Galactic Archeology with the AEGIS Survey: The Evolution of Carbon and Iron in the Galactic Halo

Abstract

Understanding the evolution of carbon and iron in the Milky Way's halo is of importance because these two elements play crucial roles in constraining star formation, Galactic assembly, and chemical evolution in the early universe. Here we explore the spatial distributions of the carbonicity, [C/Fe], and metallicity, [Fe/H], of the halo system based on medium-resolution (R∼1300) spectroscopy of ∼58,000 stars in the southern hemisphere from the AAOmega Evolution of Galactic Structure (AEGIS) survey. The AEGIS carbonicity map exhibits a positive gradient with distance, as similarly found for the Sloan Digital Sky Survey carbonicity map of Lee et al. The metallicity map confirms that [Fe/H] decreases with distance from the inner halo to the outer halo. We also explore the formation and chemical evolution history of the halo by considering the populations of carbon-enhanced metalpoor (CEMP) stars present in the AEGIS sample. The cumulative and differential frequency of CEMP-no stars (as classified by their characteristically lower levels of absolute carbon abundance, A(C)7.1, for subgiants and giants) increases with decreasing metallicity and is substantially higher than previous determinations for CEMP stars as a whole. In contrast, that of CEMP-s stars (with higher A(C)) remains almost flat, at a value of ∼10% in the range −4.0[Fe/H]−2.0. The distinctly different behaviors of the CEMP-no and CEMP-s stars relieve the tension with population synthesis models assuming a binary mass-transfer origin, which previously struggled to account for the higher reported frequencies of CEMP stars, taken as a whole, at low metallicity.

Chemical Cartography. II. The Assembly History of the Galactic Stellar Halo Traced by Carbon-enhanced Metal-poor Stars

“…The p-values for V Φ and eccentricity (e) are derived by the K-S two sample test on the distributions of V Φ and eccentricity (e) of the CEMP-s and CEMP-no star samples. We also note that the [C/Fe] value of the CEMPno stars is somewhat lower than that of the CEMP-s stars for all three regions we considered, implying that the mechanism responsible for producing the CEMP-s stars produces more carbon at a given metallicity than that responsible for the CEMP-no stars, as revealed in several previous studies (e.g., Yong et al 2013;Spite et al 2013;Bonifacio et al 2015;Hansen et al 2016a,b;Yoon et al 2016Yoon et al , 2018.…”

“…After dividing CEMP stars into likely membership in the IHP and OHP by consideration of their orbital energies, they calculated the fraction of CEMPno and CEMP-s stars in each halo population, finding that the fraction of CEMP-no stars in the OHP is higher, by about a factor of two, than for CEMP-s stars, while almost equal fractions of CEMP-s and CEMP-no stars were found in the IHP. Yoon et al (2018) also reported similar fractions of the CEMP-no stars in the IHP and OHP, using subgiant and giant stars from the AEGIS survey. They classified the CEMP stars into the CEMPs and CEMP-no stars by A(C), as adopted in this study.…”

“…Furthermore, Paper I quantitatively estimated the fraction of CEMP-no and CEMP-s stars in the outerhalo region (OHR) and inner-halo region (IHR), classified on the basis of their derived A(C) 3 , and reported that the stars in the OHR exhibit a higher proportion of CEMP-no stars compared to CEMP-s stars, in contrast to the similar fractions of CEMP-no and CEMP-s stars in the IHR, consistent with results from previous, more-local studies. Yoon et al (2018) carried out a study similar to Paper I. They constructed a carbonicity map, using ∼ 58,000 stars, which dominantly consists of MSTO stars in the Southern Hemisphere observed by the AAOmega Evolution of Galactic Structure (AEGIS) survey, and identify the inner-and outer-halo regions based on this map.…”

The SkyMapper DR1.1 search for extremely metal-poor stars

“…The CEMP-stars are dominant at the lowest metallicities (e.g. Placco et al 2014;Yoon et al 2018). Consequently, we have investigated the [C/Fe] values for the stars with abundances [Fe/H] f itter < -3.0 dex.…”

“…13 indicates that there may be a minor bias towards that inclusion of lower metallicity stars in the sample, but accounting for it would only reduce the number of more metal-poor stars relative to the number at [Fe/H] f itter = -3.0 dex. It is also necessary to keep in mind that the observed sample is likely a mixture of both inner and outer halo populations (Carollo et al 2007(Carollo et al , 2010An et al 2013;Yoon et al 2018), which may or may not have similar MDFs for [Fe/H] -3 and below: our sample likely includes stars at Galactocentric distances of up to ∼50 kpc. We intend to perform a full kinematic and spatial analysis of the current sample in a later study.…”