arXiv subjects

V. A. Marsakov

Publications and source records attributed to V. A. Marsakov.

At least 19 recordsLinked to original sources

arXiv2024

Galactic-Field Metal-Rich RR~Lyrae Variables: Features of Kinematics and Abundances of Selected Chemical Elements

We present an analysis of the relative abundance features of a number of chemical elements in the atmospheres of metal-rich ($\rm{[Fe/H]} > -1.0$) Galactic-field RR~Lyrae variable stars and the kinematic characteristics of these stars. We have previously shown that the relative abundances of some $α$-elements: magnesium, silicon, calcium, and to a greater extent of titanium, as well as yttrium and scandium in such stars are lower than in most of other types of stars, bearing similar metallicity. It is found here that some of these metal-rich RR~Lyrae stars also have very low relative abundances of sodium, aluminum and nickel. The orbital parameters of all the metal-rich RR~Lyrae variables studied in this paper are typical of the Galactic thin or thick disk objects, however, unusual chemical composition let us to suggests a possible extragalactic origin for some of them.

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arXiv2020

Physical and Chemical Properties of Galactic Global Clusters with Various Origins Identified from the Gaia DR2 Data

The differences in the relationships between the physical parameters and the chemical-element abundances in accreted globular star clusters and those formed inside the Galaxy have been investigated. The information on the supposed formation sites of the clusters based on the Gaia~DR2 data is borrowed from the literature. Those sources estimate the probability of belonging to the Galactic bulge and disk, as well as to six known events of the merger of dwarf satellite galaxies with the Milky Way, for 151~globular clusters. It is shown that all metal-poor ($\rm{[Fe/H]}<-1.0$) genetically related globular clusters have high relative abundances of $α$-elements. According to modern views, since type II supernovae release more $α$-elements into the interstellar medium with increasing mass, it has been suggested that masses of type II supernovae in the Galaxy were greater than in the accreted galaxies. It is proved that the clusters of the low-energy group, which were considered accreted, are genetically related to a single protogalactic cloud, same as the unstratified clusters UKS~1 and Liller~1, which most likely belong to the bulge. It is shown that not only the lower but also the upper limits of the clusters' masses decrease with an increase in the average radius of their orbits. The latter fact is explained by a decrease in the masses of emerging clusters with a decrease in the masses of their host galaxies. It is demonstrated that an extremely multicomponent stellar population is observed only in accreted globular clusters with an initial mass $>10^{6}M_{\odot}$. It has been suggested that these clusters retained all the matter ejected by their evolved stars, from which new generations of stars formed due to long evolution far from our Galaxy.

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arXiv2020

Peculiarities of Abundances of Selected Elements in Metal-Rich Field RR Lyrae Stars

We use the data of our extended catalog of spectroscopic determinations of elemental abundances in the atmospheres of Galactic-field RR Lyrae type variables to show that metal-rich RR Lyraes ($\rm{[Fe/H]} > -1.0$) have anomalous abundances of some elements. In particular, the relative abundances of scandium, titanium, and yttrium in metal-rich RR Lyrae type variables are lower than the corresponding abundances in field stars of similar metallicity beyond the errors. We discuss the errors of the determination of the abundances of the above elements and point out the fact that no europium, zirconium, and lanthanum abundance determinations are available for metal-rich RR Lyrae type variables. We also analyze various possible causes of the observed peculiarities of the chemical composition of metal-rich RR Lyrae type variables.

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arXiv2020

Globular Clusters of the Galaxy: Chemical Composition vs Kinematics

A comprehensive statistical analysis of the relationship between the chemical and spatially kinematic parameters of the globular clusters of the Galaxy has been performed. The data of the author's compilation catalog contain astrophysical parameters for 157 clusters and the relative abundances of α-elements for 69 clusters. For 121 clusters, the data are supplemented by spatially kinematic parameters taken from the literature. The phenomenon of reddening of horizontal branches of low-metal accreted globular clusters is discussed. We consider the contradiction between the criteria for clusters to belong to the subsystems of the thick disk and the halo in terms of chemical and kinematic properties. It consists in the fact that, regardless of belonging to the galactic subsystems by kinematics, almost all metallic ([Fe/H] >-1.0) clusters are located close to the center and plane of the Galaxy, while among the less metallic of both subsystems there are many distant ones. Differences in the abundances of α-elements in the stellar objects of the Galaxy and the surrounding low-mass dwarf satellite galaxies confirm the well-known conclusion that all globular clusters and field stars of the accreted halo are remnants of galaxies of higher mass than the current environment of the Galaxy. A possible exception is a distant low-metal cluster with low relative abundance of α-elements Rup 106.

