A Fermi-LAT Study of Globular Cluster Dynamical Evolution in the Milky Way:Millisecond Pulsars as the Probe

Using archival Fermi-LAT data with a time span of~12 yr,we study the population of Millisecond Pulsars(MSPs)in Globular Clusters(GlCs)and investigate their dependence on cluster dynamical evolution in the Milky Way.We show that theγ-ray luminosity(L_(γ))and emissivity(i.e.,ε_(γ)=L_(γ)/M,with M the cluster mass)are good indicators of the population and abundance of MSPs in GlCs,and they are highly dependent on the dynamical evolution history of the host clusters.Specifically speaking,the dynamically older GlCs with more compact structures are more likely to have larger L_(γ)andε_(γ),and these trends can be summarized as strong correlations with cluster stellar encounter rateΓand the specific encounter rate(Λ=Γ/M),with L_(γ)∝Γ^(0.7±0.11)andε_(γ)∝Λ^(0.73±0.13)for dynamically normal GlCs.However,as GlCs evolve into deep core collapse,these trends are found to be reversed,implying that strong encounters may have lead to the disruption of Low-Mass X-ray Binaries and ejection of MSPs from core-collapsed systems.Besides,the GlCs are found to exhibit largerε_(γ)with increasing stellar mass function slope(ε_(γ)∝10^((0.52±0.1)α)),decreasing tidal radius(ε_(γ)∝R_(t)^(-10±0.22))and distances from the Galactic Center(GC,ε_(γ)∝R_(gc)^(-1.13±0.21)).These correlations indicate that,as GlCs losing kinetic energy and spiral in toward the GC,tidal stripping and mass segregation have a preference in leading to the loss of normal stars from GlCs,while MSPs are more likely to concentrate to cluster center and be deposited into the GC.Moreover,we gaugeε_(γ)of GlCs is~10-1000 times larger than the Galactic bulge,the latter is thought to reside thousands of unresolved MSPs and may be responsible for the GC 7-ray excess,which supports that GlCs are generous contributors to the population of MSPs in the GC.

Variable Stars in the 50BiN Open Cluster Survey.Ⅲ.NGC 884

Open clusters are the basic building blocks that serve as a laboratory for the study of young stellar populations in the Milky Way.Variable stars in open clusters provide a unique way to accurately probe the internal structure,temporal and dynamical evolutionary stages of individual stars and the host cluster.The most powerful tool for such studies is time-domain photometric observations.This paper follows the route of our previous work,concentrating on a photometric search for variable stars in NGC 884.The target cluster is the companion of NGC869,forming the well-known double cluster system that is gravitationally bound.From the observation run in 2016 November,a total of 9247 B-band CCD images and 8218Ⅴ-band CCD images were obtained.We detected a total of 15 stars with variability in visual brightness,including five Be stars,three eclipsing binaries,and seven of unknown types.Two new variable stars were discovered in this work.We also compared the variable star content of NGC 884 with its companion NGC 869.

The Radcliffe Wave Parameters from Data on Open Star Clusters

A spectral analysis of the vertical positions and velocities of 374 open star clusters(OSCs)was carried out.We took these OSCs from the Hunt and Reffert catalog;they have an average age of about 10 million years,and are located on the galactic plane XY in a narrow zone inclined by 25°to the galactic axis Y.The following estimates of the parameters of the Radcliffe wave were obtained:(a)the maximum value in periodic perturbations of vertical coordinates Z_(max)=92±10pc with the wavelength of these perturbations λ_(z)=4.82±0.09 kpc;(b)the maximum value of the velocity of vertical disturbances Wmax=4.36±0.12km s^(-1) with disturbance wavelengthλ_(W)=1.78±0.02 kpc.Note that the results of the vertical velocity analysis are first-class in accuracy and completely new.

A Possible γ-Ray Pulsation from PSR J1740-5340B in the Globular Cluster NGC 6397

Recently, a new radio millisecond pulsar(MSP) J1740-5340B, hosted in the globular cluster(GC) NGC 6397,was reported with a 5.78 ms spin period in an eclipsing binary system with a 1.97 days orbital period. Based on a modified radio ephemeris updated by tool tempo2, we analyze the ~15 yr γ-ray data obtained from the Large Area Telescope on board the Fermi Gamma-ray Space Telescope and detect PSR J1740-5340B's γ-ray pulsation at a confidence level of ~4σ with a weighted H-test value of ~26. By performing a phase-resolved analysis, the γ-ray luminosity in on-pulse interval of PSR J1740-5340B is L_(γ)~ 3.8 × 10^(33) erg s^(-1) using NGC 6397's distance of 2.48 kpc. And γ-rays from the on-pulse part of PSR J1740-5340B contribute ~90% of the total observed γ-ray emissions from NGC 6397. No significant γ-ray pulsation of another MSP J1740-5340A in the GC is detected.Considering that the previous four cases of MSPs in GCs, more data in γ-ray, X-ray, and radio are encouraged to finally confirm the γ-ray emissions from MSP J1740-5340B, especially starving for a precise ephemeris.

