Studying young stellar populations in G345.5+1.5 molecular cloud

Using different sky surveys, we studied IRAS sources and embedded clusters located in the G345.5+1.5 region. Our analysis comprises multi-wavelength (optical to radio) data, exploited using various methods: photometry, near-infrared spectroscopy for ten stars in the embedded cluster DBS-114, and astrometry. We estimated the main parameters of the embedded stellar populations in the G345.5+1.5 molecular cloud, such as their extent, reddening, age, and mass. We also found a consistent distance value using different approaches. For each studied population, we classified several point objects as early main sequence stars, stars with infrared excess, and class I/II YSOs. For the particular case of DBS 114, our spectral classification revealed four B-type stars, and we used astrometric information from GAIA EDR3. The combination of optical and infrared information revealed an abnormal reddening law in some embedded clusters. Our analysis favored a scenario with a Lyman continuum emission excess at some of the studied stellar populations.     

New open cluster candidates discovered in the XSTPS-GAC survey

The Xuyi Schmidt Telescope Photometric Survey of the Galactic Anti-center(XSTPS-GAC)is a photometric sky survey that covers nearly 6000 deg~2 towards the Galactic Anti-center(GAC) in the g,r,i bands. Half of its survey field is located on the Galactic Anti-center disk,which makes XSTPSGAC highly suitable to search for new open clusters in the GAC region. In this paper,we report new open cluster candidates discovered in this survey,as well as properties of these open cluster candidates,such as age,distance and reddening,derived by isochrone fitting in the color-magnitude diagram(CMD).These open cluster candidates are stellar density peaks detected in the star density maps by applying the method from Koposov et al. Each candidate is inspected in terms of its true color image composed from three XSTPS-GAC band images. Then its CMD is checked,in order to identify whether the central region stars have a clear isochrone-like trend differing from background stars. The parameters derived from isochrone fitting for these candidates are mainly based on three band photometry of XSTPS-GAC.Moreover,when these new candidates are able to be seen clearly in 2 MASS data,their parameters are also derived based on the 2 MASS(J-H,J) CMD. There are a total of 320 known open clusters rediscovered and 24 new open cluster candidates discovered in this work. Furthermore,the parameters of these new candidates,as well as another 11 previously known open clusters,are properly determined for the first time.     

Stellar contents of two young open clusters: NGC 663 and 654

UBVRI CCD photometry in a wide field around two young open clusters, NGC 663 and 654, has been carried out. Hα and polarimetric observations for the cluster NGC 654 have also been obtained. We use the photometric data to construct colour–colour and colour–magnitude diagrams, from which we can investigate the reddening, age, mass and evolutionary states of the stellar contents of the these clusters. The reddening across the cluster regions is found to be variable. There is evidence for anomalous reddening law in both clusters; however, more infrared and polarimetric data are needed to conclude about the reddening law. Both clusters are situated at about a distance of 2.4 kpc. Star formation in both clusters is found to be a continuous process. In the case of NGC 663, star formation seems to have taken place sequentially, in the sense that formation of low-mass stars precedes the formation of most massive stars. Whereas, in the case of NGC 654, formation of low-mass stars did not cease after the formation of most massive stars in the cluster.     

The OPD photometric survey of open clusters II. robust determination of the fundamental parameters of 24 open clusters

In the second paper of the series we continue the investigation of open cluster fundamental parameters using a robust global optimization method to fit model isochrones to photometric data. We present optical UBVRI CCD photometry (Johnsons-Cousins system) observations for 24 neglected open clusters, of which 14 have high quality data in the visible obtained for the first time, as a part of our ongoing survey being carried out in the 0.6 m telescope of the Pico dos Dias Observatory in Brazil. All objects were then analyzed with a global optimization tool developed by our group which estimates the membership likelihood of the observed stars and fits an isochrone from which a distance, age, reddening, total to selective extinction ratio R_V (included in this work as a new free parameter) and metallicity are estimated. Based on those estimates and their associated errors we analyzed the status of each object as real clusters or not, finding that two are likely to be asterisms. We also identify important discrepancies between our results and previous ones obtained in the literature which were determined using 2MASS photometry.     

