Powerful CMD:a tool for color-magnitude diagram studies

We present a new tool for color-magnitude diagram（CMD） studies, Powerful CMD. This tool is built based on the advanced stellar population synthesis（ASPS） model, in which single stars, binary stars, rotating stars and star formation history have been taken into account. Via Powerful CMD, the distance modulus, color excess, metallicity, age, binary fraction, rotating star fraction and star formation history of star clusters can be determined simultaneously from observed CMDs. The new tool is tested via both simulated and real star clusters. Five parameters of clusters NGC 6362, NGC 6652, NGC 6838 and M67 are determined and compared to other works. It is shown that this tool is useful for CMD studies, in particular for those utilizing data from the Hubble Space Telescope（HST）. Moreover, we find that inclusion of binaries in theoretical stellar population models may lead to smaller color excess compared to the case of single-star population models.

The Study of Helium Variations in Star Clusters Using China Space Station Telescope

The China Space Station Telescope(CSST)is a 2 m space-based optical-UV telescope.Its primary goal is to carry out a high-resolution photometric imaging survey of a 17,500 square degree sky area using the on board Survey Camera.With its wide field of view(1.1 square degrees)and a mosaic imager containing 640 million pixels,studying the different populations of stars within star clusters is highly feasible.The aim of this study is to assess the CSST's ability to distinguish between stellar populations with varying helium abundance levels,with the help of Modules for Experiments in Stellar Astrophysics.The results of the CSST's photometry for these different populations are presented by transferring the models into the CSST Survey Camera photometric system.The findings confirm that helium-enriched stellar populations will have unique patterns in the color-magnitude diagrams under the CSST photometric system,compared to normal stellar populations.The CSST,with its filters and wide field of view of the Survey Camera,provides a new avenue for the study of multiple populations in star clusters.

Membership determination of open cluster NGC 188 based on the DBSCAN clustering algorithm

High-precision proper motions and radial velocities of 1046 stars are used to determine member stars using three-dimensional （3D） kinematics for open clus- ter NGC 188 based on the density-based spatial clustering of applications with noise （DBSCAN） clustering algorithm. By implementing this algorithm, 472 member stars in the cluster are obtained with 3D kinematics. The color-magnitude diagram （CMD） of the 472 member stars using 3D kinematics shows a well-defined main sequence and a red giant branch, which indicate that the DBSCAN clustering algorithm is very effective for membership determination. The DBSCAN clustering algorithm can ef- fectively select probable member stars in 3D kinematic space without any assumption about the distribution of the cluster or field stars. Analysis results show that the CMD of member stars is significantly clearer than the one based on 2D kinematics, which al- lows us to better constrain the cluster members and estimate their physical parameters. Using the 472 member stars, the average absolute proper motion and radial velocity are determined to be （PMα, PMδ） = （-2.58 ± 0.22, ＋0.17 ± 0.18） mas yr-1 and Vr = -42.35 ± 0.05 km s-1, respectively. Our values are in good agreement with values derived by other authors.

Evolutionary status of T Tauri stars

The problem of determining the masses and ages of T Tauri star （TTS） using their evolu- tionary status is discussed. We test four pre-main sequence evolutionary models using well determined observational parameters of 12 binary TTSs and two binary red dwarfs. It is shown that the masses derived using the tracks of all models are in good agreement with flhe masses obtained from the obser- vations of TTSs with masses M 〉 0.7 Me （mean error ε,- 10%）. Low-mass stars with M ≤ 0.7 M⊙ have significantly greater mean error; e - 50% for the tracks of Bressan et al. and Chen et al., and e - 30% for the other tracks. The isochrones of all tested evolutionary models diverge for stars with masses M ≤ 0.7 M⊙. The difference increases with the mass decrease and can reach 10% of Kelvin- Helmholtz time for stars with mass M = 0.2 Me. The ages of most of the considered TTSs are smaller than the Kelvin-Helmholtz time. This confirms their evolutionary status of being pre-main sequence stars.

New open clusters found by manual mining of data based on Gaia DR2

The physical nature of a series of 20 new open clusters is confirmed employing existing data on putative star members,mainly from the second Gaia Data Release(DR2).The clusters were discovered as overdensities of stars by visual inspection of either photographic DSS plates or proper motion plots of random source fields.The reported objects are not present in the most comprehensive or recent catalogs of stellar clusters and associations.For all of them,clumps of comoving stars are revealed in the proper motion space.The parallaxes of the clumped stars are compatible with the real existence of open clusters over narrow ranges of distances.Surface density calculations,free of most noise from non-member sources,allow differentiating a cluster core and an extended cluster corona in some instances.Color-magnitude diagrams generally show a definite main sequence that allows confirmation of the physical existence of the clusters and some of their characteristics.Two of the new clusters seem to form a double system with a common origin.Several of the new clusters challenge the claim of near completeness of the known OC population in the distance range from 1.0 to 1.8 kpc from the Sun(Kharchenko et al.).

