散星团。在本文中，我们利用上普罗旺斯天文台的SOPHIE摄谱仪获得了Praesepe星团（log t = 8.85）16颗正常A型和F型星以及一颗Am星的高分辨率、高信噪比光谱。对于所有观测到的恒星，我们推导出基本参数和化学丰度。此外，我们讨论了属于同一星团的另外8颗Am星，它们的丰度分析已在以前的论文中给出。我们发现Am星的特性和Vsini之间有很强的相关性。Am星中丰度不足的元素的丰度随着Vsini的增加而增加，而丰度过剩的元素的丰度则随着Vsini的增加而减少。各种元素的化学丰度似乎与铁丰度相关。
A Photometric Survey for Variables and Transits in the Field of Praesepe with KELT
KELT（Kilodegree Extremely Little Telescope）项目是一个小口径、广角的太阳型恒星行星凌日搜索项目。在本文中，我们提出的委托活动的结果与KELT望远镜观测疏散星团Praesepe的34个夜晚在2005年初。获得了69，337颗恒星的光变曲线，其中我们确定了58个长周期变星和152个周期变星。其中16颗以前被称为变星，产生了194颗新发现的变星，我们提供了它们的性质和光变曲线。我们还搜索了类似行星的凌日，找到了四个凌日候选者。后续观测表明，其中两个候选者是天体物理学的假阳性，还有两个候选者是潜在的行星凌日。
Late stages of the evolution of A-type stars on the main sequence: comparison between observed chemical abundances and diffusion models for 8 Am stars of the Praesepe cluster
