Discovery of the First Five Carbon-Enhanced Metal-Poor Stars in the LMC

TL;DR

This work investigates whether Carbon-Enhanced Metal-Poor (CEMP) stars occur in the Large Magellanic Cloud (LMC) and how their formation channels might depend on environment. Using SDSS-V BOSS spectra from the Magellanic Genesis program, the authors identify and analyze five CEMP stars in the LMC, deriving $[Fe/H]$ values from $-2.1$ to $-3.2$ and evolutionary-state corrected $[C/Fe]$ from $+1.2$ to $+2.4$, with indications that they may belong to the CEMP-$s$ subclass. They validate metallicities with Ca II triplet fits and determine carbon abundances from CH G-band and C$_2$ Swan-band fits, applying evolutionary corrections per Placco ($2014$). The results establish the existence of CEMP stars in the LMC and provide a foundation for measuring CEMP occurrence rates in this environment, enabling tests of environmental effects on early chemical evolution and nucleosynthesis. Future homogeneous analyses of the full LMC/SMC SDSS-V sample will quantify CEMP frequencies and refine constraints on the early-universe nucleosynthesis channels across galaxy environments.

Abstract

A substantial fraction of metal-poor stars in the local Milky Way halo exhibit large overabundances of carbon. These stars, dubbed Carbon-Enhanced Metal-Poor (CEMP) stars, provide crucial constraints on the nature of the early universe including the earliest nucleosynthetic events. Whether these stars exist at similar rates in nearby galaxies is a major open question with implications for the environmental dependence of early chemical evolution. Here, we present the discovery of the first five CEMP stars in the Milky Way's largest dwarf companion, the LMC, using SDSS-V spectra from the BOSS instrument. We measure metallicities ranging from [Fe/H] = -2.1 to -3.2 and evolutionary state corrected carbon enhancements of [C/Fe] = +1.2 to +2.4, placing these stars among the most metal-poor and carbon-rich ever identified in the LMC. This discovery demonstrates that CEMP stars do exist in the LMC despite previous null detections, and establishes the foundation for measuring the CEMP occurrence rate in this system. Such measurements will provide critical tests of whether environmental differences affect the formation channels and frequencies of these ancient, carbon-rich stars.

Figures (9)

• Figure 1: Distribution of sky positions (left panel), and proper motions (right panel) in Magellanic Stream coordinates Nidever2008 of the five CEMP stars (red stars) relative to the rest of the Magellanic Clouds sample in SDSS-V DR20. All five stars have sky coordinates and proper motions consistent with LMC membership.
• Figure 2: The MINESweeper stellar parameters for our five CEMP stars (black stars) relative to the rest of the SDSS-V DR20 Magellanic Genesis sample which is colored by metallicity. The five stars lay on the hotter edge of the red giant branch (RGB), consistent with having some of the lowest metallicities in the sample.
• Figure 3: A region of the BOSS spectra (grey) for the five CEMP stars with the best-fit synthetic spectral model (blue) overlaid. The spectral models shown are used for fitting the carbon abundance while the $\rm{T_{eff}}$ and log $g$ are fixed to the MINESweeper parameters. This region contains the CH G-band at 4305 Å and the $\rm{C_2}$ Swan bands, with the strongest feature at 5165 Å. The MINESweeper analysis also uses a subset of this region (4750-5550 Å) for the stellar parameter analysis with a mask for the $\rm{C_2}$ band at $5060$--$5180$ Å.
• Figure 4: Carbon abundances ([C/Fe]) as a function of [Fe/H] for the five CEMP stars compared to literature samples. Specifically, we compare to results from Chiti2024 which reports abundances for some of the most metal-poor stars known in the LMC, along with results from APOGEE spectra in SDSS-IV DR17. The commonly-used definition for CEMP ([C/Fe]>0.7) is shown as a black dashed line, with the five stars from this work being the only stars above said line.
• Figure 5: The absolute Carbon abundance (A(C)) as a function of [Fe/H] for the five CEMP stars compared to a literature sample from the local Milky Way halo where the [Ba/Fe] abundances are also measured. The literature sample is seperated by [Ba/Fe] where stars with [Ba/Fe]>1 are labeled CEMP-s stars and are shown in blue whilte stars with [Ba/Fe]$\leq$ 0 are labeled CEMP-no and are shown in red. The black dashed line indicates [C/Fe]=0.7. The grey dashed lines indicate the classification system of CEMP introduced by Yoon2016, where Group I is the top right section and are mainly CEMP-s, Group II and II are the bottom and top left sections and are mainly CEMP-no stars.