A SURVEY OF PROPER MOTION STARS .13. THE HALO POPULATION(S)

Based on our expanded sample of metallicities and kinematics for a lar ge sample of stars selected from the Lowell Proper Motion Catalog, we study several questions relating to the halo stellar population(s) in our Galaxy. For [m/H] less than or equal to -1.4, there does not seem to be any variation with [m/H] in the mean values of the V velocity (i ,e., angular momentum related to that in the disk) or the Galactic orb ital eccentricities. Further, in spite of the strong kinematical biase s in our sample, stars with very low metallicities are found that have small V velocities (high orbital angular momenta) and low orbital ecc entricities. These results contradict the model that the metal-poor st ars are a single population that is only the relic of the earliest sta ges of the Galaxy's collapse. There are signs that some of the metal-p oor stars in the solar neighborhood are due to accretion events and, p erhaps, also to the earliest stages of the formation of the Galactic d isk. Regarding accretion, we confirm Majewski's [ApJS, 78, 87 (1992)] finding of a retrograde rotation among stars that reach 5 kpc or more from the plane. These stars do not show any radial metallicity gradien t, and may be younger on average than dynamically hot, metal-poor star s closer to the plane. These latter stars show net prograde rotation a nd a radial metallicity gradient, suggestive of a dissipative process in the earliest stages of disk formation. The correlation between meta llicity and perigalacticon found by Ryan & Norris [AJ, 101, 1835 (1991 a)] disappears when care is taken to exclude the stars that may have b een accreted by our Galaxy, The field star results complement those fo r globular clusters found by other workers, notably Zinn (1993), who a rgued for two populations of metal-poor clusters, one apparently in re trograde rotation with no radial metallicity gradient and slightly you nger ages, and the other with prograde rotation, a weak radial metalli city gradient, and slightly older ages, The field stars and globular c lusters do differ slightly, however. Their metallicity distributions d iffer, with the field stars showing a larger fraction of the most meta l-poor stars. This could be caused by accretion of Draco dwarf galaxy- like objects, with very low metallicities and no globular clusters, We see in our data, particularly in the V vs [R(apo)] plane, possible si gns of large-scale kinematic substructure suggestive of specific accre tion events. We also see signs for the Preston er al. [AJ, 108, 538 (1 994)] low-metallicity,intermediate kinematics, and younger age stellar population. However, the strength of the signal in our data suggests that a fairly large fraction of its stars may be old. On the other han d, the ''away'' versus ''toward'' mystery of Croswell et al. [AJ, 93, 1445 (1987)] has disappeared: the numbers of stars approaching and rec eding from the plane agree with expectations. Finally, we point out th at the model of Norris [ApJ, 431, 645 (1994)] for a proto-disk populat ion that is hotter dynamically than the accreted halo components does not agree with our expanded data sample. We suggest that the proto-dis k component was dynamically cooler when the mean metallicity was very low. (C) 1996 American Astronomical Society.