41 research outputs found
Stars of extragalactic origin in the solar neighborhood
We computed the spatial velocities and the galactic orbital elements using
Hipparcos data for 77 nearest main-sequence F-G-stars with published the iron,
magnesium, and europium abundances determined from high dispersion spectra and
with the ages estimated from theoretical isochrones. A comparison with the
orbital elements of the globular clusters that are known was accreted by our
Galaxy in the past reveals stars of extragalactic origin. We show that the
relative elemental abundance ratios of r- and \alpha- elements in all the
accreted stars differ sharply from those in the stars that are genetically
associated with the Galaxy. According to current theoretical models, europium
is produced mainly in low mass Type II supernovae (SNe II), while magnesium is
synthesized in larger amounts in high mass SN II progenitors. Since all the old
accreted stars of our sample exhibit a significant Eu overabundance relative to
Mg, we conclude that the maximum masses of the SNII progenitors outside the
Galaxy were much lower than those inside it are. On the other hand, only a
small number of young accreted stars exhibit low negative ratios .
The delay of primordial star formation burst and the explosions of high mass
SNe II in a relatively small part of extragalactic space can explain this
situation. We provide evidence that the interstellar medium was weakly mixed at
the early evolutionary stages of the Galaxy formed from a single proto-galactic
cloud and that the maximum mass of the SN II progenitors increased in it with
time simultaneously with the increase in mean metallicity.Comment: Accepted for 2004, Astronomy Letters, Vol. 30, No. 3, P.148-158 15
pages, 3 figure
COMPARATIVE ANALYSIS OF TOOLS FOR ESTIMATION OF INVESTMENT CLIMATE OF TERRITORIES
Purpose: The present paper is a study of the most well-known foreign and domestic methods for assessing the investment climate of the regions as well as identifying their main disadvantages.
Methodology: The study of regional investment issues should be based on a systematic approach taking into account the positive and negative aspects of each methodology separately.
Result: Opportunities to improve the competitiveness of the constituent entities of the Russian Federation are considered, too. The existing methods applied by various research-rating agencies are considered in detail.
Applications: This research can be used for the universities, teachers and education students.
Novelty/Originality: In this research, the model of comparative analysis of tools for estimation of the investment climate of territories is presented in a comprehensive and complete manner
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
\Delta[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 \alpha-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.Comment: accepted 2006, Astronomy Letters, Vol. 32 No. 8, P.545, 18 pages, 6
figure
Mg, Ba and Eu abundances in thick disk and halo stars
Our sample of cool dwarf stars from previous papers (Mashonkina & Gehren 2000, 2001) is extended in this study including 15 moderately metal-deficient stars. The samples of halo and thick disk stars have overlapping metallicities with [Fe/H] in the region from -0.9 to -1.5, and we compare chemical properties of these two kinematically different stellar populations independent of their metallicity. We present barium, europium and magnesium abundances for the new sample of stars. The results are based on NLTE line formation obtained in differential model atmosphere analyses of high resolution spectra observed mainly using the UVES spectrograph at the VLT of the European Southern Observatory. We confirm the overabundance of Eu relative to Mg in halo stars as reported in our previous papers. Eight halo stars show [Eu/Mg] values between 0.23 and 0.41, whereas stars in the thick and thin disk display a solar europium to magnesium ratio. The [Eu/Ba] values found in the thick disk stars to lie between 0.35 and 0.57 suggest that during thick disk formation evolved low-mass stars started to enrich the interstellar gas by s-nuclei of Ba, and the s-process contribution to barium thus varies from 30% to 50%. Based on these results, and using the chemical evolution calculations by Travaglio et al. (1999), we estimate that the thick disk stellar population formed on a timescale between 1.1 to 1.6 Gyr from the beginning of the protogalactic collapse. In the halo stars the [Eu/Ba] values are found mostly between 0.40 and 0.67, which suggests a duration of the halo formation of about 1.5 Gyr. For the whole sample of stars we present the even-to-odd Ba isotope ratios as determined from hyperfine structure seen in the Ba II resonance line λ4554. As expected, the solar ratio 82:18 (Cameron 1982) adjusts to observations of the Ba II lines in the thin disk stars. In our halo stars the even-to-odd Ba isotope ratios are close to the pure r-process ratio 54:46 (Arlandini et al. 1999), and in the thick disk stars the isotope ratio is around 65:35 (±10%). Based on these data we deduce for thick disk stars the ratio of the s/r-process contribution to barium as 30:70 (±30%), in agreement with the results obtained from the [Eu/Ba] values
Formation of Galactic Systems in Light of the Magnesium Abundance in Field Stars: The Thick Disk
The space velocities and Galactic orbital elements of stars calculated from
the currently available high-accuracy observations in our summary catalog of
spectroscopic magnesium abundances in dwarfs and subgiants in the solar
neighborhood are used to identify thick-disk objects. The relative magnesium
abundances in thick-disk stars are shown to lie within the range
0.0<[Mg/Fe]<0.5 and to decrease with increasing metallicity starting from
[Fe/H]=-1.0. This is interpreted as evidence for a longer duration of the star
formation process in the thick disk. We have found vertical gradients in
metallicity (grad_Z[Fe/H]=-0.13\pm 0.04 kpc^{-1}) and relative magnesium
abundance (grad_Z [Mg/Fe]=0.06\pm 0.02 kpc^{-1}), which can be present in the
subsystem only in the case of its formation in a slowly collapsing protogalaxy.
