The halo of the Milky Way provides unique elemental abundance and kinematic information on the first objects to form in the Universe, and this information can be used to tightly constrain models of galaxy formation and evolution. Although the halo was once considered a single component, evidence for its dichotomy has slowly emerged in recent years from inspection of small samples of halo objects. Here we show that the halo is indeed clearly divisible into two broadly overlapping structural components-an inner and an outer halo-that exhibit different spatial density profiles, stellar orbits and stellar metallicities (abundances of elements heavier than helium). The inner halo has a modest net prograde rotation, whereas the outer halo exhibits a net retrograde rotation and a peak metallicity one-third that of the inner halo. These properties indicate that the individual halo components probably formed in fundamentally different ways, through successive dissipational (inner) and dissipationless (outer) mergers and tidal disruption of proto-Galactic clumps.Astronomers have long sought to constrain models for the formation and evolution of the Milky Way (our Galaxy) on the basis of observations of the stellar and globular cluster populations that it contains. These populations are traditionally defined as samples of objects that exhibit common spatial distributions, kinematics and metallicities (the age of a population, when available, is also sometimes used). Metallicity is taken by astronomers to represent the abundances of elements heavier than helium, which are only created by nucleosynthesis in stars-either internally via nuclear burning in their cores or externally during explosive nucleosynthesis at the end of their lives. The earliest generations of stars have the lowest metallicities, because the gas from which they formed had not been enriched in heavy elements created by previous stars and distributed throughout the primordial interstellar medium by stellar winds and supernovae.Previous work has provided evidence that the halo of the Milky Way may not comprise a single population, primarily from analysis of the spatial profiles (or inferred spatial profiles) of halo objects [1][2][3][4] . A recent example of such an analysis is the observation of two different spatial density profiles for distinct classes of RR Lyrae variable stars in the halo 5 . In addition, tentative claims for a net retrograde motion of halo objects by previous authors supports the existence of a likely dual-component halo [6][7][8][9][10] . The central difficulty in establishing with confidence whether or not a dichotomy of the halo populations exists is that the past samples of tracer objects have been quite small, and usually suitable only for consideration of a limited number of the expected signatures of its presence.In the present work, we examine this question in detail using a large, homogeneously selected and analysed sample of over 20,000 stars, originally obtained as calibration data during the course of the Sloan Digital Sk...

The chemical compositions of 26 metal-poor stars that exhibit strong CH and/or C 2 molecular bands are determined based on high-resolution spectroscopy. We define carbon-enhanced stars taking account of the carbon abundance ratio ([C/Fe]) and the evolutionary status, which is a slight modification over previous definitions. Twenty two stars in our sample satisfy our modified definition for Carbon-Enhanced Metal-Poor (CEMP) stars. In addition, we measure Na abundances for nine other carbon-enhanced stars for which abundances of other elements have been previously reported. Combining our new sample with the results of previous work, we investigate the abundance and evolutionary status of a total of 64 CEMP stars. The following results are obtained:(1) All but one of the 37 stars with [Fe/H] ≥ −2.6 exhibit large excesses of barium ([Ba/Fe] > +0.5), while the other 27 stars with lower metallicity exhibit a large scatter in their barium abundance ratios (−1.2 < [Ba/Fe]<+3.3). (2) A correlation between the carbon and barium abundance ratios ([C/Fe] and [Ba/Fe]) is found in Ba-enhanced objects (comprising 54 stars), suggesting that the origin of the observed carbon excess in Ba-enhanced stars is nucleosynthesis in asymptotic giant branch (AGB) stars, where the main s-process occurs. The correlation between the barium abundance ratio and that of carbon plus nitrogen ([(C+N)/Fe]) is relatively weak, because of the large excesses of nitrogen in some extremely metal-poor stars. (3) The majority of the Ba-enhanced stars have −1.0 < [C/H] < 0.0, and a clear cutoff exists at [C/H] ∼ 0, which we take as the limit of carbon-enrichment by metal-poor AGB stars. Within the above range,

The structure and kinematics of the recognized stellar components of the Milky Way are explored, based on well-determined atmospheric parameters and kinematic quantities for 32360 "calibration stars" from the Sloan Digital Sky Survey (SDSS) and its first extension, (SDSS-II), which included the sub-survey SEGUE: Sloan Extension for Galactic Understanding and Exploration. Full space motions for a sub-sample of 16920 stars, exploring a local volume within 4 kpc of the Sun, are used to derive velocity ellipsoids for the inner-and outerhalo components of the Galaxy, as well as for the canonical thick-disk and proposed metal-weak thick-disk populations. This new sample of calibration stars represents an increase of 60% relative to the numbers used in a previous analysis. We first examine the question of whether the data require the presence of at least a two-component halo in order to account for the rotational behavior of likely halo stars in the local volume, and whether more than two components are needed. We also address the question of whether the proposed metalweak thick disk is kinematically and chemically distinct from the canonical thick disk, and point out that the Galactocentric rotational velocity inferred for the metal-weak thick disk, as well as its mean metallicity, appear quite similar to the values derived previously for the Monoceros stream, suggesting a possible association between these structures. In addition, we consider the fractions of each component required to understand the nature of the observed kinematic behavior of the stellar populations of the Galaxy as a function of distance from the plane. Scale lengths and scale heights for the thick-disk and metal-weak thick-disk components are determined. Spatial density profiles for the inner-and outer-halo populations are inferred from a Jeans Theorem analysis. The full set of calibration stars (including those outside the local volume) is used to test for the expected changes in the observed stellar metallicity distribution function with distance above the Galactic plane in-situ, due to the changing contributions from the underlying stellar populations. The above issues are considered, in concert with theoretical and observational constraints from other Milky-Way-like galaxies, in light of modern cold dark matter galaxy formation models.