Relationship between the velocity ellipsoids of galactic-disk stars and their ages and metallicities |
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Authors: | V V Koval’ V A Marsakov T V Borkova |
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Institution: | 1.Southern Federal University,Rostov-on-Don,Russia |
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Abstract: | 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 velocity, obey power laws with indices 0.25, 0.29, 0.32,
and 0.27 (with uncertainties ±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. Elimination of stars in the most numerous moving groups from the sample slightly reduces the corresponding
indices (0.22, 0.26, 0.27, and 0.24). Limiting the sample to stars within 60 pc of the Sun, so that the sample can be considered
to be complete, leaves both the velocity ellipsoids and their age dependences virtually unchanged. With increasing age, the
velocity ellipsoid increases in size and becomes appreciablymore 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, recorded from the
close to the Sun, is most likely due to the radial migration of stars. |
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