Kinematics of Metal-poor Stars in the Galaxy. III. Formation of the Stellar Halo and Thick Disk as Revealed from a Large Sample of Nonkinematically Selected Stars

Abstract

We present a detailed analysis of the space motions of 1203 solar-neighborhood stars with metal abundances [Fe/H] ¹ [0.6, on the basis of a catalog, of metal-poor stars selected without kinematic bias recently revised and supplemented by Beers et al. This sample, having available proper motions, radial velocities, and distance estimates for stars with a wide range of metal abundances, is by far the largest such catalog to be assembled to date. We show that the stars in our sample with [Fe/H] ¹[2.2, which likely represent a "" pure ÏÏ halo component, are characterized by a radially elongated velocity ellipsoid 106^9, 94^8) km s~1 and small prograde rotation to 50 km, consistent with previous analysis of this sample by Beers and Sommer-Larsen based on radial velocity information alone. In contrast to the previous analysis, we Ðnd a decrease in with increasing SV Õ T distance from the Galactic plane for stars that are likely to be members of the halo population km s~1 kpc~1), which may represent the signature of a dissipatively formedUnlike essentially all previous kinematically selected catalogs, the metal-poor stars in our sample exhibit a diverse distribution of orbital eccentricities, e, with no apparent correlation between [Fe/H] and e. This demonstrates, clearly and convincingly, that the evidence o †ered in 1962 by Eggen, Lynden-Bell, & Sandage for a rapid collapse of the Galaxy, an apparent correlation between the orbital eccentricity of halo stars with metallicity, is basically the result of their proper-motion selection bias. However, even in our nonkinematically selected sample, we have identiÐed a small concentration of high-e stars at [Fe/H] D [1.7, which may originate, in part, from infalling gas during the early formation of the Galaxy. We Ðnd no evidence for an additional thick disk component for stellar abundances We also analyze the global kinematics of the stars constituting the halo component of the Galaxy. The outer part of the halo, which we take to be represented by local stars on orbits reaching more than 5 kpc from the Galactic plane, exhibits no systematic rotation. In particular, we show that previous suggestions of the presence of a "" counter-rotating high halo ÏÏ are not supported by our analysis. The density distribution of the outer halo is nearly spherical and exhibits a power-law proÐle that is accurately described as o P R~3.55B0.13. The inner part of the halo is characterized by a prograde rotation and a highly Ñattened density distribution. We Ðnd no distinct boundary between the inner and outer halo. We conÐrm the clumping in angular-momentum phase space of a small number of local metalpoor stars noted in 1999 by Helmi et al. We also identify an additional elongated feature in angularmomentum phase space extending from the clump to regions with high azimuthal rotation. The number of members in the detected clump is not signiÐcantly increased from that reported by Helmi et al., even though the total number of the sample stars we consider is almost triple that of the prev...

Spectroscopic Identification of White Dwarfs in the New Luyten Two-Tenths Survey

“…AV S consistent with a thick disk population membership, and the dispersion in W, km s , is consistent with an old thin Ϫ1 j p 26 W disk population as defined by Yoss, Bell, & Detweiler (1991) based on a correlation between the metallicity of G-K stars and their kinematics (see also Chiba & Beers 2000). However, the radial velocity errors are large, and new high-dispersion measurements are required to refine the kinematics.…”

Stellar Populations of the Galactic Disk: Metallicity Distribution and Kinematics

“…This fact provides evidence for the presence of a vertical gradient in the rotational velocity, which, if confirmed with additional work, may imply that a gradual heating of a precursor thin disk might have taken place. Such a kinematic gradient was found in the thick disk by Majewski (1992) and Chiba & Beers (2000), but was not observed by Soubiran et al (2003) in their sample of Tycho-2 stars at the north Galactic pole.…”

Exploring Halo Substructure with Giant Stars. III. First Results from the Grid Giant Star Survey and Discovery of a Possible Nearby Sagittarius Tidal Structure in Virgo

“…If we assume a probability of finding similar outliers in the three other Table 1 fields is also ≈0.3, then the probability of finding a set of outliers in eight consecutive fields is 0.0006. Similar simulations using the thick-disk velocity ellipsoid from Chiba & Beers (2000) in conjunction with a constant circular velocity of 200 km s Ϫ1 yield probabilities of finding a random outlier from less than 0.001 to 0.35 in each of our fields, with a formal probability of a pure disk population indistinguishable from 0. Choosing the larger of the halo or thick-disk outlier probability in each field yields a probability of at most 0.001 for finding outliers in eight consecutive fields.…”