The role of faint population III supernovae in forming CEMP stars in ultra-faint dwarf galaxies

Abstract

CEMP-no stars, a subset of carbon enhanced metal poor (CEMP) stars ($\rm [C/Fe]\geq0.7$ and $\rm [Fe/H]\lesssim-1$) have been discovered in ultra-faint dwarf (UFD) galaxies, with $M_{\rm vir} \sim 10^8$ Msun and $M_{\ast}\sim10^3-10^4$ Msun at $z=0$, as well as in the halo of the Milky Way (MW). These CEMP-no stars are local fossils that may reflect the properties of the first (Pop~III) and second (Pop~II) generation of stars. However, cosmological simulations have struggled to reproduce the observed level of carbon enhancement of the known CEMP-no stars. Here we present new cosmological hydrodynamic zoom-in simulations of isolated UFDs that achieve a gas mass resolution of $m_{\rm gas}\sim60$ Msun. We include enrichment from Pop~III faint supernovae (SNe), with $ E_{\rm SN}=0.6\times10^{51}$ erg, to understand the origin of CEMP-no stars. We confirm that Pop~III and Pop~II stars are mainly responsible for the formation of CEMP and C-normal stars respectively. New to this study, we find that a majority of CEMP-no stars in the observed UFDs and the MW halo can be explained by Pop~III SNe with normal explosion energy ($ E_{\rm SN}=1.2\times10^{51}$~erg) and Pop~II enrichment, but faint SNe might also be needed to produce CEMP-no stars with $\rm [C/Fe]\gtrsim2$, corresponding to the absolute carbon abundance of $\rm A(C)\gtrsim6.0$. Furthermore, we find that while we create CEMP-no stars with high carbon ratio $\rm [C/Fe]\approx3-4$, by adopting faint SNe, it is still challenging to reproduce CEMP-no stars with extreme level of carbon abundance of $\rm A(C)\approx 7.0-7.5$, observed both in the MW halo and UFDs.