Astronomy and Astrophysics – Astronomy
Scientific paper
Aug 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996aj....112..668c&link_type=abstract
Astronomical Journal v.112, p.668
Astronomy and Astrophysics
Astronomy
152
Galaxy: Halo, Galaxy: Stellar Content, Galaxy: Kinematics And Dynamics
Scientific paper
Based on our expanded sample of metallicities and kinematics for a large 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]≤-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 orbital eccentricities. Further, in spite of the strong kinematical biases in our sample, stars with very low metallicities are found that have small V velocities (high orbital angular momenta) and low orbital eccentricities. These results contradict the model that the metal-poor stars are a single population that is only the relic of the earliest stages of the Galaxy's collapse. There are signs that some of the metal-poor stars in the solar neighborhood are due to accretion events and, perhaps, also to the earliest stages of the formation of the Galactic disk. Regarding accretion, we confirm Majewski's [ApJS, 78, 87 (1992)] finding of a retrograde rotation among stars that reach S kpc or more from the plane. These stars do not show any radial metallicity gradient, and may be younger on average than dynamically hot, metal-poor stars closer to the plane. These latter stars show net prograde rotation and a radial metallicity gradient, suggestive of a dissipative process in the earliest stages of disk formation. The correlation between metallicity and perigalacticon found by Ryan & Norris [AJ, 101, 1835 (1991a)] disappears when care is taken to exclude the stars that may have been accreted by our Galaxy. The field star results complement those for globular clusters found by other workers, notably Zinn (1993), who argued for two populations of metal-poor clusters, one apparently in retrograde rotation with no radial metallicity gradient and slightly younger ages, and the other with prograde rotation, a weak radial metallicity gradient, and slightly older ages. The field stars and globular clusters do differ slightly, however. Their metallicity distributions differ, with the field stars showing a larger fraction of the most metal-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 >Rapo< plane, possible signs of large-scale kinematic substructure suggestive of specific accretion events. We also see signs for the Preston et al. [AJ, 108, 538 (1994)] 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 hand, the "away" versus "toward" mystery of Croswell et al. [Al, 93, 1445 (1987)] has disappeared: the numbers of stars approaching and receding from the plane agree with expectations. Finally, we point out that the model of Norris [ApJ, 431, 645 (1994)] for a proto-disk population that is hotter dynamically than the accreted halo components does not agree with our expanded data sample. We suggest that the proto-disk component was dynamically cooler when the mean metallicity was very low.
Aguilar Arevalo L.
Carney Bruce W.
Laird John B.
Latham David W.
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