CEMP

CEMP stands for carbon-enhanced metal-poor stars, a class of metal-poor stars characterized by a notable enhancement of carbon relative to iron. They are typically defined by a metallicity [Fe/H] below about -1.0 to -2.0 and a carbon-to-iron abundance ratio [C/Fe] greater than about +1.0.

CEMP stars are subdivided according to abundances of neutron-capture elements into several groups: CEMP-s, with enriched s-process elements such as barium; CEMP-r, with enhanced r-process elements; CEMP-r/s, with both; and CEMP-no, lacking neutron-capture element enhancement. CEMP-s stars often show high Ba and other s-process elements and are frequently found in binary systems. CEMP-no stars generally do not show s- or r-process enrichment.

Formation and interpretation: The leading explanation for CEMP-s is mass transfer of carbon- and s-process-rich material from an asymptotic giant branch companion in a binary system. CEMP-no stars are thought to reflect the composition of the early interstellar medium, polluted by Population III stars or faint supernovae with mixing and fallback. The origins of CEMP-r and CEMP-r/s are tied to specific early neutron-capture nucleosynthesis events and may trace different enrichment channels.

Observational context: CEMP stars dominate the metal-poor tail of the halo stellar population and are also identified in dwarf spheroidal galaxies. They have been found through wide-field spectroscopic surveys such as the Hamburg/ESO survey and the Sloan Digital Sky Survey, with follow-up high-resolution spectroscopy to determine detailed abundances.

Significance: Studying CEMP stars provides constraints on early Galactic chemical evolution, nucleosynthesis in the first generations of stars, and the distribution of carbon and heavy elements in the early Milky Way.