Orbits of 152 globular clusters of the Milky Way galaxy constructed from Gaia DR2

We present orbits and their properties for 152 globular clusters of the Milky Way galaxy obtained using average Gaia DR2 proper motions and other astrometric data from the list of Vasiliev. For orbital integration we have applied the axisymmetric model of the Galactic potential based on the Navarro-Frenk-White dark halo, and modified by Bajkova & Bobylev utilizing circular velocities of Galactic objects in a wide region of Galactocentric distances(up to 200 kpc) from the Bhattacharjee et al. catalog. Based on the analysis of the obtained orbits, we have modified the composition of the subsystems of globular clusters presented in Massari et al..

Determination of Distance,Extinction,Mass,and Age for Stars in LAMOST DR7

Large scale spectroscopic surveys such as that using Large-sky Area Multi-Object Fiber Spectroscopic Telescope(LAMOST) have collected spectra of millions stars in the Milky Way.Utilizing this huge sample of stars to study the assembling history and structure of our Galaxy requires accurate estimates of distance,extinction,age,and mass for individual stars.Combining the parallax constraint from Gaia EDR3 with Bayesian inference,we have estimated the distance and extinction for stars observed in LAMOST DR7,as well as the stellar mass and age for evolved stars in this data release.We validated the accuracies of the stellar parameters by comparing our results against various measurements,including the star-pair technique,asteroseismology,globular clusters,and isochrone fits to main sequence stars and subgiants.This is a valuable catalog of stellar parameters under a Bayesian framework estimated using the data from Gaia EDR3 and LAMOST spectroscopic data.With this data set we explored the stellar population of the Galactic massive substructure Gaia-Sausage-Enceladus(GSE).The kinematically selected members of GSE have a median metallicity of [Fe/H] =-1.29 and a median age of 11.6 Gyr.

Classifying Globular Clusters and Applying them to Estimate the mass of the Milky Way

We combine the kinematics of 159 globular clusters(GCs) provided by the Gaia Early Data Release 3(EDR3) with other observational data to classify the GCs,and to estimate the mass of the Milky Way(MW).We use the agemetallicity relation,integrals of motion,action space and the GC orbits to identify the GCs as either formed in situ(Bulge and Disk) or ex situ(via accretion).We find that 45.3% have formed in situ,while 38.4% may be related to known merger events:Gaia-Sausage-Enceladus,the Sagittarius dwarf galaxy,the Helmi streams,the Sequoia galaxy and the Kraken galaxy.We also further identify three new sub-structures associated with the Gaia-Sausage-Enceladus.The remaining 16.3% of GCs are unrelated to the known mergers and thought to be from small accretion events.We select 46 GCs which have radii 8.0 <r<37.3 kpc and obtain the anisotropy parameter β=0.315_(-0.049)^(+0.055),which is lower than the recent result using the sample of GCs in Gaia Data Release 2,but still in agreement with it by considering the error bar.By using the same sample,we obtain the MW mass inside the outermost GC as M(<37.3 kpc)=0.423_(-0.02)^(+0.02)×10^(12)M_(⊙),and the corresponding M_(200)=1.11_(-0.18)^(+0.25)×10^(12)M_(⊙).The estimated mass is consistent with the results in many recent studies.We also find that the estimated β and mass depend on the selected sample of GCs.However,it is difficult to determine whether a GC fully traces the potential of the MW.

Variable Star Detection in the Field of Open Cluster NGC 188

This work presents the charge-coupled device(CCD)photometric survey of the old open cluster NGC 188.Timeseries V-band photometric observations were conducted for ten nights in 2017 January using the Nanshan Onemeter Wide-field Telescope to search for variable stars in the field of the cluster.A total of 25 variable stars,including one new variable star,were detected in the target field.Among the detected variables,16 are cluster member stars,and the others are identified as field stars.The periods,radial velocities,effective temperatures,and classifications of the detected variables are discussed in this work.Most of the stars’effective temperatures are between 4200 and 6600 K,indicating their spectral types are G or K.The newly discovered variable is probably a W UMa system.In this study,a known cluster variable star(V21=V0769 Cep)is classified as an EA-type variable star based on the presence of an 0.5 mag eclipse in its light curve.

