The Assembly of the Halo System of the Milky Way as Revealed by SDSS/SEGUE -- The CEMP Star Connection

In recent years, massive new spectroscopic data sets, such as the over half million stellar spectra obtained during the course of SDSS (in particular its sub-survey SEGUE), have provided the quantitative detail required to formulate a coherent story of the assembly and evolution of the Milky Way. The disk and halo systems of our Galaxy have been shown to be both more complex, and more interesting, than previously thought. Here we concentrate on the halo system of the Milky Way. New data from SDSS/SEGUE has revealed that the halo system comprises at least two components, the inner halo and the outer halo, with demonstrably different characteristics (metallicity distributions, density distributions, kinematics, etc.). In addition to suggesting new ways to examine these data, the inner/outer halo dichotomy has enabled an understanding of at least one long-standing observational result, the increase of the fraction of carbon-enhanced metal-poor (CEMP) stars with decreasing metallicity.


INTRODUCTION
For decades, "the halo" of the Galaxy has been thought of as a single entity, describable in terms of a single metallicity distribution function (MDF), single density profile, and a simple kinematical distribution that distinguishes it from the disk system. Indeed, much of the effort since its recognition as an important component of the Galaxy has concentrated on the effort to better specify these characteristics [2,11,12]. However, over this same period, numerous clues have emerged that this story is incomplete. Most of these clues were obtained in the form of disjoint small-n samples, making it difficult to "see the forest for the trees." This situation has now changed significantly.
EPJ Web of Conferences a clear retrograde net rotation ( v ∼ −80 km s −1 ), a kinematically "hotter" distribution of space velocities than the inner halo, and a MDF peaked at [Fe/H] ∼ −2.2. The fundamental demonstration of these results is contained in the original papers, as well as in [3], and will not be repeated here. We note, however, that the dual-halo model for the Milky Way has received considerable recent theoretical support, based on simulations for galaxy formation including baryons (as opposed to previous pure dark-matter-only simulations), e.g., [9,19], and [22].
In this brief communication, we highlight how recognition of the inner/outer halo structure of the Milky Way's halo has provided the means for understanding at least one long-standing "mystery" concerning the nature of a specific class of very metal-poor stars, the so-called carbon-enhanced metalpoor (CEMP) stars.

THE CEMP STARS
The CEMP stellar classification was originally defined as the subset of very metal-poor ([Fe/H] ≤ −2) stars that exhibit elevated carbon abundances relative to iron, [C/Fe] > +1.0 [2]. More recently, other criteria have been used, e.g., [C/Fe] > +0.7 [1], which appear to be better supported by the bulk of the available data.
In the last two decades it has been recognized that roughly 20% of stars with [Fe/H] < −2.0 exhibit enhanced [C/Fe] ratios (which we refer to as "carbonicity"), up to several orders of magnitude larger than the solar ratio [16,17] [4]); definitive explanations for the origin of this increase have yet to be offered. Regardless of the ultimate reason, these results indicate that significant amounts of carbon were produced in the early stages of chemical evolution in the Universe.
There exist a number of sub-classes of CEMP stars, some of which have been associated with proposed progenitor objects. The CEMP-s stars (those with s-process-element enhancement), for example, are the most commonly observed type to date. High-resolution spectroscopic studies have revealed that around 80% of CEMP stars exhibit s-process-element enhancement [1]. The favored mechanism invoked to account for these stars is mass transfer of carbon-enhanced material from the envelope of an asymptotic giant-branch (AGB) star to its binary companion; it is this surviving companion that is now observed as a CEMP-s star.
The sub-class of CEMP-no stars (which exhibit no strong neutron-capture-element enhancements) is particularly prevalent among the lowest metallicity stars (Fe/H < −2.5). Possible progenitors for this class include massive, rapidly rotating, mega metal-poor ([Fe/H] < −6.0) stars, which models suggest have greatly enhanced abundances of CNO due to distinctive internal burning and mixing episodes, followed by strong mass loss [20]. Another suggested mechanism for the production of the material incorporated into CEMP-no stars is pollution of the interstellar medium by so-called faint supernovae associated with the first generations of stars, which experience extensive mixing and fallback during their explosions [24].
The increasing frequency of CEMP stars with declining metallicity, as well as the suggested increase of the fraction of CEMP stars with increasing distance from the Galactic plane [10], can be explained in the context of an inner/outer halo dichotomy, and the dominance of different carbon production mechanisms (the processes associated with the progenitors of the CEMP-s and CEMP-no stars) being linked to these two populations.