The astrophysical parameters of Majaess-95 and Majaess-225

Most astrophysical parameters of the two stellar clusters Majaess 95 and Majaess 225 have been investigated here for the first time using the third data release of the Gaia space mission (Gaia-DR3). The membership probabilities of stars were assigned using the pyUPMASK algorithm. The parallaxes offset has been corrected using the zero-point correction code of Lindegren et al. (Lindegren et al. 2021, A&A, 649, A4). Using the clusters' members with membership probabilities >0.50, we calculated the distance to the clusters of 3311 and 3020 (±130) pc, respectively, which agree with the results from our isochrone fitting on the color-magnitude diagram. The ages are found to be 282 (±20) and 12.6 (±2) Ma. Also, the reddening, heliocentric distances, luminosity function, mass function, and total mass were estimated. On studying the dynamic state (relaxation time), we found that Majaess-95 is a relaxed cluster, while Majaess-225 is not relaxed yet.     

A robust statistical estimation of the basic parameters of single stellar populations – I. Method

The colour–magnitude diagrams of resolved single stellar populations, such as open and globular clusters, have provided the best natural laboratories to test stellar evolution theory. Whilst a variety of techniques have been used to infer the basic properties of these simple populations, systematic uncertainties arise from the purely geometrical degeneracy produced by the similar shape of isochrones of different ages and metallicities. Here we present an objective and robust statistical technique which lifts this degeneracy to a great extent through the use of a key observable: the number of stars along the isochrone. Through extensive Monte Carlo simulations we show that, for instance, we can infer the four main parameters (age, metallicity, distance and reddening) in an objective way, along with robust confidence intervals and their full covariance matrix. We show that systematic uncertainties due to field contamination, unresolved binaries, initial or present-day stellar mass function are either negligible or well under control. This technique provides, for the first time, a proper way to infer with unprecedented accuracy the fundamental properties of simple stellar populations, in an easy-to-implement algorithm.     

A polarimetric study of the B[e] star HD 45677

We aim to characterize the current status of knowledge on the accuracy of open-cluster parameters such as the age, reddening and distance. These astrophysical quantities are often used to study the global characteristics of the Milky Way down to the very local stellar phenomena. In general, the errors of these quantities are neglected or set to some kind of heuristic standard value. We attempt to give some realistic estimates for the accuracy of available cluster parameters by using the independently derived values published in the literature. In total, 6437 individual estimates for 395 open clusters were used in our statistical analysis. We discuss the error sources depending on theoretical as well as observational methods and compare our results with those parameters listed in the widely used catalogue by Dias et al. In addition, we establish a list of 72 open clusters with the most accurate known parameters which should serve as a standard table in the future for testing isochrones and stellar models.     

Absolute parameters of the Algol binary Z Vul

Spectra in the wavelength range 4230–9150 Å and the first light curves of Z Vul in the infrared J, H and K bands are presented. Our infrared JHK light curves and radial velocities, together with published radial velocity and UBV data are analyzed in order to determine a new set of stellar parameters. This allows us to determine new absolute parameters of the stellar components, the interstellar reddening and the distance to the system. We discuss the rotation of the primary star, finding that must be rotating faster than synchronous. From the visual–infrared photometry we find no evidence of IR excess in the system.     

Analyses of Wolf-Rayet ultraviolet spectra

Ultraviolet spectra of population I WR stars obtained from IUE archive are used to determine fundamental stellar parameters. Terminal velocities for 85 galactic and LMC Wolf-Rayet stars were obtained by means of the empirical relation between spectral quantities easily measured in low resolution and high-resolution terminal velocity measurements. Temperatures and so-called transformed radii were derived based on available contour plots of spectral characteristics for a grid of NLTE models. The effect of the reddening law on stellar far ultraviolet continua is emphasized and the revised extinction curve towards WR stars is used for dereddening. For the sample of stars attributed to open clusters or associations we construct the stellar distance scale and adopt it for the other WR stars. The remaining fundamental parameters are derived and HR diagram for population I WR stars is presented.     

关于LAMOST天体测量标准天区选择的建议

简述已有几个天体测量标准天区的大小、星数以及恒星位置和自行的精度；给出LAMOST天体测量标准天区选择的原则；介绍研究疏散星团和古德带附近星团观测的意义；列出13个LAMOST天体测量标准天区J2000．0历元的赤经和赤纬、银经和银纬，如果疏散星团位于这个标准天区，也给出它们的日心距、红化值、金属丰度和年龄。     

Spectral indices in quantitative spectral classification from stellar libraries

With the use of modern detectors stellar spectral classification libraries have been extended from the photographic regime to the near ℝ at 11000 Å. We have defined new spectral indices within this extended wavelength-range that can be used to determine the luminosity classification for G-K-M stars. An advantage of the new indices, which sample the stellar flux in and out of selected spectral features, is that they are insensitive to catalog differences. This facilitates the use of many catalogs, with varying resolution, different reddening corrections, and calibrations, hence extending the total number of stellar standards available. Furthermore, we demonstrate that the indices can be used to infer absolute magnitudes with good accuracy. The indices should prove useful for analysis of spectra from distant clusters, galaxies, and in particular for problems involving spectral synthesis of stellar populations of galaxies.     