The Analyses of Globular Cluster Pulsars and Their Detection Efficiency

Up to 2022 November,267 pulsars had been discovered in 36 globular clusters(GCs).In this paper,we present our studies on the distribution of GC pulsar parameters and the detection efficiency.The power law relation between average dispersion measure(■)and dispersion measure difference(ΔDM)of known pulsars in GCs is lgΔDM∝1.52lg■.The sensitivity could be the key to finding more pulsars.As a result,several years after the construction of a large radio telescope facility,the number of known GC pulsars will likely be increased accordingly.We suggest that currently GCs in the southern hemisphere could have higher possibilities for finding new pulsars.

Fractions of Compact Object Binaries in Star Clusters:Theoretical Predictions

The binary population in field stars and star clusters contributes to the formation of gravitational wave(GW)sources.However,the fraction of compact-object binaries(CBs),which is an important feature parameter of binary populations,is still difficult to measure and very uncertain.This paper predicts the fractions of important CBs and semi-compact object binaries(SCBs) making use of an advanced stellar population synthesis technique.A comparison with the result of N-body simulation is also presented.It is found that most CBs are formed within about 500 Myr after the starburst.The fractions of CBs and SCBs are demonstrated to correlate with stellar metallicity.The higher the metallicity becomes,the smaller the fraction of black hole binaries(BHBs),neutron star binaries(NSBs) and SCBs.This suggests that the GW sources of BHBs and NSBs are more likely to form in metal-poor environments.However,the fraction of black hole-neutron star binaries is shown to be larger for metalrich populations on average.

Unveiling the Initial Conditions of Open Star Cluster Formation

Open clusters(OCs)are infrequent survivors of embedded clusters gestated in molecular clouds.Up to now,little is known about the initial conditions for the formation of OCs.Here,we studied this issue using high-precision astrometric parameters provided by Gaia data release 3.The statistics show that the peculiar motion velocities of OCs vary slightly from infancy to old age,providing a remarkable opportunity to use OCs to trace their progenitors.Adopting a dynamical method,we derived the masses of the progenitor clumps where OCs were born,which have statistical characteristics comparable to previously known results for clumps observed in the Galaxy.Moreover,the masses of the progenitor clumps of OCs indicate they should be capable of gestating massive O-type stars.In fact,after inspecting the observed OCs and O-type stars,we found that there are many O-type stars in OCs.The destructive stellar feedback from O-type stars may disintegrate the vast majority of embedded clusters,and only those sufficiently dense ones can survive as OCs.

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.

On the metallicity gradient in the Galactic disk

The problem of the chemical composition gradient in the Galactic disk is studied based on a sample of metallicity estimates of open star clusters,using Gaia DR2-improved distance estimates.A clearly non-monotonic variation was observed in the average metallicity of clusters with increasing Galactocentric distance.One can clearly see the metallicity jump of 0.22 in[Fe/H]at a Galactocentric distance of about9.5 kpc,which appears to be linked to the outer boundary of the thinnest and youngest component of the Galactic disk.The absence of a significant metallicity gradient in the internal(R<9 kpc)and external(R>10 kpc)regions of the disk demonstrates the absence of noticeable metal enrichment at times of the order of the ages corresponding to those of the disk regions under consideration.Observational data show that the disk experiences noticeable metal enrichment only during the starburst epochs.No significant dependence was identified between the average metallicity and the age of the clusters.

The simplest way to get a cluster's parameters in the Gaia era(Dolidze 41)

The astro-photometric parameters of the open star cluster Dolidze 41, which is located in the constellation Cygnus, have been investigated using the Gaia DR2 large survey that is merged with the near infrared Two Micron All Sky Survey(2 MASS) database.The radial density distribution(limited,core and tidal radii), color-magnitude diagrams, galactocentric coordinates, distances, color excess and age of Dolidze 41 are presented.The Gaia DR2 astrometry helped us to define the membership of the cluster stars easily.The luminosity and mass functions, the entire luminosity and mass, and the relaxation time of the cluster have been estimated as well.

