Constraints on CEMP-no progenitors from nuclear astrophysics

Abstract

Listen

Translate article

The CEMP-no stars are long-lived small mass stars presenting a very low iron content and overabundances of carbon with no sign or only very weak signs for the presence of s- or r-elements. Although the origin of that abundance pattern is still a matter of debate, it was very likely inherited from a previous massive star, that we shall call here the source star. We rely on a recent classification of CEMP-no stars arguing that some of them are made of a material processed by hydrogen burning that was enriched in products of helium burning during the nuclear life of the source star. We examine the possibility of forming CEMP-no stars with this material. We study the nucleosynthesis of the CNO cycle and the Ne-Na Mg-Al chains in a hydrogen burning single zone while injecting the helium burning products $^{12}$C, $^{16}$O, $^{22}$Ne and $^{26}$Mg. We investigate the impact of changing the density and temperature, as well as the injection rate. The nuclear reaction rates involving the creation and destruction of $^{27}$Al are also examined. $^{14}$N, $^{23}$Na, $^{24}$Mg and $^{27}$Al are formed when injecting $^{12}$C, $^{16}$O, $^{22}$Ne and $^{26}$Mg in the hydrogen burning zone. The $^{12}$C/$^{13}$C ratio is constant under various conditions in the hydrogen burning zone. The predicted [Al/Fe] ratio varies up to $\sim 2$ dex depending on the prescription used for the reaction rates involving $^{27}$Al. The experiments we carried out support the view that some CEMP-no stars are made of a material processed by hydrogen burning, coming from a massive star experiencing mild-to-strong rotational mixing. During its burning, this material was likely enriched in helium burning products. No material coming from the carbon-oxygen rich core of the source star should be added to form the daughter star, otherwise the $^{12}$C/$^{13}$C ratio would be largely above the observed range of values.