We use the Wide Field Camera 3 onboard the Hubble Space Telescope to obtain deep, high-resolution photometry of the young (age ~ 300 Myr) star cluster NGC1856 in the Large Magellanic Cloud. We compare the observed colour-magnitude diagram (CMD), after having applied a correction for differential reddening, with Monte Carlo simulations of simple stellar populations (SSPs) of various ages. We find that the main sequence turn-off (MSTO) region is wider than that derived from the simulation of a single SSP. Using constraints based on the distribution of stars in the MSTO region and the red clump, we find that the CMD is best reproduced using a combination of two different SSPs with ages separated by 80 Myr (0.30 and 0.38 Gyr, respectively). However, we can not formally exclude that the width of the MSTO could be due to a range of stellar rotation velocities if the efficiency of rotational mixing is higher than typically assumed. Using a King-model fit to the surface number density profile in conjunction with dynamical evolution models, we determine the evolution of cluster mass and escape velocity from an age of 10 Myr to the present age, taking into account the possible effects of primordial mass segregation. We find that the cluster has an escape velocity Vesc ~ 17 km/s at an age of 10 Myr, and it remains high enough during a period of ~ 100 Myr to retain material ejected by slow winds of first-generation stars. Our results are consistent with the presence of an age spread in NGC1856, in contradiction to the results of Bastian & Silva-Villa (2013).
The star formation history of the Large Magellanic Cloud star clusters NGC1846 and NGC1783
NGC 1846和NGC 1783是位于大麦云星云中的两个大质量星团，它们同时拥有一个扩展的主序星关闭区和一个红巨星双团。它们的质量相似，但主要在角的大小上不同。从F435W、F555W和F814W滤波器中的高质量ACS数据和更新的恒星演化轨迹集出发，利用经典的CMD重建方法，得到了它们的恒星形成率与年龄的函数SFR（t）。该方法证实了从以前的分析相同的数据得出的恒星形成的延长期。当分析进行了一个更好的分辨率在年龄，我们发现了明确的证据，50万年长的间隙之间的最古老的高峰在SFR（t），第二个长期的恒星形成，在两个集群。对于更紧凑的星系团NGC1846，星系团中心的SFR（t）和半径在20到60角秒之间的环（从4.8到15.4秒差距）似乎没有显著的差异。同样的情况并没有发生在更扩展的NGC1783星系团中，发现外环（在33到107弧秒之间，从8.0到25.9秒差距）比中心稍年轻。我们还探讨了目前的质量函数和二进制分数的最佳拟合斜率为不同的集群区域，发现在NGC1783的中心和外环之间的质量函数不同的提示。这些研究结果进行了讨论，在目前的情况下，形成集群与多个关闭。
Hubble Tarantula Treasury Project: Unraveling Tarantula's Web. II. Optical and Near Infrared Star Formation History of the Starburst Cluster NGC 2070 in 30 Doradus
A Study on the Evolution of NGC6822. I. Investigating the Star Formation History through Evolved Stellar Populations
School of Astronomy, Institute for Research in Fundamental Sciences (IPM)（基础科学研究所天文学院）；Astrophysics Group, Lennard-Jones Laboratories, Keele University（基尔大学天体物理组伦纳德-琼斯实验室）；Konkoly Observatory, HUN-REN Research Centre for Astronomy and Earth Sciences（孔科利天文台匈牙利研究与教育网络天文学与地球科学研究中心）
Evidence for Extended Star Formation in the Old, Metal-Rich Open Cluster, NGC 6791?
NGC 6791是一个古老的富含金属的星团，通常被认为是一个盘状疏散星团。它的红巨星分支是广泛的颜色，但迄今为止，没有证据表明它的恒星之间的金属分布。主层序的关闭区域也比宽波段光度误差所预期的要宽。颜色-星等图的分析揭示了集群的核心和外围之间的颜色梯度，我们评估了这种趋势的潜在解释。虽然二进制和光度误差似乎不太可能，但不能排除整个星团表面的变红变化。我们认为，这种颜色趋势的一个可行的替代解释是由于星团形成时间延长而导致的年龄分布; NGC 6791内部区域的恒星似乎比外部区域的恒星平均年龄大1 Gyr。
