71 research outputs found
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 \alpha-elements in their metal-poor stars
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 ( =
-0.22), badly mixed (\sigma_[Fe/H] = 0.21), and the average relative magnesium
abundance was comparatively high ( = 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
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
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