Relics of Metal-free Low-Mass Stars Exploding as Thermonuclear Supernovae

Abstract

Renewed interest in the first stars that were formed in the universe has led to the discovery of extremely iron-poor stars. Since several competing scenarios exist, our understanding of the mass range that determines the observed elemental abundances remains unclear. In this study, we consider three well-studied metal-poor stars in terms of the theoretical supernova (SN) model. Our results suggest that the observed abundance patterns in the metal-poor star BD +80 245 and the pair of stars HD 134439/40 agree strongly with the theoretical possibility that these stars inherited their heavy-element abundance patterns from SNe initiated by thermonuclear runaways in the degenerate carbon-oxygen cores of primordial asymptotic giant branch stars with masses of ~3.5-5 M☉. Recent theoretical calculations have predicted that such SNe could be originated from metal-free stars in the intermediate-mass range. On the other hand, intermediate-mass stars containing some metals would end their lives as white dwarfs after expelling their envelopes in the wind due to intense momentum transport from outgoing photons to heavy elements. This new pathway for the formation of SNe requires that stars be formed from the primordial gas. Thus, we suggest that stars of a few solar masses were formed from the primordial gas and that some of them caused thermonuclear explosions when the mass of their degenerate carbon-oxygen cores increased to the Chandrasekhar limit without experiencing efficient mass loss.