Population studies. XII. The duality of the Galactic halo

Abstract

Consideration of the abundance distribution and the complicated dependence of kinematics on abundance in high-proper-motion samples of main-sequence stars, together with the velocities of non-kinematically selected objects in the direction of the Galactic poles, leads to the conjectures (1) that for [Fe/H] < -1.0, two distinct components are present in the samples, each of which spans the range -3.0 ≤ [Fe/H] ≤ -1.0, (2) that one of the components has the properties of the accretion process postulated for the formation of the halo by Searle & Zinn (1978) and amplified by Rodgers & Paltoglou (1984), van den Bergh (1993), and Zinn (1993), and (3) that the second component encompasses at high abundance ([Fe/H] > -1.5) the disklike entity variously referred to as the thick disk, the metal-weak thick disk, the extended disk, and Intermediate Population II, together with, at lower abundances, material which has an abundance distribution similar to that of the so-called Best Accretion Model of Lynden-Bell and kinematics which have a very hot spheroidal signature (σU = σV = σW ∼ 130 km s-1 and low systemic rotation Vrot) at the lowest values ([Fe/H]∼ -2.0 to -3.0). The second component has many of the properties espoused by Eggen, Lynden-Bell, & Sandage (1962) in the context of the contraction of the proto-Galaxy. Monte Carlo simulations are presented which demonstrate that, within the framework of these conjectures and the rather large uncertainties associated with the second component, it is possible to explain fully the basic features of the abundance distribution and kinematics of the high-proper-motion samples. The crux of the issue is that such a model can explain the observed relative constancy of σU and Vrot, for [Fe/H] ≤ -1.5, together with the constancy of σV and the steady increase of σW from 50 to 120 km s-1 as [Fe/H] decreases from -1.0 to -2.5. It is emphasized that within the suggested framework neither the precepts of Searle & Zinn nor those of Eggen et al. can by themselves fully explain the totality of the observations. For kinematically selected samples the combination of both as essential ingredients can provide an explanation. The possibility exists that a double accretion scenario as described by Freeman (1987) following Quinn & Goodman (1986) might also explain the binarity conjectured upon here, but more information on the details of the accretion is needed to facilitate the comparison between theory and observation. Show more