arXiv subjects

Anton F. Seleznev

Publications and source records attributed to Anton F. Seleznev.

17 recordsLinked to original sources

arXiv2026

Photometric Identification of Unresolved Binary Stars in Nearby Open Star Clusters

This paper introduces a new method to search for unresolved binary stars in open star clusters. The work aims at improving the approach introduced previously, which employs the (H-W2)-W1 versus W2-(BP-K) photometric diagram. This diagram, in tandem with the Gaia Color Magnitude Diagram (CMD) and using theoretical isochrones as reference sequences, is used to estimate the binary star fraction and the distribution of the component mass ratio $q$ in eight nearby open star clusters, including Pleiades, Alpha Per, and Praesepe, which we investigated in previous studies. In this study, to alleviate the uncertainties associated with the use of theoretical isochrones, we propose an empirical isochrones approach. We show that this is an effective approach to exploring a wider primary-mass interval, in particular for the region of low-mass sources. Box-and-whisker plots are used to present the distribution of the component mass ratio $q$. The mode of distribution turns out to be in the range $0.43-0.83$ and $0.38-0.63$ for Gaia and infrared-visible photometry, respectively. In addition, we update the algorithm to obtain the binary fraction, whose estimate lies in the range $0.16 - 0.36$ and $0.21 - 0.44$, depending on the adopted method, and show that in previous studies the binary fraction was overestimated. We do not find evidence that the variable spatial resolution of the employed catalogs (Gaia, 2MASS, and WISE) affects the precision of the binary fraction estimate.

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arXiv2025

Old open clusters NGC 188 and M 67 in Gaia DR3

We performed a statistical study of two old open clusters NGC 188 and M 67 using Gaia DR3 data. No tidal tails of the clusters were detected, which most likely had been destroyed when the cluster passed through the Galactic plane. The size estimates of the clusters depend on the range of astrometric parameters and stellar magnitudes of the stars used for star counts. The mass spectra of two clusters differ significantly. NGC 188 shows a deficit of low-mass stars compared to M 67. In the halo region of NGC 188 (compared to the core region of the cluster), there is a relative excess of the low-mass stars (just as in the case of M 67) and a deficit of stars in the mass range from 0.66 to 0.9 solar masses. Comparison with the Hunt & Reffert samples showed that almost all the stars from these samples are contained among the stars we selected for counting. Moreover, the group probability of these stars belonging to clusters, estimated by the uniform background method, is higher than 60%. It is shown that the velocity dispersion of single stars (selected according to the `stellar magnitude -- color index' diagrams) is significantly smaller than the velocity dispersion of unresolved binary stars.

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arXiv2025

Statistic of stars with poor astrometric solutions of Gaia DR3 in open star clusters

We performed counts of stars with poor astrometric solutions of Gaia DR3 in the regions of open star clusters: NGC 188, NGC 1039, NGC 2287, NGC 2301, NGC 2360, NGC 2420, NGC 2527, NGC 2548, NGC 2682 (M 67), NGC 3114, NGC 3766, NGC 5460, NGC 6649. The selection of the possible cluster members is based on the Gaia photometry using the Hess diagram. We look for stars that fall within the region of the Hess diagram plotted from probable cluster members based on Hunt & Reffert data. We take stars with two-parameter solutions, with the parameter RUWE>1.4, as well as with large relative parallax errors that fall within the Hess diagram region for probable cluster members. The radii of clusters based on stars with poor astrometric solutions and the number of such possible cluster members were estimated. The number of stars with poor astrometric solutions relative to the number of stars from the Hunt & Reffert sample N_bad/N varies very widely with a median average of approximately 30%. This means that when one selects probable cluster members based on precise astrometric data from Gaia DR3, an average of about 23% of cluster members may be lost. Among the lost stars there may be a significant number of unresolved binary and multiple systems. We investigated the dependence of the relative number of stars with poor astrometric solutions on the galactic latitude and on the average number density of stars. The brightness functions with and without stars with poor solutions differ significantly in the region of faint stars 14 =0.15.