Discovery of Extended Structure Around Open Cluster COIN-Gaia 13 Based on Gaia EDR3

COIN-Gaia 13 is a newly discovered open cluster revealed by Gaia DR2 data.It is a nearby open cluster with a distance of about 513 pc.Combined with the five-dimensional astrometric data of Gaia EDR3 with higher accuracy,we use the membership assignment algorithm(pyUPMASK)to determine the membership of COINGaia 13 in a large extended spatial region.The cluster has 478 identified candidate members.After obtaining reliable cluster members,we further study its basic properties and spatial distribution.Our results show that there is an obvious extended structure of the cluster in the X-Y plane.This elongated structure is distributed along the spiral arm,and the whole length is about 270 pc.The cluster age is 250 Myr,the total mass is about 439 M_(⊙) and the tidal radius of the cluster is about 11 pc.Since more than half of member stars(352 stars)are located outside twice the tidal radius,it is suspected that this cluster is undergoing the dynamic dissolution process.Furthermore,the spatial distribution and kinematic analysis indicate that the extended structure in COIN-Gaia 13 is more likely to be caused by differential rotation of the Galaxy.

A Search for Exoplanets in Open Clusters and Young Associations based on TESS Objects of Interest

We report the results of our search for planet candidates in open clusters and young stellar associations based on the Transiting Exoplanet Survey Satellite(TESS) Objects of Interest Catalog.We find one confirmed planet,one promising candidate,one brown dwarf and three unverified planet candidates in a sample of 1229 open clusters from the second Gaia data release.We discuss individual planet-star systems based on their basic parameters,membership probability and the observation notes from the ExoFOP-TESS website.We also find ten planet candidates(P> 95%) in young stellar associations by using the BANYAN ∑ Multivariate Bayesian Algorithm.Among the ten candidates,five are known planetary systems.We estimate the rotation periods of the host stars using the TESS light curves and estimate their ages based on gyrochronology.Two candidates with periodic variations are likely to be young planets,but their exact memberships to young stellar associations remain unknown.

An upgraded interpolator of the radial basis function network for spectral calculation based on empirical stellar spectral library

Stellar population synthesis is an important method in galaxy and star-cluster studies.In stellar population synthesis models,a stellar spectral library is necessary for the integrated spectra of the stellar population.Usually,the stellar spectral library is utilized for the transformation between the stellar atmospheric parameters and stellar spectrum.The empirical stellar spectral library has distinct advantages over the theoretical library.However,for the empirical spectral library,the distribution of stars is irregular in the stellar atmospheric parameter space,which makes the traditional interpolator difficult to get accurate results.In this work,we will provide an improved radial basis function interpolator which is implemented to obtain the interpolated stellar spectra based on the empirical stellar spectral library.For this interpolator,we use the relation between the standard varianceσin the Gaussian radial basis function and the density distribution of stars in the stellar atmospheric parameter space to give the prior constraint on thisσ.Moreover,we also consider the anisotropic radial basis function by the advantage of the local dispersion of stars in the stellar atmospheric parameter space.Furthermore,we use the empirical stellar spectral library MILES to test this interpolator.On the whole,the interpolator has a good performance except for the edge of the low-temperature region.At last,we compare this interpolator with our work in 2018,and the interpolation result shows an obvious improvement.Users can apply this interpolator to retrieve the interpolated spectra based on the stellar spectral library quickly and easily.

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.).

Detection of the Farthest Globular Cluster NGC 6715 and Two Other GCs in Gamma-Rays with Fermi-LAT

In this paper,~12 yr long-term Pass 8 data from Fermi Large Area Telescope for the 157 globular clusters are carefully re-analyzed.Besides the 31 globular clusters reported in the fourth Fermi Large Area Telescope catalog Data Release 2,NGC 1851 is identified as a gamma-ray emitter and the significant gamma-ray emissions from NGC 6715 and NGC 6723 are detected.Especially NGC 6715 is located at a distance of 26.8 kpc,so far it is the farthest globular cluster detected in gamma-rays.A detailed analysis for these three globular clusters has been performed,but their gamma-ray pulsation emissions or flux variabilities are not found.The numbers of millisecond pulsars(MSPs)in these globular clusters are estimated under the assumption that each MSP inside globular clusters emits a similar amount of gamma-rays.Some possible origins of gamma-ray emission from globular clusters,such as MSPs,pulsar binary systems and/or dark matter,are discussed.

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.