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arXiv2020

Inconsistency of Chemical Properties of Stellar Populations in the Thick Disk Subsystem of Our Galaxy

Using modern published data on velocities and spectroscopic definitions of chemical elements in stellar objects of the Galaxy, we investigated the relationship of chemical composition with the kinematics of different populations. The paper shows that the old stellar populations of the Galaxy, belonging (by the kinematic criterion) to the thick disk subsystem---globular clusters, field variables of the type RR Lyrae (lyrids), as well as close F--G dwarfs and field giants, have different chemical composition. In particular, the dwarfs and giants of the field are on average more metallic than the globular clusters and lyrids of the field. Moreover, the relative abundances of $α$-elements in the range $\rm{[Fe/H]} > -1.0$ are the highest for globular clusters, and are the lowest for for field variables of the RR Lyrae type. Based on the analysis of the nature of the dependences of [$α$/Fe] on [Fe/H] for these objects it was suggested that the thick disk subsystem in the Galaxy is composite and at least three components exist independently within it. The oldest one includes metal-rich globular clusters that formed from a single proto-galactic cloud shortly after the start of type Ia supernovae outbursts. Then the subsystem of field stars of a thick disk was formed as a result of ``heating'' of stars of already formed thin disk of the Galaxy by a rather massive dwarf satellite galaxy that fell on it. And finally, subsystems of field stars with the kinematics of not only a thick, but even a thin disk that fell on the Galaxy from this captured satellite galaxy.

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arXiv2019

The Chemical Composition of Globular Clusters of Different Nature in our Galaxy

A catalog of Galactic globular clusters has been compiled and used to analyze relations between the chemical and kinematic parameters of the clusters. The catalog contains positions, distances, luminosities, metallicites, and horizontal-branch morphology indices for 157~globular clusters, as well as space velocities for 72~globular clusters. For 69~globular clusters, these data are suppleented with the relative abundances of 28~chemical elements produced in various nuclear-synthesis processes, taken from 101~papers published between 1986 and 2018. The tendency for redder horizontal branches in low-metallicity accreted globular clusters is discussed. The discrepancy between the criteria for cluster membership in the thick-disk and halo subsystems based on chemical and kinematic properties is considered. This is manifest through the fact that all metal-rich ($\rm{[Fe/H]} > -1.0$) clusters are located close to the center and plane of the Galaxy, regardless of their kinematic membership in particular Galaxy subsystems. An exception is three accreted clusters lost by a dwarf galaxy in Sagittarius. At the same time, the fraction of more distant clusters is high among metal-poorer clusters in any kinematically selected Galactic subsystem.

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arXiv2019

Masses of RR Lyrae Stars with Different Chemical Abundances in the Galactic Field

The surface gravities and effective temperatures have been added to a compilative catalog published earlier, which includes the relative abundances of several chemical elements for 100 field RR Lyrae stars. These atmoshperic parameters and evolutionary tracks from the Dartmouth database are used to determine the masses of the stars and perform a comparative analysis of the properties of RR Lyrae stars with different chemical compositions. The masses of metal-rich ($\rm{[Fe/H]}> -0.5$) RR Lyrae stars with thin disk kinematics are in the range $(0.51-0.60)M_{\odot}$. Only stars with initial masses exceeding $1M_{\odot}$ can reach the horizontal branch during the life time of this subsystem. To become an RR Lyrae variable, a star must have lost approximately half of its mass during the red-giant phase. The appearance of such young, metal-rich RR Lyrae stars is possibly due to high initial helium abundances of their progenitors. According to the Dartmouth evolutionary tracks for Y = 0.4, a star with an initial mass as low as $0.8M_{\odot}$ could evolve to become an RR~Lyrae variable during this time. Such stars should have lost $(0.2-0.3)M_{\odot}$ in the red-giant phase, which seems quite realistic.