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arXiv2025

Search for the possible members of the open cluster NGC 3532 with poor astrometric solutions of Gaia DR3

We performed star counts in the region of the open cluster NGC 3532. The ranges of trigonometric parallaxes and proper motions containing all the stars of the cluster were determined using the stars with 5- and 6-parameter Gaia DR3 solutions. The estimated radius of the cluster was R_c=178+-3 arcminutes and the number of cluster stars was N_c=2200+-40. We estimate the number of stars with poor astrometric solutions that may be members of the cluster. For this purpose, we analyze the surface density distribution of stars with two-parameter Gaia DR3 solutions, stars with the parameter RUWE>1.4, and stars with large relative errors of trigonometric parallaxes in the vicinity of the cluster. We are looking for stars that fall within the area of the color-magnitude diagram occupied by probable members of the NGC 3532 cluster from the Hant&Reffert sample. The radial surface density profile plotted with such stars shows the concentration of stars toward the cluster center. An analysis of the profile yields an estimate of 2150+-230 stars that may be cluster members. Thus, nearly one half of cluster members can be lost when the probable members are selected only by exact astrometric data of Gaia DR3. Among these lost stars, there may be a significant number of unresolved binary and multiple systems.

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arXiv2024

Velocity Dispersion of the open cluster NGC 2571 by Radial Velocities and Proper Motions

We use a Kernel Density Estimator method to evaluate the stellar velocity dispersion in the open cluster NGC 2571. We derive the 3-D velocity dispersion using both proper motions as extracted from Gaia DR3 and single epoch radial velocities as obtained with the instrument FLAMES at ESO VLT. The mean-square velocity along the line-of-sight is found to be larger than the one in the tangential direction by a factor in the interval [6,8]. We argue that the most likely explanation for such an occurrence is the presence of a significant quantity of unresolved binary and multiple stars in the radial velocity sample. Special attention should be paid to single line spectroscopic binaries (SB1) since in this case we observe the spectral lines of the primary component only, and therefore the derived radial velocity is not the velocity of the binary system center of mass. To investigate this scenario, we performed numerical experiments at varying the fractional abundance of SB1 in the observed sample. These experiments show that the increase of the mean-square radial velocity depends actually on the fractional abundance of SB1 to a power in the range of [0.39,0.45]. We used the 3-D velocity dispersion obtained by the dispersions in the tangential directions and the assumption that the radial velocity dispersion is the same as a tangential one to estimate the virial cluster mass and the cluster mass taking into account the gravitational field of the Galaxy and the non-stationarity of the cluster. These estimates are $650\pm30 \; M_\odot$ and $310\pm80 \; M_\odot$, respectively, and they are in substantial agreement with the photometric cluster mass.

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arXiv2022

Unresolved Binaries and Multiples in the Intermediate Mass Range in open clusters: Pleiades, Alpha Per, Praesepe, and NGC 1039

In this study, we continue our project to search for unresolved binary and multiple systems in open clusters exploiting the photometric diagram (H-W2)-W1 vs W2-(BP-K) firstly introduced in \citet{Malofeeva+2022}. In particular, here we estimate the binary and multiple star ratios and the distribution of the component mass ratio $q$ in the Galactic clusters Alpha Persei, Praesepe, and NGC 1039. We have modified the procedure outlined in our first study \citep{Malofeeva+2022} making star counts automatic and by introducing bootstrapping for error estimation. Basing on this, we re-investigated the Pleiades star cluster in the same mass range as in our previous work and corrected an inaccuracy in the mass ratio $q$ distribution. The binary and multiple star ratio in the four clusters is then found to lie between 0.45$\pm$0.03 and 0.73$\pm$0.03. On the other hand, the ratio of systems with multiplicity more than 2 is between 0.06$\pm$0.01 and 0.09$\pm$0.02. The distribution of the component mass ratio $q$ is well fitted with a Gaussian having the mode between 0.22$\pm$0.04 and 0.52$\pm$0.01 and the dispersion between 0.10$\pm$0.02 and 0.35$\pm$0.07. All clusters show a large number of the very low-mass secondary components in the binary systems with primary components below 0.5 $M_{\odot}$.

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arXiv2021

Unresolved multiple stars and Galactic clusters' mass estimates

If not properly accounted for, unresolved binary stars can induce a bias in the photometric determination of star cluster masses inferred from star counts and the luminosity function. A correction factor close to 1.15 (for a binary fraction of 0.35) was found in \citep{Boro19}, which needs to be applied to blind photometric mass estimates. This value for the correction factor was found to be smaller than literature values. In an attempt to lift this discrepancy, in this work the focus is on higher order multiple stars with the goal of investigating the effect of triple and quadruple systems adopting the same methodology and data-set as in the quoted work. Then the result is found that when triple and quadruple together with binary systems are properly accounted for, the actual cluster mass (computed as all stars were single) should be incremented by a factor of 1.18$-$1.27, depending on the cluster and when the binary fraction $α$ is 0.35. Fitting formulae are provided to derive the increment factor for different binary star percentages.