THE INNER/OUTER HALO, CEMP-STAR CONNECTION
The recent study of Carollo et al. (2011, [7]) illuminates the intimate connection between the nature of the halo system and the observed facts concerning CEMP stars. Among other results, these authors find that:

IMPLICATIONS FOR GALAXY FORMATION
The large fractions of CEMP stars in both halo components indicates that significant amounts of carbon were produced in the early stages of chemical evolution in the Universe. The observed contrast of CEMP star fractions in the inner-and outer-halo populations strengthen the picture that the halo components had different origins, and supports a scenario in which the outer-halo component has been assembled by the accretion of small subsystems. In this regard, it is interesting that the MDF of the inner halo The fact that, in the metal-poor regime, the outer halo exhibits a fraction of CEMP stars that is larger than the inner halo suggests that multiple sources of carbon, besides the nucleosynthesis of AGB stars in binary systems, were present in the pristine environment of the outer-halo progenitors (lower mass subhalos). These sources could be the fast massive rotators and/or the faint supernovae mentioned above. If the CEMP stars in the outer halo are predominantly CEMP-no stars (which has yet to be established), it might suggest that non-AGB-related carbon production took place in the primordial mini-halos. The predominance of CEMP-s stars in the inner halo, if found, would suggest that the dominant source of carbon was the nucleosynthesis in AGB stars in a binary system. This would place important constraints on the primordial IMF of the sub-systems responsible for the formation of the two halo components. Recent efforts to model, from the population synthesis standpoint, the fractions of observed CEMP stars in the halo have struggled to reproduce results as high as 15-20% for metallicities [Fe/H] < −2.0 [13,21]. Note, however, that these predictions are based solely on carbon production by AGB stars. While such calculations may prove meaningful for the inner-halo population, they may not be telling The green and red filled circles show the location of the observed CEMP star fractions for the inner halo and outer halo, respectively, based on the kinematic deconvolution method described by [7]. the full story for carbon production associated with the progenitors of the outer-halo population. The lower CEMP fraction in the inner halo may relieve some of the tension with current model predictions.
The fact that the CEMP star fraction exhibits a clear increase with |Z| suggests that the relative numbers of CEMP stars in a stellar population is not driven by metallicity alone. The proposed coupling of the cosmic microwave background to the IMF [14,23] is one mechanism for imposing a temporal dependence on the IMF. This effect, coupled with chemical evolution models, predicts that the CEMP fraction would be expected to increase as the metallicity decreases, but with similar metallicity regimes forming carbon according to the expected yields of the predominant mass range available at that time.
In the hierarchical context of galaxy formation, star-forming regions are spatially segregated, and their chemical evolution can proceed at different rates, with stars at the same metallicity forming at different times. Depending on the source(s) of carbon, this trend could lead to a spatial variation of the CEMP fraction at the same metallicity, increasing in older populations and decreasing in younger ones.

THE PATH FORWARD
We have used this article to illustrate one application of the dual-halo model to understand the formerly unexplained increase in the fraction of CEMP stars with declining metallicity, demonstrating that it arises from the differing nature of the stellar populations associated with the inner/outer halo.
Further exploration of the CEMP stars, in particular the CEMP-no sub-class, which may well have arisen from non-AGB sources of carbon production, should prove of great interest. Connections of such stars with chemical production in early epochs are now being made. For example, the recently reported high-redshift (z = 2.3), extremely metal-poor Damped Lyman-system ( [8], [Fe/H] ∼ −3.0) exhibits enhanced carbonicity ([C/Fe] = +1.5) and other elemental abundance signatures that are quite similar to those of the CEMP-no stars, and which [15] associate with production by faint supernovae. It is also of interest that evidence for strong carbon production in the early Universe has been reported [18], based on analysis of the optical spectrum of the most distant known radio galaxy, TN J0924-2201, with z = 5.19. When viewed in this light, the CEMP-no stars may well prove to be our local touchstone to the abundance patterns produced by the very FIRST generations of stars, an exciting prospect!