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arXiv2017

Relationship between the Elemental Abundances and the Kinematics of Galactic-Field RR Lyrae Stars

Data of our compiled catalog containing the positions, velocities, and metallicities of 415 RR~Lyrae variable stars and the relative abundances [el/Fe] of 12~elements for 101 RR~Lyrae stars, including four $α$~elements (Mg, Ca, Si, and Ti), are used to study the relationships between the chemical and spatial--kinematic properties of these stars. In general, the dependences of the relative abundances of $α$~elements on metallicity and velocity for the RR~Lyrae stars are approximately the same as those for field dwarfs. Despite the usual claim that these stars are old, among them are representatives of the thin disk, which is the youngest subsystem of the Galaxy. Attention is called to the problem of low-metallicity RR~Lyrae stars. Most RR~Lyrae stars that have the kinematic properties of thick disk stars have metallicities ${\rm [Fe/H]} < -1.0$ and high ratios [$α$/Fe]$ \approx 0.4$, whereas only about 10\,\% of field dwarfs belonging to the so-called "low-metallicity tail" have this chemical composition. At the same time, there is a sharp change in [$α$/Fe] in RR~Lyrae stars belonging just to the thick disk, providing evidence for a long period of formation of this subsystem. The chemical compositions of SDSS J1707+58, V455 Oph, MACHO 176.18833.411, V456 Ser, and BPS CS 30339--046 do not correspond to their kinematics. While the first three of these stars belong to the halo, according to their kinematics, the last two belong to the thick disk. It is proposed that they are all most likely extragalactic, but the possible appearance of some of them in the solar neighborhood as a result of the gravitational action of the bar on field stars cannot be ruled out.

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arXiv2016

Peculiarities of α-element abundances in Galactic open clusters

A catalog compiling the parameters of 346 open clusters, including their metallicities, positions, ages, and velocities has been composed. The elements of the Galactic orbits for 272 of the clusters have been calculated. Spectroscopic determinations of the relative abundances, [el/Fe], for 14 elements synthesized in various nuclear processes averaged over data from 109 publications are presented for 90 clusters. Since no systematic effects distorting the relative abundances of the studied elements in these clusters have been found, these difference suggest real differences between clusters with high, elongated orbits and field stars. In particular, this supports the earlier conclusion, based on an analysis of the elements of the Galactic orbits, that some clusters formed as a result of interactions between high-velocity, metal-poor clouds and the interstellar medium of the Galactic thin disk. On average, clusterswith high, elongated orbits and metallicities ${\rm [Fe/H]} < - 0.1$ display lower relative abundances of the primary á elements than do field stars. The low [O,Mg/Fe] ratios of these clusters can be understood if the high-velocity clouds that gave rise to them were formed of interstellar material from regions where the star-formation rate and/or the masses of Type II supernovae were lower than near the Galactic plane. It is also shown that, on average, the relative abundances of the primary $α$-elements are higher in relatively metal-rich clusters with high, elongated orbits than in field stars. This can be understood if clusters with ${\rm [Fe/H]} > - 0.1$ formed as a result of interactions between metal-rich clouds with intermediate velocities and the interstellar medium of the Galactic disk; such clouds could form from returning gas in a so-called "Galactic fountain".

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arXiv2016

Peculiarities of the abundances of neutron-capture elements in Galactic open clusters

The properties of the relative abundances of rapid and slow neutron-capture elements are studied using a catalog containing spectroscopic abundance determinations for 14~elements produced in various nuclear-synthesis processes for 90~open clusters. The catalog also contains the positions, ages, velocities, and elements of the Galactic orbits of the clusters. The relative abundances of both $r$-elements (Eu) and $s$-elements (Y, Ba, La, and Ce) in clusters with high, elongated orbits and in field stars of the Galactic thin disk display different dependences on metallicity, age, Galactocentric distance, and the elements of the Galactic orbits, supporting the view that these objects have different natures. In young clusters, not only barium, but also the three other studied $s$-elements display significantly higher relative abundances than field stars of the same metallicity. The relative abundances of Eu are lower in high-metallicity clusters (${\rm [Fe/H]} > -0.1$) with high, elongated orbits than in field giants, on average, while the [Eu/Fe] ratios in lower-metallicity clusters are the same as those in field stars, on average, although with a large scatter. The metallicity dependence of the [O, Mg/Eu] ratios in clusters with high, elongated orbits and in field stars are substantially different. These and other described properties of the Eu abundances, together with the properties of the abundances of primary $α$-elements, can be understood in a natural way if clusters with high, elongated orbits with different metallicities formed as a result of interactions of two types of high-velocity clouds with the interstellar medium of the Galactic disk: low-metallicity high-velocity clouds that formed from "primordial" gas, and high-metallicity clouds with intermediate velocities that formed in "Galactic fountains".

