Origins of carbon-enhanced metal-poor stars

TL;DR

This paper uses cosmological simulations to explore the origins of carbon-enhanced metal-poor stars in the Milky Way, revealing two main formation pathways linked to different stellar enrichment processes.

Contribution

It introduces a novel simulation-based scenario explaining the formation of CEMP stars through enrichment by SNe and AGB stars, matching observed abundance trends.

Findings

Two classes of CEMP stars identified: CEMP-no and CEMP-s.

Simulation reproduces observed abundance patterns and trends.

CEMP-no stars linked to early galaxy reionization contributors.

Abstract

We investigate the nature of carbon-enhanced metal poor (CEMP) stars in Milky Way (MW) analogues selected from the EAGLE cosmological hydrodynamical simulation. The stellar evolution model in EAGLE includes the physics of enrichment by asymptotic giant branch (AGB) stars, winds from massive stars, and type I and type II supernovae (SNe). In the simulation, star formation in young MW progenitors is bursty due to efficient stellar feedback, which causes poor metal mixing leading to the formation of CEMP stars with extreme abundance patterns. In this scenario, two classes of CEMP stars emerge: those mostly enriched by low-metallicity type II SNe with low Fe yields that drive galactic outflows, and those mostly enriched by AGB stars when a gas-poor progenitor accretes pristine gas. The first class resembles CEMP-no stars with high [C/Fe] and low [C/O], the second class resembles CEMP-s stars overabundant in s-process elements and high values of [C/O]. This scenario explains several trends seen in data: (i) the increase in the scatter and median of [C/O] at low and decreasing [O/H], (ii) the trend of stars with very low [Fe/H] or [C/H] to be of type CEMP-no, and (iii) the reduction in the scatter of [{\alpha}/Fe] with atomic number in metal poor stars. In this interpretation CEMP-no stars were enriched by the stars that enabled galaxies to reionise the Universe.