Discovery of the First Five Carbon-enhanced Metal-poor Stars in the LMC

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 fifth-generation Sloan Digital Sky Survey (SDSS-V) spectra from the Baryon Oscillation Spectroscopic Survey 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. Their absolute carbon abundances and metallicities classify them as Group I CEMP stars, suggesting binary mass-transfer origins, though neutron-capture abundance measurements are required to confirm whether this classification scheme applies beyond the Milky Way. Although these stars were selected as the most promising CEMP candidates from the SDSS-V sample, likely biasing this initial sample toward higher absolute carbon abundances, their discovery suggests that previous null detections of CEMP stars in the LMC were caused by metallicity-sensitive photometric targeting biases against high [C/H] stars. A forthcoming analysis of the full spectroscopic sample will push to lower carbon abundances, providing a more complete census and enabling critical tests of whether environmental differences shape the formation channels and frequencies of CEMP stars in this system.