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arXiv2012

Heterogeneity of the Population of Open Star Clusters in the Galaxy

Based on published data, we have compiled a catalogue of fundamental astrophysical parameters for 593 open clusters of the Galaxy. In particular, the catalogue provides the Galactic orbital elements for 500 clusters, the masses, central concentrations, and ellipticities for 424 clusters, the metallicities for 264 clusters, and the relative magnesium abundances for 56 clusters. We describe the sources of initial data and estimate the errors in the investigated parameters. The selection effects are discussed. The chemical and kinematical properties of the open clusters and field thin-disk stars are shown to differ. We provide evidence for the heterogeneity of the population of open clusters.

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arXiv2012

Two Populations of Open Star Clusters in the Galaxy

Based on our compiled catalogue of fundamental astrophysical parameters for 593 open clusters, we analyze the relations between the chemical composition, spatial positions, Galactic orbital elements, age, and other physical parameters of open star clusters. We show that the population of open clusters is heterogeneous and is divided into two groups differing by their mean parameters, properties, and origin. One group includes the Galactic clusters formed mainly from the interstellar matter of the thin disk with nearly solar metallicities ([Fe/H] > -0.2) and having almost circular orbits a short distance away from the Galactic plane, i.e., typical of the field stars of the Galactic thin disk. The second group includes the peculiar clusters formed through the interaction of extragalactic objects (such as high--velocity clouds, globular clusters, or dwarf galaxies) with the interstellar matter of the thin disk, which, as a result, derived abnormally low (for field thin-disk stars) metallicities and/or Galactic orbits typical of objects of the older Galactic subsystems.

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arXiv2011

The Age-Metallicity Relation in the Thin Disk of the Galaxy

HST trigonometric distances, photometric metallicities, isochronic ages from the second revised version of the Geneva--Copenhagen survey, and uniform spectroscopic Fe and Mg abundances from our master catalog are used to construct and analyze the age--metallicity and age-relative Mg abundance relations for stars of the thin disk. The influences of selection effects are discussed in detail. It is demonstrated that the radial migration of stars does not lead to appreciable distortions in the age dependence of the metallicity. During the first several billion years of the formation of the thin disk, the interstellar material in this disk was, on average, fairly rich in heavy elements (<[Fe/H]> ~-0.2) and poorly mixed. However, the metallicity dispersion continuously decreased with age, from σ_{[Fe/H]}~0.22 to ~0.13. All this time, the mean relative abundance of Mg was somewhat higher than the solar value (<[Mg/Fe]>~0.1). Roughly four to five billion years ago, the mean metallicity began to systematically increase, while retaining the same dispersion; the mean relative Mg abundance began to decrease immediately following this. The number of stars in this subsystem increased sharply at the same time. These properties suggest that the star-formation rate was low in the initial stage of formation of the thin disk, but abruptly increased about four to five billion years ago.

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arXiv2011

Evolution of the Velocity Ellipsoids in the Thin Disk of the Galaxy and the Radial Migration of Stars

Data from the revised Geneva--Copenhagen catalog are used to study the influence of radial migration of stars on the age dependences of parameters of the velocity ellipsoids for nearby stars in the thin disk of the Galaxy, assuming that the mean radii of the stellar orbits remain constant. It is demonstrated that precisely the radial migration of stars, together with the negative metallicity gradient in the thin disk,are responsible for the observed negative correlation between the metallicities and angular momenta of nearby stars, while the angular momenta of stars that were born at the same Galactocentric distances do not depend on either age or metallicity. (abridged)

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arXiv2009

Relationship between the Velocity Ellipsoids of Galactic-Disk Stars and their Ages and Metallicities

The dependences of the velocity ellipsoids of F-G stars of the thin disk of the Galaxy on their ages and metallicities are analyzed based on the new version of the Geneva-Copenhagen Catalog. The age dependences of the major, middle, and minor axes of the ellipsoids, and also of the dispersion of the total residual veltocity, obey power laws with indices 0.25,0.29,0.32, and 0.27 (with uncertainties \pm 0.02). Due to the presence of thick-disk objects, the analogous indices for all nearby stars are about a factor of 1.5 larger. Attempts to explain such values are usually based on modeling relaxation processes in the Galactic disk. With increasing age, the velocity ellipsoid increases in size and becomes appreciably more spherical, turns toward the direction of the Galactic center, and loses angular momentum. The shape of the velocity ellipsoid remains far from equilibrium. With increasing metallicity, the velocity ellipsoid for stars of mixed age increases in size, displays a weak tendency to become more spherical, and turns toward the direction of the Galactic center (with these changes occurring substantially more rapidly in the transition through the metallicity [Fe/H]= -0.25). Thus, the ellipsoid changes similarly to the way it does with age; however, with decreasing metallicity, the rotational velocity about the Galactic center monotonically increases, rather than decreases(!). Moreover, the power-law indices for the age dependences of the axes depend on the metallicity, and display a maximum near [Fe/H]=-0.1. The age dependences of all the velocity-ellipsoid parameters for stars with equal metallicity are roughly the same. It is proposed that the appearance of a metallicity dependence of the velocity ellipsoids for thin-disk stars is most likely due to the radial migration of stars.