Evaluating Helium Variations By Modeling Red Giant Branch Bump of Large Magellanic Cluster NGC 1978

Many evidences show that the Multiple Population(MP)features exist not only in old clusters but also in intermediate-age clusters in the Megallanic Clouds(MCs),which are characterized by star-to-star abundance scatter of several elements,including helium(He).The red giant branch bump(RGBB)’s photometric properties are proved to be related to the variation in helium abundances of the member stars in star clusters.We use the"Modules for Experiments in Stellar Astrophysics"(MESA)stellar evolution code to calculate the evolution sequences of stars along the red giant branch(RGB)with changing helium content.Following the RGB sequences,we then generate a luminosity function of the RGB stars within the grid of input helium abundances,which are compared with the observational data of an intermediate-age MC cluster NGC 1978.

Effects of Binaries on Open Cluster Age Determination in Bayesian Inference

We investigate the effects of binaries on the cluster age determination for 561 open clusters in the Galactic disk via the Bayesian statistical framework.Stellar properties of these star clusters,including age,metallicity,distance modulus,color excess,binary fraction,and rotating star fraction,are derived from color–magnitude-diagrams(CMDs)via isochrone fitting to high-precision Gaia EDR3 data.Across the simple stellar population of binary and single-star,age differences can be found with the same star cluster.A Bayesian applied regression modeling software,Stan,is employed to explore how much binaries affect the age determination of open clusters.Our results present less statistically significant difference between the binary-star simple populations(bs SSPs)and the singlestar simple populations(ss SSPs)for cluster age determination.For all clusters in our sample,the ages estimated using the bs SSPs models are younger than those estimated using the ss SSPs,with a mean value of~70 Myr.However,it is found that for 52.5%clusters in our sample,ages are relatively sensitive to the presence of binaries,at least~25%younger.This suggests that in studies of open cluster age determinations,the effects of binary interactions on the whole sample are not prominent,but its effects on some clusters should still be included as an essential ingredient.

3D cluster members and near-infrared distance of open cluster NGC 6819

In order to obtain clean members of the open cluster NGC 6819, the proper motions and radial velocities of 1691 stars are used to construct a three-dimensional （3D） velocity space. Based on the DBSCAN clustering algorithm, 537 3D cluster members are obtained. From the 537 3D cluster members, the average radial velocity and absolute proper motion of the cluster are Vr = ＋2.30 ±0.04 km s-1 and （PMRA, PMDec） = （-2.5 ±0.5, -4.3 ± 0.5） mas yr-1, respectively. The proper motions, radial velocities, spatial positions and color-magnitude diagram of the 537 3D members indicate that our membership determination is effective. Among the 537 3D cluster members, 15 red clump giants can be easily identified by eye and are used as reliable standard candles for the distance estimate of the cluster. The distance modulus of the cluster is determined to be （m - M）0 -- 11.86 ± 0.05 mag （2355 ±54 pc）, which is quite consistent with published values. The uncertainty of our distance mod- ulus is dominated by the intrinsic dispersion in the luminosities of red clump giants （--0.04 mag）.

Discovery of high-velocity EHB stars in the globular cluster ωCentauri(NGC 5139)

Abstract We report the discovery of 45 high-velocity extreme horizontal branch （EHB） stars in the globular cluster Omega Centauri （NGC 5139）. The tangential ve- locities of these EHB stars are determined to be in the range 93-313 km s^-1, with an average uncertainty of -27 km s^-1. The central escape velocity of the cluster is determined to be in the range 60~105 km s^-1. These EHB stars are significantly more concentrated toward the cluster core compared with other cluster members. The formation mechanisms of these EHB stars are discussed. Our conclusions can be sum- marized as follows： （1） A comparison of the tangential velocities of these EHB stars to the central escape velocity of the cluster shows that most if not all of these EHB stars are unbound to the cluster; （2） These EHB stars obtained high velocities in the central cluster region no longer than - 1 Myr ago and may be subsequently ejected from the cluster in the next -1 Myr; （3） If the progenitors of these EHB stars were single stars, then they may have experienced a fast mass-loss process. If the progen- itors were in close binaries, then they may have formed through disruptions by the intermediate-mass black hole in the cluster center.