The large spread in relative magnesium abundance (-0.3<[Mg/Fe]<0.5) in the
stars of the metal-poor "tail" of the thick disk which constitute 8% of the
subsystem, can be explained in terms of their formation inside isolated
interstellar clouds that interacted weakly with the matter of a single
protogalactic cloud. We have found a statistically significant negative radial
gradient in relative magnesium abundance in the thick disk (grad_R
[Mg/Fe]=-0.03\pm 0.01 kpc^{-1}) instead of the expected positive gradient. The
smaller perigalactic orbital radii and the higher eccentricities for
magnesium-richer stars, which among other stars, are currently located in a
small volume of the Galactic space near the Sun are assumed to be responsible
for the gradient inversion. A similar but statistically less significant
inversion is also observed in the subsystem for the radial metallicity
gradient.Comment: Accepted for 2005, Astronomy Letters, Vol. 31, No. 8, P.515-527; 14
pages, 6 figure
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.Comment: 15 pages, 6 figures, accepted 2009, Astronomy Reports, Vol. 53 No. 9,
P.785-80
Mg, Ba and Eu abundances in thick disk and halo stars
Our sample of cool dwarf stars from previous papers is extended in this study
including 15 moderately metal-deficient stars. The samples of halo and thick
disk stars have overlapping metallicities with [Fe/H] in the region from -0.9
to -1.5, and we first compare chemical properties of these two kinematically
different stellar populations independent on their metallicity. We present
barium, europium and magnesium abundances for the new sample of stars. The
results are based on NLTE line formation obtained in differential model
atmosphere analyses of high resolution spectra observed mainly using the UVES
spectrogragh at the VLT of ESO. We confirm the overabundance of Eu relative to
Mg in the halo stars. Eight halo stars show the [Eu/Mg] ratios between 0.23 and
0.41, whereas stars in the thick and thin disk display a solar Eu to Mg ratio.
The [Eu/Ba] values found in the thick disk stars to lie between 0.35 and 0.57
suggest that during the thick disk formation evolved low-mass stars started to
enrich the interstellar gas by s-nuclei of Ba, and the s-process contribution
to Ba varies from 30% to 50%. Based on these results, and using the chemical
evolution calculations by Travaglio et al. (1999), we estimate that the thick
disk stellar population formed on a timescale from 1.1 to 1.6 Gyr from the
beginning of the protogalactic collapse. In the halo stars the [Eu/Ba] ratios
are found mostly between 0.40 and 0.67, which suggests a duration of the halo
formation of about 1.5 Gyr. For the whole sample of stars we present the
even-to-odd Ba isotope ratios as determined from hyperfine structure seen in
the Ba II resonance line lambda4554. Based on these data we deduce for the
thick disk stars the ratio of the s/r-process contribution to barium as 30 : 70
(+- 30%), in agreement with the results obtained from the [Eu/Ba] ratios.Comment: Accepted for A&A, 10 pages, 6 figure
A Compiled Catalogue of Spectroscopically Determined Elemental Abundances for Stars with Accurate Parallaxes. I.Magnesium
We present a compiled catalogue of effective temperatures, surface gravities,
iron and magnesium abundances, distances, velocity components, and orbital
elements for stars in the solar neighborhood. The atmospheric parameters and
iron abundances are averages of published values derived from model synthetic
spectra for a total of about 2000 values in 80 publications. Our relative
magnesium abundances were found from 1412 values in 31 publications for 876
dwarfs and subgiants using a three-step iteration averaging procedure, with
weights assigned to each source of data as well as to each individual
determination and taking into account systematic deviations of each scale
relative to the reduced mean scale. The estimated assumed completeness for data
sources containing more than five stars, up to late December 2003, exceeds 90%.
For the vast majority of stars in the catalogue, the spatial velocity
components were derived from modern high-precision astrometric observations,
and their Galactic orbit elements were computed using a three-component model
of the Galaxy, consisting of a disk, a bulge, and a massive extended halo.Comment: Accepted for Astron. Rep. 2005, v.49, No.5, p.405-416, 12 pages, 10
figure
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 ( ~-0.2) and poorly mixed. However, the metallicity dispersion
continuously decreased with age, from \sigma_{[Fe/H]}~0.22 to ~0.13. All this
time, the mean relative abundance of Mg was somewhat higher than the solar
value (~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.Comment: 16 page, 7 figures, accepted 2011, Astron. Rep., v.55, No.8,
p.667-68