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arXiv2020

On the Motion of Stars in the Pleiades according to Gaia DR2 Data

We used Gaia DR2 data in order to estimate parameters of the Pleiades. With the data on stars with m_G<=18 mag, we constructed the density maps and profile, luminosity and mass functions, determined the cluster radius, 10.9+/-0.3 deg (26.3+/-0.7 pc), and the radius of its core, 2.62 deg (6.24 pc), and obtained estimates for the number of stars in the cluster, 1542+/-121, and their mass, 855+/-104 solar mass. Distribution of stars with m_G<16 mag at distances r_s from the cluster center in three-dimensional space of r_s<1 pc and at r_s~1.4-5 pc contains radial density waves. We determined the average `prograde' rotation velocity of the core of the cluster v_c=0.56+/-0.07 km/s at distances d in the sky plane d<=4.6 pc from its center. The rotation velocity of the cluster core at a distance of d~5.5 pc from its center is close to zero: v_c=0.1+/-0.3 km/s. The velocity of the `retrograde' rotation of the cluster at a distance of d~7.1 pc from its center is v_c=0.48+/-0.20 km/s. The dependences of moduli of the tangential and radial components of the cluster core star velocities in the sky plane on the distance d to the cluster center contain the periodic oscillations. The velocity dispersions of the cluster core stars increase on average with an increase in r_s, which, like the radial density waves and the waves of oscillations of the velocity field in the sky plane, indicates the non-stationarity of the cluster in the regular field. The Jeans wavelength in the cluster core decreases, and the velocity dispersion of the core stars under the Jeans instability increases after taking into account the Galactic gravitational field. The region of gravitational instability in the Pleiades cluster is located in the interval r_s=2.2-5.7 pc and contains 39.4-60.5% of the total number of stars in the considered samples.

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arXiv2020

The relation of the Alpha Persei star cluster with the nearby stellar stream

A map of 100 degrees on a side extracted from Gaia DR2 and centred on Alpha Persei reveals two distinct structures | the Alpha Persei star cluster and a conspicuous stellar stream, as widely documented in recent literature. In this work we employ DBSCAN to assess individual stars' membership and attempt at separating stars belonging to the cluster and to the stream from the general field. In turn, we characterize the stream and investigate its relation with the cluster. The stream population turned out to be significantly older (5+/-1Gyr) than the cluster, and to be positioned ~90 pc away from the cluster, in its background. The stream exhibits a sizeable thickness of ~180 pc in the direction of the line of view. Finally, the stream harbours a prominent population of white dwarf stars. We estimated an upper limit of the stream mass of ~6000 solar masses?. The stream would therefore be the leftover of a relatively massive old cluster. The surface density map of the Alpha Persei evidences the presence of tidal tails. While it is tempting to ascribe them to the interaction with the disrupting old star cluster, we prefer to believe, conservatively, they are of Galactic origin.

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arXiv2019

Unresolved binaries and Galactic clusters' mass estimates

Binary stars are present in all stellar systems, yet their role is far from being fully understood. We investigate the effect of unresolved binaries in the derivation of open clusters' mass by star counts. We start from the luminosity functions of five open clusters: IC 2714, NGC 1912, NGC 2099, NGC 6834 and NGC 7142. Luminosity functions are obtained via star counts extracted from the 2MASS database. The fraction of binaries is considered to be independent on stellar magnitude. We take into account different assumptions for the binary mass ratio distribution and assign binary masses using the so-called {\it luminosity-limited pairing} method and Monte-Carlo simulations. We show that cluster masses increase when binary stars are appropriately taken into account.

