Early Evolution of the Galactic Halo Revealed from [ITAL]Hipparcos[/ITAL] Observations of Metal-poor Stars

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Abstract
The kinematics of 122 red giant and 124 RR Lyrae stars in the solar neighborhood are studied using accurate measurements of their proper motions obtained by the Hipparcos astrometry satellite, combined with their published photometric distances, metal abundances, and radial velocities. A majority of these sample stars have metal abundances of [Fe/H] ≤ -1 and thus represent the old stellar populations in the Galaxy. The halo component, with [Fe/H] ≤ -1.6, is characterized by a lack of systemic rotation [(U, V, W) = (16 ± 18, -217 ± 21, -10 ± 12) km s-1] and a radially elongated velocity ellipsoid [(σU, σV, σW) = (161 ± 10, 115 ± 7, 108 ± 7) km s-1]. About 16% of such metal-poor stars have low orbital eccentricities (e < 0.4), and we see no evidence of a correlation between [Fe/H] and e. Based on the model for the e-distribution of orbits, we show that this fraction of low-e stars for [Fe/H] ≤ -1.6 is explained by the halo component alone, without introducing the extra disk component claimed by recent workers. This is also supported by the absence of a significant change in the e-distribution with height from the Galactic plane. In the intermediate-metallicity range (-1.6 < [Fe/H] ≤ -1), we find that stars with disklike kinematics have only modest effects on the distributions of rotational velocities and e for the sample at |z| < 1 kpc. This disk component appears to constitute only ~10% for -1.6 < [Fe/H] ≤ -1 and ~20% for -1.4 < [Fe/H] ≤ -1. It is also verified that this metal-weak disk has a mean rotation of ~195 km s-1 and a vertical extent of ~1 kpc, which is consistent with the thick disk's dominating at [Fe/H] = –0.6 to –1. We find no metallicity gradient in the halo, whereas there is an indication of a metallicity gradient in the metal-weak tail of the thick disk. The implications of these results for the early evolution of the Galaxy are also presented.
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