Carbon-Enhanced Metal-Poor Stars: CEMP-s and CEMP-no Sub-Classes in the Halo System of the Milky Way (1401.0574v3)

Abstract: We explore the kinematics and orbital properties of a sample of 323 very metal-poor stars in the halo system of the Milky Way, selected from the high-resolution spectroscopic follow-up studies of Aoki et al. and Yong et al. The combined sample contains a significant fraction of carbon-enhanced metal-poor (CEMP) stars (22% or 29%, depending on whether a strict or relaxed criterion is applied for this definition). Barium abundances (or upper limits) are available for the great majority of the CEMP stars, allowing for their separation into the CEMP-$s$ and CEMP-no sub-classes. A new method to assign membership to the inner- and outer-halo populations of the Milky Way is developed, making use of the integrals of motion, and applied to determine the relative fractions of CEMP stars in these two sub-classes for each halo component. Although limited by small-number statistics, the data suggest that the inner halo of the Milky Way exhibits a somewhat higher relative number of CEMP-$s$ stars than CEMP-no stars (57% vs. 43%), while the outer halo possesses a clearly higher fraction of CEMP-no stars than CEMP-$s$ stars (70% vs. 30%). Although larger samples of CEMP stars with known Ba abundances are required, this result suggests that the dominant progenitors of CEMP stars in the two halo components were different; massive stars for the outer halo, and intermediate-mass stars in the case of the inner halo.

Carbon-Enhanced Metal-Poor Stars in the Milky Way Halo

The paper "Carbon-Enhanced Metal-Poor Stars: CEMP-s and CEMP-no Sub-Classes in the Halo System of the Milky Way" by Carollo et al. examines a sample of 323 very metal-poor (VMP) stars, with an emphasis on Carbon-Enhanced Metal-Poor (CEMP) stars, within the halo system of the Milky Way. This research not only categorizes these stars into CEMP-s and CEMP-no sub-classes but also explores their kinematic properties to enhance understanding of the Galactic halo and its evolution.

Study Overview

The authors analyzed stars from high-resolution spectroscopic studies, incorporating datasets from various surveys, including SDSS and the Hamburg/ESO survey. A significant fraction of the sample stars are CEMP stars, characterized by elevated carbon-to-iron ratios. The distinguishing feature is the presence (CEMP-s) or absence (CEMP-no) of substantial barium enhancements, attributable to s-process nucleosynthesis from asymptotic giant branch stars.

Kinematic Analysis and Methodology

Innovatively, the paper introduces a method of assigning stars to the inner and outer halo populations using their integrals of motion—specifically, energy and vertical angular momentum. The authors examined the space motions of stars via comprehensive orbital analyses involving parameters like peri-Galactic and apo-Galactic distances and eccentricity. It was observed that inner-halo stars predominantly exhibit high-eccentricity, low-energy orbits, while outer-halo stars show more retrograde, higher-energy motions.

Findings

The paper posits a dichotomy in the progenitor masses of the halo's sub-components. By employing statistical analyses—such as a Z-test—they tentatively conclude that the inner halo is dominated by CEMP-s stars implying an intermediate-mass progenitor population. Conversely, the outer halo predominantly hosts CEMP-no stars, suggesting a population influenced by massive star progenitors. This distribution reflects differing star formation conditions and slight variations in the initial mass function (IMF) possibly due to environmental factors at the time of fragmentation or accretion.

Implications and Future Work

Understanding the distribution of CEMP sub-classes offers crucial insights into the chemical history and formation processes of the Galactic halo. The results imply that the halo components originate from distinct star-forming environments—one potentially dominated by more massive stars capable of creating CEMP-no stars through phenomena like faint supernovae, while the intermediate-mass progenitors associated with CEMP-s stars relate to inner-halo conditions.

This paper underscores that expanding sample sizes and improving classifications of CEMP stars are imperative for robust conclusions. Future work may explore the link between IMF variability and galaxy formation theories, considering the merger histories and nucleosynthetic outputs of halo stars to affirm these preliminary findings.

Conclusion

Carollo et al. contribute to the nuanced understanding of the Milky Way's halo by examining distinct CEMP star types and their kinematic properties. These results foster a deeper comprehension of the galaxy’s formation and evolution, providing a framework for further investigations into the chemical and structural dynamics of our and other galaxies' halos.