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arXiv2007

Star Formation History in the Galactic Thin Disk

The behavior of the relative magnesium abundances in the thin-disk stars versus their orbital radii suggests that the star formation rate in the thin disk decreases with increasing Galactocentric distance, and there was no star formation for some time outside the solar circle while this process was continuous within the solar circle. The decrease in the star formation rate with increasing Galactocentric distance is responsible for the existence of a negative radial metallicity gradient in the thin disk. At the same time the relative magnesium abundance exhibits no radial gradient. It is in detail considered the influence of selective effects on the form of both age - metallicity and age - relative magnesium abundance diagrams. It is shown that the first several billion years of the formation of the thin disk interstellar medium in it was on the average sufficiently rich in heavy elements (<[Fe/H]> = -0.22), badly mixed (σ_[Fe/H] = 0.21), and the average relative magnesium abundance was comparatively high (<[Mg/Fe]> = 0.10). Approximately 5 billion years ago average metallicity began to systematically increase, and its dispersion and the average relative magnesium abundance - to decrease. These properties may be explained by an increase in star formation rate with the simultaneous intensification of the processes of mixing the interstellar medium in the thin disk, provoke possible by interaction the Galaxy with the completely massive by satellite galaxy.

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arXiv2007

Stellar Objects of Extragalactic Origin in the Galactic Halo

We identified globular clusters and field stars of extragalactic origin and investigated their chemical, physical, and kinematical properties. This objects as supposed was captured by the Galaxy at different times from debris of the dwarf satellite galaxies disrupted by its tidal forces. The results are follows. (1) The majorities of metal-poor stellar objects in the Galaxy have an extragalactic origin. (2) The masses of the accreted globular clusters decrease with the removal from the center and the plane of the Galaxy. (3) The relative abundances of chemical elements in the accreted and genetically connected stars are essentially distinguished. (4) The accreted field stars demonstrate the decrease of the relative magnesium abundanses with an increase in sizes and inclinations of their orbits. (5) The stars of the Centaurus moving group were born from the matter, in which star formation rate was considerably lower than in the early Galaxy. On the base of these properties was made a conclusion that with the decrease of the masses of the dwarf galaxies in them simultaneously decrease the average masses of globular clusters and the maximum masses of supernova SNe II. Namely latter fact leads to the decrease of the relative abundances of α-elements in their metal-poor stars.

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arXiv2006

Formation of Galactic Systems in Light of the Magnesium Abundance in Field Stars.III.the Halo

We analyze the relations between the relative magnesium abundances, metallicities, and Galactic orbital elements for halo stars. We show that the relative magnesium abundances in protodisk halo stars are virtually independent of metallicity and lie within a fairly narrow range while presumably accreted stars demonstrate a large spread in relative magnesium abundances up to negative [Mg/Fe]. The mean metallicity of magnesium-poor ([Mg/Fe]<0.2 dex) accreted stars has been found to be displaced toward the negative values when passing from stars with low azimuthal velocities to those with high ones at Δ[Fe/H]=0.5dex. The mean apogalactic radii and inclinations of the orbits also increase while their eccentricities decrease. As a result negative radial and vertical gradients in relative magnesium abundances are observed in the accreted halo in the absence of correlations between the [Mg/Fe] ratios and other orbital elements, while these correlations are found at a high significance level for genetically related Galactic stars. We surmise that as the masses of dwarf galaxies decrease, the maximum SNII masses and hence, the yield of α-elements in them also decrease. In this case, the relation between the [Mg/Fe] ratios and the inclinations and sizes of the orbits of accreted stars is in complete agreement with numerical simulations of dynamical processes during the interaction of galaxies. Thus the behavior of the magnesium abundance in accreted stars suggests that the satellite galaxies are disrupted and lose their stars en masse only after dynamical friction reduces significantly the sizes of their orbits and drags them into the Galactic plane. Less massive satellite galaxies are disrupted even before their orbits change appreciably under tidal forces.

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