The open cluster Berkeley 53

We present a photometric study of the neglected open cluster Berkeley 53. We derived its fundamental parameters, such as the age, the interstellar reddening, and the distance from the Sun, based on BV photometry combined with near‐infrared JHK_S data. The structure and the mass function of the cluster were also studied and the total number of members and the total mass were estimated. The cluster was found to be a rich and massive stellar system, located in the Perseus Arm of the Milky Way, 3.1 ± 0.1 kpc from the Sun. Its age exceeds 1 Gy but it seems tobe very young in the context of its dynamical evolution. The analysis of the two‐color diagrams and color‐magnitude diagrams indicates that the cluster is significantly reddened. However, both methods resulted in different values of E (B – V), i.e. 1.21 ± 0.04 and 1.52 ± 0.01, respectively. This discrepancy suggests the presence of an abnormal interstellar extinction law toward the cluster (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Tracking Down the Processes that Shape the ISM: The Case of the Supergiant Shell in IC 2574

We present new X-ray data (obtained with the Chandra telescope) as well as FUV data of the supergiant shell (SGS) in IC 2574, a dwarf galaxy in the M 81 group. This region has proven to be a unique target for studying the interplay of massive star formation with the surrounding interstellar medium. The Chandra data confirm the presence of soft X-ray emission, indicative of the presence of hot gas. Using optical and FUV data, we estimate the age and the energy input of the stellar cluster inside the SGS. The age estimate agrees well with the age estimated based on the HI data alone – providing further supportfor the view that the stellar cluster interior the SGS is powering its expansion. We conclude that indeed massive stellar clusters can create supergiant shells in galaxies (even at large galactocentric distance) as predicted by the `standard' picture (creation by SN explosions and strong stellar winds), a scenario which has recently been questioned by some authors.     

Single-binary star separation by ultraviolet color index diagrams

Detection of a composite flux in photometry can serve as an indication of a photometrically unresolved binarity and can contribute to the parameterization of the components of binary systems. A main goal of the present study is to develop a method of automatic photometric detection of binaries, based on multi-color photometry, theoretical stellar spectral energy distributions and general understanding of binary evolution. In particular, we consider an ultraviolet photometry where, in combination with optical and infrared photometry, interstellar reddening can be easier distinguished from temperature reddening.     

The embedded cluster DBSB 48 in the nebula Hoffleit 18: Comparison with Trumpler 14

We derive fundamental parameters of the embedded cluster DBSB 48 in the southern nebula Hoffleit 18 and the very young open cluster Trumpler 14, by means of deep JHK_s infrared photometry. We build colour–magnitude and colour–colour diagrams to derive reddening and age, based on main sequence and pre-main sequence distributions. Radial stellar density profiles are used to study cluster structure and guide photometric diagram extractions. Field-star decontamination is applied to uncover the intrinsic cluster sequences in the diagrams. Ages are inferred from K-excess fractions. A prominent pre-main sequence population is present in DBSB 48, and the K-excess fraction f_K = 55 ± 6% gives an age of 1.1 ± 0.5 Myr. A mean reddening of A_Ks=0.9±0.03 was found, corresponding to A_V = 8.2 ± 0.3. The cluster CMD is consistent with the far kinematic distance of 5 kpc for Hoffleit 18. For Trumpler 14 we derived similar parameters as in previous studies in the optical, in particular an age of 1.7 ± 0.7 Myr. The fraction of stars with infrared excess in Trumpler 14 is f_K = 28 ± 4%. Despite the young ages, both clusters are described by a King profile with core radii R_core = 0.46 ± 0.05 pc and R_core = 0.35 ± 0.04 pc, respectively, for DBSB 48 and Trumpler 14. Such cores are smaller than those of typical open clusters. Small cores are probably related to the cluster formation and/or parent molecular cloud fragmentation. In DBSB 48, the magnitude extent of the upper main sequence is ΔK_s ≈ 2 mag, while in Trumpler 14 it is ΔK_s ≈ 5 mag, consistent with the estimated ages.     

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.).     