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arXiv2019

Ruprecht 147: a paradigm of dissolving star cluster

We employed recent Gaia/DR2 data to investigate the dynamical status of the nearby (300 pc), old (2.5 Gyr) open cluster Ruprecht~147. We found prominent leading and trailing tails of stars along the cluster orbit, which demonstrates that Ruprecht~147 is losing stars at fast pace. Star counts indicate the cluster has a core radius of 33.3 arcmin, and a tidal radius of 137.5 arcmin. The cluster also possesses an extended corona, which cannot be reproduced by a simple King model. We computed the present-day cluster mass using its luminosity and mass function, and derived an estimate of 234$\pm$52 $M_{\odot}$. We also estimated the cluster original mass using available recipes extracted from N-body simulations obtaining a mass at birth of 50000$\pm$6500 $M_{\odot}$. Therefore dynamical mass loss, mostly caused by tidal interaction with the Milky Way, reduced the cluster mass by about 99\%. We then conclude that Ruprecht~147 is rapidly dissolving into the general Galactic disc.

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arXiv2017

On the assessment of the nature of open star clusters and the determination of their basic parameters with limited data

Our knowledge of stellar evolution and of the structure and chemical evolution of the Galactic disk largely builds on the study of open star clusters. Because of their crucial role in these relevant topics, large homogeneous catalogues of open cluster parameters are highly desirable. Although efforts have been made to develop automatic tools to analyse large numbers of clusters, the results obtained so far vary from study to study, and sometimes are very contradictory when compared to dedicated studies of individual clusters. In this work we highlight the common causes of these discrepancies for some open clusters, and show that at present dedicated studies yield a much better assessment of the nature of star clusters, even in the absence of ideal data-sets. We make use of deep, wide-field, multi-colour photometry to discuss the nature of six strategically selected open star clusters: Trumpler~22, Lynga~6, Hogg~19, Hogg~21, Pismis~10 and Pismis~14. We have precisely derived their basic parameters by means of a combination of star counts and photometric diagrams. Trumpler~22 and Lynga~6 are included in our study because they are widely known, and thus provided a check of our data and methodology. The remaining four clusters are very poorly known, and their available parameters have been obtained using automatic tools only. Our results are in some cases in severe disagreement with those from automatic surveys.

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arXiv2017

On the mass of the Galactic star cluster NGC 4337

Only a small number of galactic open clusters survives for longer than few hundred million years. Longer lifetimes are routinely explained in term of larger initial masses, particularly quiet orbits, and off-plane birth-places. We derive in this work the actual mass of NGC 4337, one of the few open clusters in the Milky Way inner disk that managed to survive for about 1.5 Gyr. We derive its mass in two different ways. First, we exploit an unpublished photometric data set in the UBVI passbands to estimate -using star counts- the cluster luminosity profile, and luminosity and mass function, and hence its actual mass both from the luminosity profile and from the mass function.This data-set is also used to infer crucial cluster parameters, as the cluster half-mass radius and distance. Second, we make use of a large survey of cluster star radial velocities to derive dynamical estimates for the cluster mass. Under the assumption of virial equilibrium and neglecting the external gravitational field leads to values for the mass significantly larger than those obtained by mean of observed density distribution or with the mass function but still marginally compatible with the inferred values of the invisible mass in form of both low mass stars or remnants of high mass stars in the cluster. Finally, we derive the cluster initial mass by computing the mass loss experienced by the cluster during its lifetime, and adopting the various estimates of the actual mass.

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arXiv2016

Open-cluster density profiles derived using a kernel estimator

Surface and spatial radial density profiles in open clusters are derived using a kernel estimator method. Formulae are obtained for the contribution of every star into the spatial density profile. The evaluation of spatial density profiles is tested against open-cluster models from N-body experiments with N = 500. Surface density profiles are derived for seven open clusters (NGC 1502, 1960, 2287, 2516, 2682, 6819 and 6939) using Two-Micron All-Sky Survey data and for different limiting magnitudes. The selection of an optimal kernel half-width is discussed. It is shown that open-cluster radius estimates hardly depend on the kernel half-width. Hints of stellar mass segregation and structural features indicating cluster non-stationarity in the regular force field are found. A comparison with other investigations shows that the data on open-cluster sizes are often underestimated. The existence of an extended corona around the open cluster NGC 6939 was confirmed. A combined function composed of the King density profile for the cluster core and the uniform sphere for the cluster corona is shown to be a better approximation of the surface radial density profile.The King function alone does not reproduce surface density profiles of sample clusters properly. The number of stars, the cluster masses and the tidal radii in the Galactic gravitational field for the sample clusters are estimated. It is shown that NGC 6819 and 6939 are extended beyond their tidal surfaces.

