Three-dimensional orbits of metal-poor halo stars and the formation of the Galaxy

We present the three-dimensional orbital motions of metal-poor stars in conjunction with their metal abundances, for the purpose of getting insight into the formation process of the Galaxy. Our sample stars, which include metal-deficient red giants and RR Lyrae variables observed by the HIPPARCOS satellite, are least affected by known systematics, stemming from kinematic bias, metallicity calibration, and secondary metal contamination of stellar surface. We find, for the stars in the metallicity range of [Fe/H] ≤ -1, that there is no evidence for the correlation between [Fe/H] and their orbital eccentricities, e. Even for [Fe/H] ≤ -1.6, about 16% of the stars have e values less than 0.4. We show that the e distribution of orbits for [Fe/H] ≤ -1.6 is independent of the height |z| away from the Galactic plane, whereas for [Fe/H] > -1.6, the stars at |z| ≥ 1 kpc are systematically devoid of low-e orbits with e ≲ 0.6. This indicates that low-e stars with [Fe/H] ≤ -1.6 belong to the halo component, whereas the rapidly rotating thick disk with a scale height of ∼1 kpc has a metal-weak tail in the range of -1.6 < [Fe/H] ≤ -1. The fraction of this metal-weak thick disk appears to be less than 20%. The significance of these results for the early evolution of the Galaxy is briefly discussed.