Reddening, colour and metallicity of the M31 globular cluster system

Using metallicities from the literature, combined with the Revised Bologna Catalogue of photometric data for M31 clusters and cluster candidates [the latter of which is the most comprehensive catalogue of M31 clusters currently available, including 337 confirmed globular clusters (GCs) and 688 GC candidates], we determine 443 reddening values and intrinsic colours, and 209 metallicities for individual clusters without spectroscopic observations. This, the largest sample of M31 GCs presently available, is then used to analyse the metallicity distribution of M31 GCs, which is bimodal with peaks at [Fe/H]≈−1.7 and −0.7 dex. An exploration of metallicities as a function of radius from the M31 centre shows a metallicity gradient for the metal-poor GCs, but no such gradient for the metal-rich GCs. Our results show that the metal-rich clusters appear as a centrally concentrated spatial distribution; however, the metal-poor clusters tend to be less spatially concentrated. There is no correlation between luminosity and metallicity among the M31 sample clusters, which indicates that self-enrichment is indeed unimportant for cluster formation in M31.
The reddening distribution shows that slightly more than half of the GCs are affected by a reddening of E ( B − V ) ≲ 0.2 mag; the mean reddening value is   E ( B − V ) = 0.28^+0.23_−0.14 mag  . The spatial distribution of the reddening values indicates that the reddening on the north-western side of the M31 disc is more significant than that on the south-eastern side, which is consistent with the conclusion that the north-western side is nearer to us.     

UBV(RI) C photometry and spectroscopy of the young open cluster Haffner 18

UBV(RI) _C CCD photometry of a 2.1 × 3.3 arcmin² field centred on the young open cluster Haffner 18 is presented and discussed. Spectroscopic classification of seven stars is also provided. 44 cluster members are identified, the earliest type being O6. The distance to the cluster is found to be 6.3 kpc, corresponding to a galactocentric distance of 12.7 kpc (for a Sun galactocentric distance of 8.5 kpc). An excellent fit to the observed main sequence is achieved by a solar composition isochrone of 2 × 10⁶ yr reddened by E(B − V)  = 0.62 mag. Differential reddening of intracluster origin is present. Pre-main-sequence members are likely to be present over the 6 mag range explored by our observations (reaching down to earliest A spectral types). The presence of differential reddening and pre-main-sequence members agrees with the evidence for a bright parent nebulosity embedding the whole cluster. The radial velocity of the cluster is consistent with the Hron model of Galactic rotation.     

蓝致密星系谱的星族合成

利用星团谱样本的星族合成方法,分析了四个蓝致密星系（ＢｌｕｅＣｏｍｐａｃｔＧａｌａｘｉｅｓ,ＢＣＧ）的光学谱．给出了在各个ＢＣＧ中,年龄和金属丰度不同的星族对星系５８７ｎｍ处连续谱流量贡献的比率,同时还给出ＢＣＧ内部恒星演化的历史和星系内部的红化值．通过对星系内部星族及光谱的分析,解决了ＢＣＧ研究中存在的这类星系年龄和其内部光学不可见物质等问题．结果表明ＢＣＧ是年老星系,其内部恒星形成过程为长时间的宁静相上叠加有短暂且剧烈恒星形成的爆发,其内部存在光学不可见物质,且至少有一部分为年老的球状星团;根据由光学连续谱和发射线计算星系的内红化值的不同,得出ＢＣＧ星系内部尘埃对连续谱和发射线的产生区遮挡程度不同．     

A 'super' star cluster grown old: the most massive star cluster in the Local Group

We independently redetermine the reddening and age of the globular cluster (GC) 037−B327 in M31 by comparing independently obtained multicolour photometry with theoretical stellar population synthesis models. 037−B327 has long been known to have a very large reddening value, which we confirm to be   E ( B − V ) = 1.360 ± 0.013  , in good agreement with the previous results. We redetermine its most likely age at  12.4 ± 3.2 Gyr  .
037−B327 is a prime example of an unusually bright early counterpart to the ubiquitous 'super' star clusters presently observed in most high-intensity star-forming regions in the local Universe. In order to have survived for a Hubble time, we conclude that its stellar initial mass function (IMF) cannot have been top-heavy. Using this constraint, and a variety of simple stellar population (SSP) models, we determine a photometric mass of     , somewhat depending on the SSP models used, the metallicity and age adopted and the IMF representation. This mass, and its relatively small uncertainties, makes this object the most massive star cluster of any age in the Local Group. Assuming that the photometric mass estimate thus derived is fairly close to its dynamical mass, we predict that this GC has a (one-dimensional) velocity dispersion of the order of  (72 ± 13) km s⁻¹  . As a surviving 'super' star cluster, this object is of prime importance for theories aimed at describing massive star cluster evolution.