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arXiv2015

The complex stellar populations in the lines of sight to open clusters in the third Galactic quadrant

Multi-color photometry of the stellar populations in five fields in the third Galactic quadrant centred on the clusters NGC 2215, NGC 2354, Haffner 22, Ruprecht 11, and ESO489SC01 is interpreted in terms of a warped and flared Galactic disk, without resort to an external entity such as the popular Monoceros or Canis Major overdensities. Except for NGC 2215, the clusters are poorly or unstudied previously. The data generate basic parameters for each cluster, including the distribution of stars along the line of sight. We use star counts and photometric analysis, without recourse to Galactic-model-based predictions or interpretations, and confirms earlier results for NGC 2215 and NGC 2354. ESO489SC01 is not a real cluster, while Haffner~22 is an overlooked cluster aged about 2.5 Gyr. Conclusions for Ruprecht~11 are preliminary, evidence for a cluster being marginal. Fields surrounding the clusters show signatures of young and intermediate-age stellar populations. The young population background to NGC~2354 and Ruprecht~11 lies 8-9 kpc from the Sun and $\sim$1 kpc below the formal Galactic plane, tracing a portion of the Norma-Cygnus arm, challenging Galactic models that adopt a sharp cut-off of the disk 12-14 kpc from the Galactic center. The old population is metal poor with an age of 2-3 Gyr, resembling star clusters like Tombaugh 2 or NGC 2158. It has a large color spread and is difficult to locate precisely. Young and old populations follow a pattern that depends critically on the vertical location of the thin and/or thick disk, and whether or not a particular line of sight intersects one, both, or none.

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arXiv2011

UBVI CCD photometry and star counts in 9 inner disk Galactic star clusters

We present and discuss new CCD-based photometric material in the UBVI pass-bands for nine Galactic star clusters located inside the solar ring, for which no CCD data are currently available. They are: IC 2714, NGC 4052, ESO131SC09, NGC 5284, NGC 5316, NGC 5715, VdB-Hagen ~164, NGC 6268, and Czernik 38. We first perform star counts by combining our optical photometry wi th 2MASS, and derive cluster sizes and radial density profiles. The fundamental parameters - age, reddening and distance- are then inferred from the analysis of the star distribution in color-color and color-magnitude diagrams of only the spatially selected likely members. Our analysis shows that ESO131SC09, NGC 5284, and VdB-Hagen 164 are most probably not clusters, but random enhancements of a few bright stars along the line of sight, with prop erties much similar to open cluster remnants. The remaining clusters are physical groups, and are all younger than about 1 Gyr . We use the newly derived set of parameters, in particular distance and reddening, to investigate their position in the Galaxy in the context of the spiral structure of the Milky Way. We find that the youngest clusters (IC 2714, NGC 5316, and NGC 6268) are located close to or inside the Carina-Sagittarius arm, and are therefore {ıt bona fide} spiral structure tracers. On the other hand, the oldest (Czernik 38, NGC 4052, and NGC 5715) are floating in the inter-arm space between the Carina-Sagittarius and the more distant Scutum-Crux arm. Interestingly enough, the oldest clusters of this sample - Czernik 38 and NGC 5715- are among the few known open clusters to be older or as old as the Hyades in the inner Galactic disk, where star clusters are not expected to survive for a long time, because of the strong tidal field and the higher probability of close encounters .

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arXiv2010

An analysis of the blue straggler population in the Sgr dSph globular cluster Arp 2

We present and discuss new BVI CCD photometry in the field of the globular cluster Arp~2, which is considered a member of the Sagittarius Dwarf Spheroidal Galaxy. The main goal of this investigation is to study of the statistics and spatial distribution of blue straggler stars in the cluster. Blue stragglers are stars observed to be hotter and bluer than other stars with the same luminosity in their environment. As such, they appear to be much younger than the rest of the stellar population. Two main channels have been suggested to produce such stars: (1) collisions between stars in clusters or (2) mass transfer between, or merger of, the components of primordial short-period binaries. The spatial distribution of these stars inside a star cluster, compared with the distribution of stars in different evolutionary stages, can cast light on the most efficient production mechanism at work. In the case of Arp~2, we found that blue straggler stars are significantly more concentrated than main sequence stars, while they show the same degree of concentration as evolved stars (either red giants or horizontal branch stars). Since Arp~2 is not a very concentrated cluster, we suggest that this high central concentration is an indication that blue stragglers are mostly primordial binary stars.

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